Chapter 4:Ocean Basins: Chapter Overview
Chapter Overview Pages 102 to 103


Ocean Basins: Chapter Objectives
Ocean Basins: Chapter Overview
Ocean Basins: Chapter Objectives




Remote sensing moves to the future. The new HROV Nereus, named after a Greek sea god who could change himself into any shape, will be capable of operating in two modes: free-swimming (autonomous) and tethered. In either mode, Nereuswill be able to spend up to 36 hours working in the ocean's deepest recesses. (HROV stands for hybrid remotely operated vehicle.) Robert Elder/Woods Hole Oceanographic Institution  View PDF



Ocean Basins  Click here to view
STUDY PLAN
•	The Ocean Floor Is Mapped by Bathymetry
•	Ocean-Floor Topography Varies with Location
•	Continental Margins May Be Active or Passive
•	The Topology of Deep-Ocean Basins Differs from That of the Continental Margin
•	The Grand Tour
FIVE MAIN CONCEPTS
1.	Tectonics forces shape the seabed.
2.	The ocean floor is divided into continental margins and deep-ocean basins. The continental margins are seaward extensions of the adjacent continents and are usually underlain by granite; the deep seabeds have different features and are usually underlain by basalt.
3.	Continental margins may be active (earthquakes, volcanoes) or passive, depending on the local sense of plate movement.
4.	The mid-ocean ridge system is perhaps Earth's most prominent feature. Most of the water of the world ocean circulates through hot oceanic crust in the ridges about every 10 million years.
5.	Using remote sensing methods, oceanographers have mapped the world ocean floor in surprising detail.
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Ocean Basins: Chapter Overview
Deep and Deeper In this chapter you will read about ocean-bottom features. We put the ideas in Chapter 3 to use here—tectonic forces, erosion, and deposition have built and shaped the seabed.
We've gained our knowledge of the seabed from people working at a distance from their goal. They use remote sensors to measure sound waves, radar beams, and differences in the pull of gravity. Then they combine this information to draw details of the seafloor. Sometimes, though, there is no substitute for actually seeing—focusing a well-trained set of eyes on the ocean floor or viewing high-definition images from a remote location. Here's where research submersibles come in handy.
Alvin, the best known and oldest of the deep-diving manned research submarine now in operation, is showing its age and will soon be retired. Its replacement, HROV Nereus, is being built for the Woods Hole Oceanographic Institution's National Deep Submergence Facility. Improvements in sensor and camera technology combined with the implementation of telepresence (sensory feedback) have greatly reduced the need for humans to be aboard, so Nereus is unmanned.
Nereus will operate in two modes. For wide area surveys it will be autonomous (guided by on-board computers and sensors). For sampling and other detailed tasks, it will be tethered to a ship and remotely controlled by researchers aboard ship. This unique configuration allows one vehicle to replace two. Nereus will venture anywhere in the ocean - it's capable of reaching the 11,000-meter (36,000-foot) depths of the great trenches.
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4.1: The Ocean Floor Is Mapped by Bathymetry Pages 104 to 108


Echo Sounders Bounce Sound off the Seabed
Multibeam Systems Combine Many Echo Sounders
Satellites Can Be Used to Map Seabed Contours
As I write, Mars Reconnaissance Orbiter, an orbiting robotic spacecraft, has mapped about half the surface of our planetary neighbor at a resolution that would reveal a dinner table resting on the sand. There are no oceans and few storms to spoil the view.




Figure 4.1 Seamen handling the steam winch aboard HMS Challenger. The winch was used to lower a weight on the end of a line to the seabed to find the ocean depth. The work was difficult and repetitive—a quarter of the 269 crew members eventually deserted during the four-and-a-half year journey! This illustration is from the Challenger Report (1880). Archive: Scripps Institute of Oceanography  View PDF

Mapping Earth is much more difficult because water and clouds hide more than three-quarters of the surface. Until surprisingly recently we have known more about the global contours of the moon and the inner planets than we knew about our own home. Thanks to modern bathymetry, our view is clearing.
The discovery and study of ocean floor contours is called bathymetry  (bathy, “deep”; meter, “measure”). The earliest-known bathymetric studies were carried out in the Mediterranean by a Greek named Posidonius in 85 B.C.E. He and his crew let out nearly 2 kilometers (1.25 miles) of rope until a stone tied to the end of the line touched bottom. Bathymetric technology had not improved by the time Sir James Clark Ross obtained soundings of 4,893 meters (16,054 feet) in the South Atlantic in 1818. In the 1870s, the researchers aboard HMS Challenger added the innovation of a steam-powered winch to raise the line and weight, but the method was the same (Figure 4.1). The Challenger crew made 492 bottom soundings and confirmed Matthew Maury's earlier discovery of the Mid-Atlantic Ridge.
Echo Sounders Bounce Sound off the Seabed
The sinking of the RMS Titanic in 1912 stimulated research that finally ended slow, laborious weight-on-a-line efforts. By April 1914, one of Thomas Edison's former employees had developed the “Iceberg Detector and Echo Depth Sounder.” The detector directed a powerful underwater sound pulse ahead of a ship and then listened for an echo from the submerged portion of an iceberg. It was easy to direct the beam downward to sense the distance to the bottom. It might take most of a day to lower and raise a weighted line, but echo sounders could take many bottom recordings in a minute.
In June 1922, an echo sounder based on his designs made the first continuous profile across an ocean basin aboard the USS Stewart, a U.S. Navy vessel. Using an improved echo sounder, the German research vessel Meteor made 14 profiles across the Atlantic from 1925 to 1927. The wandering path of the Mid-Atlantic Ridge was revealed, and its obvious coincidence with coastlines on both sides of the Atlantic stimulated the discussions that culminated in our present understanding of plate tectonics.
Echo sounding wasn't perfect. The ship's exact position was sometimes uncertain. The speed of sound through seawater varies with temperature, pressure, and salinity, and those variations made depth readings slightly inaccurate. Simple depth sounder images (such as that shown in Figure 4.2) were also unable to resolve the fine detail that oceanographers needed to explore seabed features. Even so, researchers using depth sounder tracks had painstakingly compiled the first comprehensive charts of the ocean floor by 1959 (Figure 4.3).1
Since then, two new techniques—made possible by improved sensors and fast computers—have been perfected to minimize inaccuracies and speed the process of bathymetry. Multibeam echo sounder systems and satellite altimetry (as well as other systems) have been used to study the features discussed in this chapter. Any of them is surely an improvement over lowering rocks into the ocean on ropes!
Multibeam Systems Combine Many Echo Sounders
Like other echo sounders, a multibeam system bounces sound off the seafloor to measure ocean depth. Unlike a simple echo sounder, a multibeam
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Figure 4.2 a The accuracy of an echo sounder can be affected by water conditions and bottom contours. The pulses of sound energy, or “pings,” from the sounder spread out in a narrow cone as they travel from the ship. When depth is great, the sounds reflect from a large area of seabed. As you'll see in Figure 4.4, a solution exists for this problem. b An echo sounder trace. A sound pulse from a ship is reflected off the seabed and returns to the ship. Transit time provides a measure of depth. For example, it takes about 2 seconds for a sound pulse to strike the bottom and return to the ship when water depth is 1,500 meters (4,900 feet). Bottom contours are revealed as the ship sails a steady course. In this trace, the horizontal axis represents the course of the ship, and the vertical axis represents water depth. The ship has sailed over a small submarine canyon. Robert Profeta  View PDF





Figure 4.3 Marie Tharp and geologist Bruce Heezen study the final version of Heinrich Berann's World Ocean Floor Panorama in Berann's studio, Austria, 1977. Berann was engaged by the United States Navy and the National Geographic Society to incorporate Tharp's work and data from other sources in a unified form. One of Berann's beautiful maps is included as Figure 4.22. LDEO, Columbia University  View PDF

system may have as many as 121 beams radiating from a ship's hull. Fanning out at right angles to the direction of travel, these beams can cover a 120º arc (Figure 4.4a). Typically, a pulse of sound energy is sent toward the seabed every 10 seconds. Listening devices record sounds reflected from the bottom, but only from the narrow corridors corresponding to the outgoing pulse. Successive observations build a continuous swath of coverage beneath the ship. By “mowing the lawn”—moving the ship in a coverage pattern similar to one you would follow in cutting grass—researchers can build a complete map of an area (Figure 4.4b). Further processing can yield remarkably detailed images like those of Figures 4.10 and 4.11. Fewer than 200 research vessels are equipped with multibeam systems. At the present rate, charting the entire seafloor in this way would require more than 125 years.
Satellites Can Be Used to Map Seabed Contours
Satellites cannot measure ocean depths directly, but they can measure small variations in the elevation of surface water. Using about a thousand radar pulses each second, the U.S. Navy's Geosat satellite (Figure 4.5a) measured its distance from the ocean surface to within 0.03 meter (1 inch)! Because the precise position of the satellite can be calculated, the average height of the ocean surface can be known with great accuracy.
Disregarding waves, tides, or currents, researchers have found the ocean surface can vary from the ideal
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Figure 4.4 How a multibeam system collects data. a A multibeam echo sounder uses as many as 121 beams radiating from a ship's hull. Fanning out at right angles to the direction of travel, these beams can cover a 120° arc and measure a swath of bottom about 3.4 times as wide as the water is deep. Typically, a “ping” is sent toward the seabed every 10 seconds. Listening devices record sounds reflected from the bottom, but only from the narrow corridors corresponding to the outgoing pulse. Thus a multi-beam system is much less susceptible to contour error than the single-beam device shown in Figure 4.2. bA multibeam record of a fragment of seafloor near the East Pacific Rise south of the tip of Baja California, Mexico. The uneven coverage reflects the path of the ship across the surface. Detailed analysis requires sailing a careful back-and-forth pattern. Computer processing provides extraordinarily detailed images, such as those in Figures 4.5e, 4.12 – 4.14. Andrew Goodwillie, SIO  View PDF





Figure 4.5 Measuring the seabed from space a Geosat, a U.S. Navy satellite that operated from 1985 through 1990, provided measurements of sea surface height from orbit. Moving above the ocean surface at 7 kilometers (4 miles) a second, Geosat bounced 1,000 pulses of radar energy off the ocean every second. Height accuracy was within 0.03 meter (1 inch)! Other satellites have taken its place. b Distortion of the sea surface above a seabed feature occurs when the extra gravitational attraction of the feature “pulls” water toward it from the sides, forming a mound of water over itself. U.S. Department of the Navy  View PDF

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Figure 4.5 (Continued) c A swath of the South Pacific seabed imaged by multibeam (echo) sounding. d The same swath imaged from space—note the added detail. e South America viewed by satellite altimetry. Note the high Andes Mountains, the Peru–Chile trench running the length of the continent's active west coast, the transform faults and fracture zones of the Chile Rise (at lower left), and the very large continental shelf on the passive (trailing) edge of the southern part of the continent. NOAA/NGDC/Walter H.F. Smith and David T. Sandwell NGDC/NOAA  View PDF

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Figure 4.6 Cross sections of the Atlantic Ocean basin and the continental United States, showing the range of elevations. The vertical exaggeration is 100:1. Although ocean depth is clearly greater than the average height of the continent, the general range of contours is similar.  View PDF

smooth (ellipsoid) shape by as much as 200 meters (660 feet). The reason is that the pull of gravity varies across Earth's surface depending on the nearness (or distance away) of massive parts of Earth. An undersea mountain or ridge “pulls” water toward it from the sides, forming a mount of water over itself (Figure 4.5b). For example, a typical undersea volcano with a height of 2,000 meters (6,600 feet) above the seabed and a radius of 20 kilometers (32 miles), would produce a 2 meter (6.6 foot) rise in the ocean surface. (This mound cannot be seen with the unaided eye because the slope of the surface is very gradual.) The large features of the seabed are amazingly and accurately reproduced in the subtle standing irregularities of the sea surface (Figure 4.5d)!

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CONCEPT CHECK
1.	How was bathymetry accomplished in years past? How do scientists do it now?
2.	Echo sounders bounce sound off the seabed to measure depth. How does that work?
3.	Satellites orbit in space. How can a satellite conduct oceanographic research? Why does the surface of the ocean “bunch up” over submerged mountains and ridges?
To check your answers, see the book's website. The website address is printed at the end of the chapter.
Geosat and its successors, TOPEX/Poseidon and Jason-1 and Jason-2, have allowed the rapid mapping of the world ocean floor from space. Hundreds of previously unknown features have been discovered through the data they have provided.
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4.2: Ocean-Floor Topography Varies with Location Pages 108 to 109


CONCEPT CHECK
Most people think an ocean basin is shaped like a giant bathtub. They imagine that the continents drop off steeply just beyond the surf zone and that the ocean is deepest somewhere out in the middle. As is clear inFigure 4.6, bathymetric studies have shown that this picture is wrong.
Why? As you read in the last chapter, the theory of plate tectonics suggests that Earth's surface is not a static arrangement of continents and ocean but a dynamic mosaic of jostling lithospheric plates. The lighter continental lithosphere floats in isostatic equilibrium above the level of the heavier lithosphere of the ocean basins. The great density of the seabed partly explains why more than half of Earth's solid surface is at least 3,000 meters (10,000 feet) below sea level
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Figure 4.7 A graph showing the distribution of elevations and depths on Earth. This curve is not a land-to-sea profile of Earth, but rather a plot of the area of Earth's surface above any given elevation or depth below sea level. Note that more than half of Earth's solid surface is at least 3,000 meters (10,000 feet) below sea level. The average depth of the ocean is much greater than the average elevation of the continents: The average depth of the world ocean (3,796 meters or 12,451 feet) is much greater than the average height of the continents (840 meters or 2,760 feet).  View PDF

(Figure 4.7). Continental crust is thicker than oceanic crust, and continental lithosphere is less dense than oceanic lithosphere. Therefore, the less dense litho-sphere containing the continents floats in isostatic equilibrium above the level of the denser lithosphere containing ocean basins. The seabed topography we observe is the result of this dynamic balance and the jostling of tectonic plates.

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CONCEPT CHECK
4. How would you characterize the general shape of an ocean basin?
5. If you could walk down into the seabed, the transition from granite to basalt would mark the true edge of the continent and would divide ocean floors into two major provinces. What are they?
6. How does a continental margin differ from a deep-ocean basin?
To check your answers, see the book's website. The website address is printed at the end of the chapter.
Notice in Figure 4.8 the transition between the thick (and less dense) granitic rock of the continents and the relatively thin (and denser) basalt of the deep-sea floor. Near shore the features of the ocean floor are similar to those of the adjacent continents because they share the same granitic basement. The transition to basalt marks the true edge of the continent and divides ocean floors into two major provinces. The submerged outer edge of a continent is called the continental margin . The deep-sea floor beyond the continental margin is properly called the ocean basin . Figure 4.9 shows the relative amounts of Earth's surface composed of continental and oceanic structures and the important subdivisions of each.
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4.3: Continental Margins May Be Active or Passive Pages 109 to 116


Continental Shelves Are Seaward Extensions of the Continents
Continental Slopes Connect Continental Shelves to the Deep-Ocean Floor
Submarine Canyons Form at the Junction between Continental Shelf and Continental Slope
Continental Rises Form As Sediments Accumulate at the Base of the Continental Slope
You learned in Chapter 3 that lithospheric plates converge, diverge, or slip past one another. As you might expect, the submerged edges of continents—continental margins—are greatly influenced by this tectonic activity. Continental margins facing the edges of diverging plates are called passive margins  because relatively little earthquake or volcanic activity is now associated with them. Because they surround the Atlantic, passive margins are sometimes referred to as Atlantic-type margins. Continental margins near the edges of converging plates (or near places where plates are slipping past one another) are called active margins  because of their earthquake and volcanic activity. Because of their prevalence in the Pacific, active margins are sometimes referred to as Pacific-type margins.
Figure 4.10 shows active and passive margins west and east of South America. Note that active margins coincide with plate boundaries but passive margins do not. Passive margins are also found outside the Atlantic, but active margins are confined mostly to the Pacific.
Continental margins have three main divisions: a shallow, nearly f at continental shelf close to shore; a more steeply sloped continental slope seaward; and an apron of sediment—the continental rise—that blends the continental margins into the deep-ocean basins.
Continental Shelves Are Seaward Extensions of the Continents


   The shallow submerged extension of a continent is called the continental shelf . Continental shelves, an extension of the adjacent continents, are underlain by granitic continental crust. They are much more like the continent than like the deep-ocean floor, and they may have hills, depressions, sedimentary rocks, and mineral and oil deposits similar to those on the dry land nearby. Taken together, the area of the continental shelves is 7.4% of Earth's ocean area.
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Figure 4.8 Cross section of a typical ocean basin flanked by passive continental margins. (The vertical scale has been greatly exaggerated to emphasize the basin contours.)  View PDF





Figure 4.9 Features of Earth's solid surface shown as percentages of the planet's total surface.  View PDF

Figure 4.11 shows a passive-margin continental shelf characteristic of Atlantic Ocean edges. The broad shelf extends far from shore in a gentle incline, typically 1.7 meters per kilometer (0.1º, or about 9 feet per mile), much more gradual than the slope of a well-drained parking lot. Shelves along the margin of the Atlantic Ocean often reach 350 kilometers (220 miles) in width and end at a depth of about 140 meters (460 feet), where a steeper drop-off begins.
The passive-margin shelves of the Atlantic Ocean formed as the fragments of Pangaea were carried away from each other by seafloor spreading. The continental lithosphere, thinned during initial rifting, cooled and contracted as it moved away from the spreading center, submerging the trailing edges of the continents and forming the shelves.
Most of the material composing a shelf comes from erosion of the adjacent continental mass. Rivers assist in passive shelf building by transporting huge amounts of sediments to the shore from far inland. In some places the sediments accumulate behind natural dams formed by ancient reefs or ridges of granitic crust (see again Figure 4.7). The weight of the sediment isostatically depresses the continental edges and allows the sediment load to grow even thicker. Sediment at the outer edge of a shelf can be up to 15 kilometers (9 miles) thick and 150 million years old at the bottom.
The width of a shelf is usually determined by its proximity to a plate boundary. You can see in Figure 4.8 that the shelf at the passive margin (east of South America) is broad, but the shelf at the active margin (west of South America) is very narrow. The widest shelf, 1,280 kilometers (800 miles) across, lies north of Siberia in the tectonically quiet Arctic Sea. Shelf width depends not only on tectonics but also on marine processes: Fast-moving ocean currents can sometimes prevent sediments from accumulating. For example, the east coast of Florida has a very narrow shelf because there is no natural offshore dam formed by ridges of granitic crust and because the swift current of the nearby Gulf Stream scours surface sediment away. Florida's west coast, however, has a broad shelf with a steep terminating slope (Figure 4.12).
A broad shelf has resulted from sediment accumulation along the sheltered edges of the Gulf of Mexico (Figure 4.13). Sediment eroded from the Rocky Mountains has been carried by the Mississippi River and deposited south of Texas and Louisiana. This sediment overlies salt deposited about 180 million years ago when much of the water in the Gulf evaporated. The weight of the overlying sediment causes domes of salt to rise,
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Figure 4.10 Typical continental margins bordering the tectonically active (Pacific-type) and tectonically passive (Atlantic-type) edges of a moving continent. (The vertical scale has been exaggerated.)  View PDF





Figure 4.11 The features of a passive continental margin. a Vertical exaggeration 50:1. b With no vertical exaggeration.  View PDF

spread out, and dissolve; collapsed salt domes create the pockmarked bottom characteristic of the area.
The shelves of the active Pacific margins are generally not as broad and f at as the Atlantic shelves. An example is the abbreviated shelf off the west coast of South America, where the steep western slope of the Andes Mountains continues nearly uninterrupted beneath the sea into the depths of the Peru–Chile Trench (see again Figures 4.4e and 3.18d). Active-margin shelves have more varied topography than passive-margin shelves; the character of continental shelves at an active margin may be determined more by faulting, volcanism, and tectonic deformation than by sedimentation (Figure 4.14).
A few Pacific shelves are broad, however. As in the Atlantic, natural offshore dams trap sediments and form shelves; but in the Pacific, the sediment-trapping dams are more commonly offshore chains of volcanoes or lines of coral reefs. Volcanic activity east of China and Southeast Asia has formed a broad basin that is now filling with sediments and is one of the largest shelves in the Pacific.
Because of their gentle slope, continental shelves are greatly influenced by changes in sea level. Around
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Figure 4.12 A cliff more than 1.6 kilometers (1 mile) high marks the edge of the continental shelf west of central Florida. The very steep continental slope seen here is unusual. Perhaps fresh water seeping from the adjacent land has undermined the slope and caused it to collapse. Currents have removed much of the material that would otherwise be found at the base of the slope. William Haxby, Lamont–Doherty Earth Observatory of Columbia  View PDF





Figure 4.13 The broad continental shelf along the edge of the Gulf of Mexico south of Texas and Louisiana (looking east). Sediments carried by the Mississippi have overlain ancient salt deposits. The weight of the sediments causes salt domes to form and dissolve, leaving pockmarks in the continental shelf. William Haxby, Lamont–Doherty Earth Observatory of Columbia  View PDF





Figure 4.14 The complex continental shelf off central California, typical of an active margin. Compare to Figure 4.12. NESDIS/National Geophysical Data Center/Dr. Lincoln Pratson  View PDF

18,000 years ago—at the height of the last ice age  (period of widespread glaciation)—massive ice caps covered huge regions of the world's continents. The water that formed these thick ice sheets came from the ocean, and sea level fell about 125 meters (410 feet) below its present position (Figure 4.15).2 The continental shelves were almost completely exposed, and the surface area of the continents was about
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Figure 4.15 Changes in sea level over the last 250,000 years, as traced by data taken from ocean floor cores. The rise and fall of sea level is due largely to the coming and going of ice ages—periods of increased and decreased glaciation, respectively. Water that formed the ice-age glaciers came from the ocean, and this caused sea level to drop. Point aindicates a low stand of 2125 meters (2410 feet) at the climax of the last ice age some 18,000 years ago. Point b indicates a high stand of 16 meters (119.7 feet) during the last interglacial period about 120,000 years ago. Point c shows the present sea level. Sea level continues to rise as we emerge from the last ice age and enter an accelerating period of global warming. For more detail of the last 20,000 years, please look ahead to Figure 12.2  View PDF





Figure 4.16 An offshore oil drilling platform in the relatively shallow continental shelf waters of Trading Bay near Anchorage, Alaska. Deep-water platforms may be seen in Figure 17.4. AP Photo/U.S. Coast Guard  View PDF

18% greater than as it is today. Rivers and waves cut into the sediments that had accumulated during periods of higher sea level, and they transported some coarse sediments to their present locations at the shelves’ outer edges. Sea level began to rise again when the ice caps melted, and sediments again began to accumulate on the shelves. More on the history and effects of sea-level change will be found in the discussion of coasts in Chapter 12 and of environmental issues in Chapter 18.
The continental shelves have been the focus of intense exploration for natural resources. Because shelves are the submerged margins of continents, any deposits of oil or minerals along a coast are likely to continue offshore. Water depth over shelves averages only about 75 meters (250 feet), so large areas of the shelves are accessible to mining and drilling activities. Many of the techniques used to find and exploit natural resources on land can also be used on the continental shelves. Resource development requires intensive scientific investigations, and our understanding of the geology of the shelves has benefited greatly from the search for offshore oil and natural gas (Figure 4.16).3
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Figure 4.17 (a) A submarine canyon. (b) A multibeam image of Hudson Canyon east of New Jersey. The shelf in this area is broad. The canyon can be seen nicking the shelf-slope junction and then continuing toward the abyssal plain to the southwest. The underwater topology has been exaggerated by a factor of 5 relative to the land topography. The fine black line marks sea level. Ray Sterner/The Johns Hopkins University, Applied Physics Laboratory/North Star Science & Technology, LLC  View PDF

Continental Slopes Connect Continental Shelves to the Deep-Ocean Floor
The continental slope  is the transition between the gently descending continental shelf and the deep-ocean floor. Continental slopes are formed of sediments that reach the built-out edge of the shelf and are transported over the side. At active margins a slope may also include marine sediments scraped off a descending plate during subduction. The inclination of a typical continental slope is about 4º (70 meters per kilometer, or 370 feet per mile), slightly steeper than the steepest road slope allowed on the interstate highway system. As Figure 4.9b implied, even the steepest of these slopes is not precipitous: a 25º slope is the greatest incline yet discovered. In general, continental slopes at active margins are steeper than those at passive margins. Continental slopes average about 20 kilometers (12 miles) wide and end at the continental rise, usually at a depth of about 3,700 meters (12,000 feet). The bottom of the continental slope is the true edge of a continent.
The shelf break  marks the abrupt transition from continental shelf to continental slope. The depth of water at the shelf break is surprisingly constant—about 140 meters (460 feet) worldwide—but there are exceptions. The great weight of ice on Antarctica, for example, has isostatically depressed that continent, and the depth of water at the shelf break is 300–400 meters (1,000–1,300 feet). The shelf break in Greenland is similarly depressed.
Submarine Canyons Form at the Junction between Continental Shelf and Continental Slope
Submarine canyons  cut into the continental shelf and slope, often terminating on the deep-sea floor in a fan-shaped wedge of sediment (Figure 4.17a). More than a hundred submarine canyons nick the edge of nearly all of Earth's continental shelves. The canyons generally trend at right angles to the shoreline (and shelf edge),
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Figure 4.18 A turbidity current flowing down a submerged slope off the island of Jamaica. A turbidity current is not propelled by the water within it, but by gravity. The propeller of a submarine caused the turbidity current by disturbing sediment along the slope. Society for Sedimentary Geology/Dr. Lynton Land  View PDF





Figure 4.19 A continuous cascade of sediment at the head of San Lucas submarine canyon (off the coast of Baja California, Mexico), which may be eroding the narrow gorge in conjunction with occasional turbidity currents. About 100,000 cubic meters (130,000 cubic yards) of sand slip down this canyon every year. U.S. Department of the Navy  View PDF

sometimes beginning very close to shore. Congo Canyon actually extends into the African continent as a deep estuary at the mouth of the Congo River. These enigmatic features can be quite large. In fact, submarine canyons similar in size and profile to Arizona's Grand Canyon have been discovered!
Hudson Canyon, a typical large canyon on a passive margin, is shown in Figure 4.17b. Like many submarine canyons, Hudson Canyon is located just offshore of the mouth of a river or stream—in this case, New York's Hudson River. Because of their similarity to canyons on land, submarine canyons appear to have been created by erosion, so marine geologists initially thought the canyons were carved into the shelves by stream erosion at times of lower sea level. But most researchers agree that sea level has never fallen more than 200 meters (660 feet) below its present level in the last 600 million years. Stream erosion could account for the shape of the uppermost parts of the canyons. However, since submarine canyons can be traced to depths in excess of 3,000 meters (10,000 feet) below sea level, stream erosion could not have played a direct role in cutting their lower depths.
What, then, caused the submarine canyons to form? Local landslides or sediment liquefaction triggered by earthquakes sometimes causes an abrasive underwater “avalanche” of sediments. These mass-movements of sediment, called turbidity currents , occur when turbulence mixes sediments into water above a sloping bottom. The sediment-filled water is denser than the surrounding water, so the thick muddy fluid runs down the slope at speeds up to 27 kilometers (17 miles) per hour. Figure 4.18 is a rare photo of a turbidity current.
What is the connection between turbidity currents and submarine canyons? Sediments may cascade continuously down the canyons (Figure 4.19), but earthquakes can shake loose huge masses of boulders and sand that rush down the edge of the shelf, scouring the canyon deeper as they go. Most geologists believe that the canyons have been formed by abrasive turbidity currents plunging down the canyons. In this way the canyons can be cut to depths far below the reach of streams even during the low sea levels of the ice ages.
Continental Rises Form As Sediments Accumulate at the Base of the Continental Slope
Along passive margins, the oceanic crust at the base of the continental slope is covered by an apron of accumulated sediment called the continental rise  (see again Figures 4.8 and 4.11). Sediments from the shelf slowly descend to the ocean floor along the whole continental slope, but most of the sediments that form the continental rise are transported to the area by turbidity currents. The width of the rise varies from 100 to 1,000 kilometers (63 to 630 miles), and its slope is gradual—about one-eighth that of the continental slope. One of the widest and thickest continental rises has formed in the Bay of Bengal at the mouths of the Ganges–Brahmaputra River, the most sediment-laden of the world's great rivers.
Deep ocean currents are an important factor in shaping continental rises, especially along the western boundaries of most ocean basins. Deep boundary currents held against the continental slopes by Coriolis Effect (news of which awaits you in Chapter 8) pick up volcanic debris and sediments transported by turbidity currents, and sweep it along the ocean floor. As the currents are forced around bends and across depressions, they slow and deposit the
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Figure 4.20 Large stationary waves of sediment deposited at the base of the continental rise in the western North Atlantic. Deep boundary currents pick up sediments and sweep them across the ocean floor. As the currents are forced around bends and across depressions, they slow and deposit the suspended material at the base of the rises in the form of ridges or mud waves. Charles D. Hollister, WHOI  View PDF


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CONCEPT CHECK
7. What are the features of the continental margins?
8. How is an active tectonic margin different from a passive tectonic margin?
9. How do the widths of continental shelves differ between active margins and passive margins?
10. How has sea level varied with time? Is sea level unusually high or low at present?
11. What are submarine canyons? Where are they found, and how are they thought to have been formed?
12. Where would you look for a continental rise? What forms continental rises?
To check your answers, see the book's website. The website address is printed at the end of the chapter.
suspended material on the rises in the form of ridges and mud waves (Figure 4.20).
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4.4: The Topology of Deep-Ocean Basins Differs from That of the Continental Margin Pages 116 to 125


Oceanic Ridges Circle the World
Hydrothermal Vents Are Hot Springs on Active Oceanic Ridges
Abyssal Plains and Abyssal Hills Cover Most of Earth's Surface
Volcanic Seamounts and Guyots Project above the Seabed
Trenches and Island Arcs Form in Subduction Zones
Away from the margins of continents the structure of the ocean floor is quite different. Here the seafloor is a blanket of up to 5 kilometers (3 miles) thick overlying basaltic rocks. Deep-ocean basins constitute sediments more than half of Earth's surface.
The deep-ocean floor consists mainly of oceanic ridge systems and the adjacent sediment-covered plains. Deep basins may be rimmed by trenches or by masses of sediment. Flat expanses are interrupted by islands, hills, active and extinct volcanoes, and active zones of seafloor spreading. The sediments on the deep-ocean floor reflect the history of the surrounding continents, the biological productivity of the overlying water, and the ages of the basins themselves.
Oceanic Ridges Circle the World
If the ocean evaporated, the oceanic ridges would be Earth's most remarkable and obvious feature. Anoceanic ridge  is a mountainous chain of young basaltic rock at the active spreading center of an ocean. Stretching 65,000 kilometers (40,000 miles), more than 1½ times Earth's circumference, oceanic ridges girdle the globe like seams surrounding a softball (Figure 4.21a). The rugged ridges, which often are devoid of sediment, rise about 2 kilometers (1.25 miles) above the seafloor. In places, they project above the surface to form islands such as Iceland, the Azores, and Easter Island. Oceanic ridges and their associated structures account for 22% of the world's solid surface area (all the land above sea level accounts for 29%). Although these features are often called mid-ocean ridges, less than 60% of their length actually exists along the centers of ocean basins.
As we saw in our discussion of plate tectonics, the rift zones associated with oceanic ridges are sources of new ocean floor where lithospheric plates diverge. The oceanic ridges are widest where they are most active. The youngest rock is located at the active ridge center, and rock becomes older with distance from the center. As the lithosphere cools, it shrinks and subsides. Slowly spreading ridges have a steeper profile than rapidly spreading ones because slowly diverging seafloor cools and shrinks closer to the spreading center. Figure 4.21b shows these distinctly different ridge profiles.
Figure 4.22, a bathymetric map of the North Atlantic, clearly shows the great extent of the Mid-Atlantic Ridge, a typical oceanic ridge. Figure 4.23 (page 119), a multibeam image, provides a detailed look at the young central rift.
As can be seen in Figure 4.22, the Mid-Atlantic Ridge does not run in a straight line. It is offset at more or less regular intervals by transform faults. A fault, recall, is a fracture in the lithosphere along which movement has occurred, and transform faults  are fractures along which lithospheric plates slide horizontally (Figure 4.24, page 120). As you saw in Figure 3.18c and 3.27a, when segments of a ridge system are offset, the fault connecting the axis of the ridge is a transform fault. Shallow earthquakes are common along transform faults. Because the ocean floor cannot expand evenly on the surface of a sphere, plate divergence on the spherical Earth can only be irregular and asymmetrical, and transform faults and fracture zones result.
Transform faults are the active part of fracture zones . Extending outward from the ridge axis, fracture
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Figure 4.21 a The oceanic ridge system (in colors) stretches some 65,000 kilometers (40,000 miles) around Earth. If the ocean evaporated, the ridge system would be Earth's most remarkable and obvious feature. The thickness of the red lines indicate the rate of spreading for some of the most rapidly spreading sections, and the numbers give spreading rates in centimeters per year. The East Pacific Rise typically spreads about six times faster than the Mid-Atlantic Ridge.b Rapid spreading at the East Pacific Rise (lower image) spreads ridge features over a greater area. The slower spreading along the Mid-Atlantic Ridge concentrates the features in a smaller area with more pronounced contours. The relatively slow-spreading ridge is shown in more detail in Figure 4.17b.  View PDF

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Figure 4.22 A hand-drawn map of a portion of the Atlantic Ocean floor showing some major oceanic features: mid-ocean ridge, transform faults, fracture zones, submarine canyons, seamounts, continental rises, trenches, and abyssal plains. Depths are in feet. The map is vertically exaggerated. ALCOA Aluminum Company  View PDF

zones are seismically inactive areas that show evidence of past transform fault activity. While segments of a lithospheric plate on either side of a transform fault move in opposite directions from each other, the plate segments adjacent to the outward segments of a fracture zone move in the same direction, as Figure 4.24 shows.
Hydrothermal Vents Are Hot Springs on Active Oceanic Ridges
Some of the most exciting features of the ocean basins are the hydrothermal vents . In 1977 Robert Bal-lard and J. F. Grassle of the Woods Hole Oceanographic Institution discovered hot springs on oceanic ridges. Diving in Alvin at 3 kilometers (1.9 miles) near the Galápagos Islands along the East Pacific Rise (an oceanic ridge), they came across rocky chimneys up to 20 meters (66 feet) high, from which dark, mineral-laden water was blasting at 350ºC (660ºF) (Figure 4.25, page 120). Only the great pressure at this depth prevented the escaping water from flashing to steam. These black smokers, as they were quickly nicknamed, fascinate marine geologists. It is believed that water descends through fissures and cracks in the ridge floor until it comes into contact with very hot rocks associated with active seafloor spreading. There the superheated, chemically active water dissolves minerals and gases and escapes upward through the vents by convection (Figure 4.26, page 121).
Since that first discovery, vents have been found on the Mid-Atlantic Ridge east of Florida, in the Sea of Cortez east and south of Baja California, and on the Juan de Fuca Ridge off the coast of Washington and Oregon. Scientists now believe that hydrothermal vents may be very common on oceanic ridges, especially in zones of rapid seafloor spreading. In July 1990, vents were discovered in fresh water, at the bottom of Lake Baikal in southern Siberia. This discovery suggests that the world's oldest and deepest lake may someday become part of the ocean as Asia slowly breaks apart.
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Figure 4.23 a The fine structure of the central portion of the Mid-Atlantic ridge between Florida and western Africa. The depressed central valley (the spreading center, shown in blue) is clearly visible in this computergenerated multi-beam image.b As this seismic profile shows, the rugged relief of the North Atlantic's oceanic ridge is gradually being buried by slowly accumulating sediments. Charles D. Hollister, WHOI  View PDF

In Iceland, these vents may be seen on dry land! As you may recall, the country of Iceland rests uneasily on a mid-ocean ridge lifted above sea level (see again Chapter 3's opener). Figure 4.27 is an extraordinary view of the central rift of that ridge (similar to that seen as the blue-colored contour in Figure 4.23a). The rift rises from an Icelandic lake at the left, traverses to right, and supports many thermal vents visible as jets of steam. Notice the hills paralleling the rift on the far side. Crustal spreading in this area averages about 10 centimeters (4 inches) a year.
Not all vents form chimneys of mineral deposits—some are simply cracks in the seabed, or porous mounds, or broad segments of ocean ridge floor through which warm, mineral-laden water percolates upward. Cooler vents result when hot, rising water mixes with cold bottom water before reaching the surface. Water temperature in the vicinity of most hydrothermal vents
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Figure 4.24 Transform faults and fracture zones along an oceanic ridge. Transform faults are fractures along which lithospheric plates slide horizontally past one another. Transform faults are the active part of fracture zones. For a review of this process in larger scale, please see Figure 3.27.  View PDF





Figure 4.25 A black smoker discovered at a depth of about 2,800 meters (9,200 feet) along the East Pacific Rise. Woods Hole Oceanographic Institution  View PDF

averages 8º–16ºC (46º–61ºF), much warmer than usual for ocean-bottom water, which has an average temperature of 3º–4ºC (37º–39ºF). We will study the unusual communities of marine animals that populate these vents in Chapter 14.



Fault Zones  Click here to view
A volume of water equal to the volume of the world ocean is thought to circulate through the hot oceanic crust at spreading centers about every 10 million years! The water coming from the vents and seeping from the floor is more acidic, is enriched with metals, and has higher concentrations of dissolved gas than seawater. The heat and chemicals issuing from these structures may play important roles in the chemical composition of seawater and the atmosphere and in the formation of mineral deposits.
Abyssal Plains and Abyssal Hills Cover Most of Earth's Surface
A quarter of Earth's surface consists of abyssal plains and abyssal hills. Abyssal is an adjective derived from a Greek word meaning “without bottom.” Although this is obviously not literally true, you can appreciate how the term came into use following the
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Figure 4.26 A cross section of the central part of a mid-ocean ridge—similar to that shown in Figure 4.17b—showing the origin of hydrothermal vents. Cool water (blue arrows) is heated as it descends toward the hot magma chamber, leaching sulfur, iron, copper, zinc, and other materials from the surrounding rocks. The heated water (red arrows) returning to the surface carries these elements upwards, discharging them at the hydrothermal springs on the seafloor. The areas around the vents support unique communities of organisms (see Chapter 14).  View PDF

Challenger expedition's laborious soundings of these extremely deep areas!
Abyssal plains  are flat, featureless expanses of sediment-covered ocean floor found on the periphery of all oceans. They are most common in the Atlantic, less so in the Indian Ocean, and relatively rare in the active Pacific, where peripheral trenches trap most of the sediments flowing from the continents. They lie between the continental margins and the oceanic ridges about 3,700 to 5,500 meters (12,000 to 18,000 feet) below the surface (see again Figure 4.21a). The Canary Abyssal Plain, a huge plain west of the Canary Islands in the North Atlantic, has an area of about 900,000 square kilometers (350,000 square miles).



Figure 4.27 The Mid-Atlantic ridge comes ashore in southwestern Iceland. The central rift (similar to that seen as the blue-colored contour in Figure 4.17b) is the valley in the middle distance. Notice the linear hills paralleling the rift and the steam issuing from thermal vents. If this part of the rift were submerged, these fumaroles would be hydrothermal vents similar to the one seen in Figure 4.19. Reykjavik, Iceland's largest city, is supplied with domestic hot water, hot water for space heating, and geothermally generated electricity from this valley. Tom Garrison  View PDF

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Figure 4.28 The deep, smooth sediments of the Atlantic's Northern Madeira Abyssal Plain bury 100-million-year-old mountains. This image was generated by a powerful echo sounder. Charles D. Hollister, WHOI  View PDF

Abyssal plains are extraordinarily flat. A 1947 survey by the Woods Hole Oceanographic Institution shipAtlantis found that a large Atlantic abyssal plain varies no more than a few meters in depth over its entire area. Such flatness is caused by the smoothing effect of the layers of sediment, which often exceed 1,000 meters (3,300 feet) in thickness. Most of the sediment that forms the abyssal plains appears to be of terrestrial or shallow-water origin, not derived from biological activity in the ocean above. Some of it may have been transported to the plains by winds or turbidity currents. These deep sediment layers mask irregularities in the underlying ocean crust, but a powerful type of echo sounder can “see” through this sediment to reveal the complex topography of the basaltic basin floor below (Figure 4.28). The broad basaltic shoulders of the Mid-Atlantic Ridge extend beneath this cloak of sediment almost as far as the bordering continental slopes.
Abyssal plain sediments may not be thick enough to cover the underlying basaltic floor near the edges bordering the oceanic ridges. Here the plains are punctuated by abyssal hills —small, sediment-covered extinct volcanoes or intrusions of once-molten rock, usually less than 200 meters (650 feet) high (one is seen extending above the f at surface in Figure 4.28). These abundant features are associated with seafloor spreading; they form when newly formed crust moves away from the center of a ridge, stretches, and cracks. Some blocks of the crust drop to form valleys, and others remain higher as hills. Lava erupting from the ridge flows along the fractures, coating the hills. This helps explain why abyssal hills occur in lines parallel to the flanks of the nearby oceanic ridge and why they occur most abundantly in places where the rate of seafloor spreading is fastest. Abyssal plains and hills account for nearly all the area of deep-ocean floor that is not part of the oceanic ridge system. They are Earth's most common “landform.”
Volcanic Seamounts and Guyots Project above the Seabed
The ocean floor is dotted with thousands of volcanic projections that do not rise above the surface of the sea. These projections are called seamounts. Seamounts  are circular or elliptical, more than 1 kilometer (0.6 mile) in height, with relatively steep slopes of 20º to 25º. (Abyssal hills, in contrast, are much more abundant, less than a kilometer high, and not as steep.)
Seamounts may be found alone or in groups of 10 to 100. Though many form at hot spots, most are thought to be submerged inactive volcanoes that formed at spreading centers (Figure 4.29). Movement of the lithosphere away from spreading centers has carried them outward and downward to their present positions, and will eventually cause them to bond with the edge of a continent or disappear into a trench (Figure 4.30, page 124). As many as 10,000 seamounts are thought to exist in the Pacific, about half the world total.
Guyots  are flat-topped seamounts that once were tall enough to approach or penetrate the sea surface. Generally, they are confined to the west-central Pacific. The flat top suggests that they were eroded by wave action when they were near sea level. Their plateau-like tops eventually sank too deep for wave erosion to continue wearing them down. Like the more abundant seamounts, most guyots were formed near spreading centers and transported outward and downward as the seafloor moved away from a spreading center and cooled.
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Figure 4.29 Guyots and seamounts. a The process by which guyots (G) and seamounts (S) form. Guyots have flat tops because they were once tall enough to be eroded by waves at the ocean's surface. Seamounts have a similar origin but retain their more pointed volcano shape because they never reached the surface. b An undersea volcano east of the easternmost island of the Samoan chain in the Pacific. Vailulu'u, as it has been named, rises from an ocean depth of 4,800 meters (15,700 feet) to within 590 meters (1,900 feet) of the ocean surface. A new volcano grew inside Vailulu'u between 1999 and 2005. Image courtesy of Vailulu'u Exploration, NOAA-OE Stanley Hart, Woods Hole Oceanographic Institution  View PDF

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Figure 4.30 Seamounts slowly approach a trench near in the Pacific Basin west of Costa Rica. They will collapse into the trench, and may trigger large earthquakes in the process. Dr. Wilhelm Weinrebe, Leibniz-Institut für Meereswissenschaften  View PDF

Trenches and Island Arcs Form in Subduction Zones
A trench  is an arc-shaped depression in the deep-ocean floor. These creases in the seafloor occur where a converging oceanic plate is subducted. The water temperature at the bottom of a trench is slightly cooler than the near-freezing temperatures of the adjacent f at ocean floor, reflecting the fact that trenches are underlain by old, relatively cold ocean crust sinking into the upper mantle. Trenches (and their associated island arcs topped by erupting volcanoes) are among the most active geological features on Earth. Great earthquakes and tsunami (huge waves we will discuss in Chapter 9) often originate in them. Figure 4.31shows the distribution of the ocean's major trenches. It is not surprising that most are around the edges of the active Pacific.
Trenches are the deepest places in Earth's crust, 3 to 6 kilometers (1.9 to 3.7 miles) deeper than the adjacent basin floor. The ocean's greatest depth is the Mariana Trench of the western Pacific, where the ocean bottom is 11,022 meters (36,163 feet) below sea level, 20% deeper than Mount Everest is high (Figure 4.32). The Mariana Trench is about 70 kilometers (44 miles) wide and 2,550 kilometers (1,600 miles) long, typical dimensions for these structures.
Trenches are curving chains of V-shaped indentations. The trenches are curved because of the geometry of plate interactions on a sphere. The convex sides of these curves generally face the open ocean (see again Figure 4.24). The trench walls on the island side of the depressions are steeper than those on the seaward side, indicating the direction of plate subduction. The sides of trenches become steeper with depth, normally reaching angles of about 10º–16º before flattening to a floor underlain by thick sediment. (Parts of the concave wall of the Kermadec–Tonga Trench are the world's steepest at 45º.) No continental rise occurs along coasts with trenches because the sediment that would form the rise ends up at the bottom of the trench.
Island arcs , curving chains of volcanic islands and seamounts, are almost always found parallel to the concave edges of trenches. As you may remember from Chapter 3, trenches and island arcs are formed by tectonic and volcanic activity associated with sub-duction. The descending lithospheric plate contains some materials that melt as the plate sinks into the mantle. These materials rise to the surface as magmas and lavas that form the chain of islands behind the trench. The Aleutian Islands, most Caribbean islands, and the Mariana Islands are island arcs. (See Figure 3.25 for a review of the processes involved in their construction.)

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CONCEPT CHECK
13. What are typical features of deep-ocean basins?
14. What is the extent of the mid-ocean ridge system? Are mid-ocean ridges always literally in mid-ocean?
15. Draw a cross section through an active mid-ocean ridge. Where are the hydrothermal vents located? Where is new seabed being formed?
16. What are fracture zones? What causes these lateral breaks?
17. What are abyssal plains? What is unique about them?
18. Why are abyssal plains relatively rare in the Pacific?
19. How do guyots form? How were lines of guyots and seamounts important in deciphering plate tectonics?
20. How are the ocean's trenches formed? How are earthquakes related to their formation?
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21. Why are trenches and island arcs curved? Is the descent to the bottom steeper on the convex side of the arc or on the concave side? Why do you think most trenches are in the western Pacific?
To check your answers, see the book's website. The website address is printed at the end of the chapter.




Figure 4.31 Oceanic trenches of the world. Note their prevalence in the active Pacific.  View PDF





Figure 4.32 The Mariana Trench a Comparing the Challenger Deep and Mount Everest at the same scale shows that the deepest part of the Mariana Trench is about 20% deeper than the mountain is high. b The Mariana Trench shown without vertical exaggeration.  View PDF

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4.5: The Grand Tour Pages 125 to 128


CONCEPT CHECK
Researchers at the National Oceanic and Atmospheric Administration have generated a map of the world ocean floor based on satellite observations of the shape of the sea surface (Figure 4.33). The graphic
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Figure 4.33 A technological tour de force: a map that shows all the features discussed in this chapter, derived from data provided to the National Geophysical Data Center from satellites and shipborne sensors. These features—and a basic understanding of the geological reasons for their existence—will help you recall the dramatic nature and history of the seafloor that we have discussed in the past two chapters. David Sandwell and Walter Smith, NGDC/SIO  View PDF

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shows all the features discussed in this chapter. These features—and a basic understanding of the geological reasons for their existence—will help you recall the dramatic nature and history of the seafloor that we have discussed in the past two chapters.

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CONCEPT CHECK
20. How do you think graphics like The Grand Tour have assisted our understanding of geological processes?
To check your answers, see the book's website. The website address is printed at the end of the chapter.
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End of Chapter Review Pages 128 to 129


Ocean Basins: Questions from Students
Ocean Basins: Chapter in Perspective
Ocean Basins: Terms and Concepts to Remember
Ocean Basins: Study Questions
Ocean Basins: Questions from Students
1. Shouldn't the ocean be deeper in the middle? And why is Iceland one of only a few places in the world where oceanic crust is found above sea level?
Understanding why there are ocean basins explains their contours. A basin usually contains an expanding ridge that is higher than the surrounding bed. Oceanic crust is thin and dense. Because of isostasy, the ocean floor lies at a lower elevation than the thicker, less dense, higher continents. Water filled the lower elevations first, submerging nearly all the basaltic basement we now call ocean floor. In areas of rapid seafloor spreading, or at hot spots, peaks are occasionally pushed toward the ocean surface by the large volume of mantle material rising in plumes from below. Large quantities of erupted magma (lava) then build the crests above sea level to form islands, as in Iceland. The Azores is another place on the Mid-Atlantic Ridge where this is happening.
2. Turbidity currents seem important in forming canyons and distributing deep sediments over abyssal plains. Has anybody ever seen a turbidity current in action?
Yes, surprisingly. In the late 1940s the Dutch geologist Philip Kuenen produced turbidity currents in his laboratory by pouring muddy water into a trough with a sloping bottom. His observations confirmed nineteenth-century reports that the muddy Rhône River continued to flow in a dense stream along the bottom of Lake Geneva. In the 1960s, Robert Dill and Francis Shepard viewed sandfalls in Scripps Canyon from the diving saucer, and French researchers have recently photographed these currents in the Mediterranean.
3. Is Alvin the deepest-diving human-carrying research submersible?
No, that honor belongs to Shinkai 6500. Operated by a consortium of Japanese research institutions and industries, Shinkai 6500 is the newest deep-diving vehicle capable of carrying a human crew. Now the deepest-diving submersible in operation, Shinkai 6500 safely descended to a depth of 6,527 meters (21,409 feet) on 11 August 1989, and can reach all but the deepest trench floors, or about 98% of the world ocean bottom.
4. Wouldn't wave action and tides hopelessly clutter the radar signals sent from satellites to determine sea surface height?
Satellite altimetry is one of the most sophisticated oceanographic uses of high-speed computer processing. Imagine the processing power needed to reduce the data generated by more than 1,000 radar pulses from orbit each second! Programmers subtract predicted tidal height from the measurements, and then use algorithms to average and cancel wave crests and troughs. The remaining sea surface height—determined to perhaps 2 centimeters (slightly less than one inch)—is due to gravitational variations caused by submerged features. Still more processing is needed to generate graphic images like that of Figure 4.4d or 4.4e.
The procedures were pioneered by the U. S. Navy and the Office of Naval Research. The initial goal was to provide detailed seafloor maps for use in anti-submarine defense.
5. How far away is the horizon when I stand on the beach and look out to sea?
That depends on how tall you are. If you're about six feet tall, the distance to the horizon is about 4.8 kilometers (3 miles). For a more precise estimate, subtract 10 centimeters (4 inches or a third of a foot) to find the height of your eyes above the ground, then divide that number by 0.5736.4 Take the square root of the result, and there's the number of miles to the horizon. Lifeguards standing on a 3 meter (10 foot) tower see a horizon roughly 8.5 kilometers (5.3 miles) distant.
Ocean Basins: Chapter in Perspective
In this chapter you learned how diffcult it has been to discover the shape of the seabed. Even today, the surface contours of Mars are better known than those of our ocean floor.
We now know that seafloor features result from a combination of tectonic activity and the processes of erosion and deposition. The ocean floor can be divided into two regions: continental margins and deep ocean basins. The
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continental margin, the relatively shallow ocean floor nearest the shore, consists of the continental shelf and the continental slope. The continental margin shares the structure of the adjacent continents, but the deep-ocean floor away from land has a much different origin and history. Prominent features of the deep-ocean basins include rugged oceanic ridges, flat abyssal plains, occasional deep trenches, and curving chains of volcanic islands. The processes of plate tectonics, erosion, and sediment deposition have shaped the continental margins and ocean basins.
In the next chapter you will learn that nearly all the ocean floor is blanketed with sediment. Except for the spreading centers themselves, the broad shoulders of the oceanic ridge systems are buried according to their age—the older the seabed, the greater the sediment burden. Some oceanic crust near the trailing edges of plates may be overlain by sediments more than 1,500 meters (5,000 feet) thick. Sediments have been called the “memory of the ocean.” The memory, however, is not a long one. Before continuing, can you imagine why that is so?
Ocean Basins: Terms and Concepts to Remember
•	abyssal hill
•	abyssal plain
•	active margin
•	bathymetry
•	continental margin
•	continental rise
•	continental shelf
•	continental slope
•	epicenter
•	fracture zone
•	guyot
•	hydrothermal vent
•	ice age
•	island arc
•	ocean basin
•	oceanic ridge
•	passive margin
•	seamount
•	shelf break
•	submarine canyon
•	transform fault
•	trench
•	turbidity current
Ocean Basins: Study Questions
Thinking Critically
1.	Why did people think an ocean was deepest at its center? What changed their minds?
2.	What do the facts that (a) granite underlies the edges of continents and (b) basalt underlies deep-ocean basins, suggest? (Hint: Consider thicknesses and densities.)
3.	The terms leading and trailing are also used to describe continental margins. How do you suppose these words relate to active and passive, or Atlantic-type and Pacific-type used in the text?
4.	What forces control the shape of a continental shelf? A continental slope? A continental rise?
5.	Answer this question if you have already read Chapter 3: Your time machine has been programmed to deliver you to Frankfurt, Germany, on a chilly evening in January 1912, to hear Wegener's lectures on continental drift. What two illustrations from this chapter would you take with you to cheer him up after the lecture? Why did you select those particular illustrations?

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Visit the Oceanography Resource Center at www.cengage.com/login for more assets, including animations, videos, audio clips, and more.
Thinking Analytically
1.	The speed of sound through seawater is about 1,500 meters per second. If a ship equipped with a multibeam mapping system is surveying a feature 3,500 meters below the surface, and if the researches wish to obtain an image of the feature at a resolution of 10 meters, what is the maximum speed the ship can steam?
2.	Review the speed of a turbidity current. In the unlikely event that a fast-running current formed near the shoreline of a trailing edge coast, how long would it take for the current to traverse a typical continental shelf and arrive at the shelf break? Would you expect the current to move at a constant speed during this traverse?
3.	How much wider has Iceland become because of seafloor spreading since the last sequence of major eruptions in the 1500s? [Hint: Use the data in Figure 4.21a.]
4.	What would be the approximate age of a seamount produced at the East-Pacific Rise at the position marked (6) in Figure 4.33 when it collapses into the Peru-Chile Trench?
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Chapter Overview Pages 130 to 132


Sediments: Chapter Objectives
Sediments: Chapter Overview
Sediments: Chapter Objectives




Portrait of a disaster: 65 million years ago on one very bad day! (a) An artist's conception of a catastrophic asteroid strike. The 10-kilometer object would have vaporized above Earth's surface and struck with catastrophic force. The energy of the collision (imagined here about 45 seconds after impact) would have sent shock waves and debris around Earth. (b) This cross section of a seabed core shows clear evidence of the impact and its aftermath. Don Davies/NASA  View PDF




Sediments  Click here to view
STUDY PLAN
•	Sediments Vary Greatly in Appearance
•	Sediments May Be Classified by Particle Size
•	Sediments May Be Classified by Source
•	Neritic Sediments Overlie Continental Margins
•	Pelagic Sediments Vary in Composition and Thickness
•	Scientists Use Sensitive Tools to Study Ocean Sediments
•	Sediments Are Historical Records of Ocean Processes
•	Marine Sediments Are Economically Important
FOUR MAIN CONCEPTS
1.	Sediments are loose accumulations of particulate material. Their depth and composition tell us of relatively recent events in the ocean basin above.
2.	The most abundant sediments are terrigenous (from land) and biogenous (from once-living things). The volume of terrigenous sediment exceeds that of biogenous sediment, but biogenous material covers a greater area of seabed.
3.	Marine sediments have been uplifted and exposed on land. Arizona's Grand Canyon is made of marine sediment.
4.	Because marine sediments are usually subducted along with the seabed on which they lie, the oldest sediments are relatively young—rarely older than 180 million years.
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Sediments: Chapter Overview
The Memory Of the Ocean A glance at the first few photographs in this chapter will show you the true face of the ocean floor. The basalt and lava we have been discussing are nearly always hidden—covered by dust and gravel, silt and mud. This sediment includes particles from land, from biological activity in the ocean, from chemical processes within water, and even from space. Analysis of this sedimentary material can tell us the recent history of an ocean basin, and sometimes the recent history of the whole Earth. In a sense, seafloor sedimentary deposits are the memory of the ocean.
Sometimes that memory records catastrophic events. For example, many lines of evidence suggest that Earth was struck 65 million years ago by an asteroid about 10 kilometers (6 miles) across. The cataclysmic collision is thought to have propelled shock waves and huge clouds of seabed and crus’ all over Earth, producing a time of cold and dark that contributed to the extinction of many species, including the dinosaurs. The accompanying photograph of a deep-sea core shows evidence of this disaster, and you'll learn more about it in Chapter 13.
Because seafloors are recycled by tectonic forces, the ocean's sedimentary memory is not long. Records of events older than about 180 million years are recycled as oceanic crust and its overlying sediment reaches subduction zones. The ocean has forgotten much in the last 4 billion years.




b Richard Norris/Scripps Institute of Oceanography  View PDF

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Chapter 5:Sediments: 5.1: Sediments Vary Greatly in Appearance
5.1: Sediments Vary Greatly in Appearance Pages 132 to 134


CONCEPT CHECK
Sediment  is particles of organic or inorganic matter that accumulate in a loose, unconsolidated form. The particles originate from the weathering and erosion of rocks, from the activity of living organisms, from volcanic eruptions, from chemical processes within the water itself, and even from space. Most of the ocean floor is being slowly dusted by a continuing rain of sediments. Accumulation rates on the deep-sea floor vary from a few centimeters per year to the thickness of a dime every thousand years.
Marine sediments occur in a broad range of sizes and types. Beach sand is sediment; so are the muds of a quiet bay and the mix of silt and tiny shells found on the continental margins. Less familiar sediments are the f ne clays of the deep-ocean floor, the biologically derived oozes of abyssal plains, and the nodules and coatings that form around hard objects on the seafloor. The origin of these materials—and the distribution and sizes of the particles—depends on a combination of physical and biological processes.
What do sediments look like? That depends on where you look. Figure 5.1 shows a sea anemone on the Mid-Atlantic Ridge. The young rocky outcrop on which it rests is only lightly powdered with sediment. Contrast that rough ridge with the smooth seafloor shown in Figure 5.2. The sediment there is about 35 meters (116 feet) thick, and marked by the tracks of brittle stars. These widely distributed organisms feed on surface bacteria and fallen particles of organic sediment. Note that the surface of the sediment is not always smooth. Where bottom currents are swift and persistent, they can cause ripples like those on a stream-bed(Figure 5.3).




Figure 5.1 Sediment near the crest of the Mid-Atlantic Ridge. A sea anemone clings to newly formed rock outcrops only lightly dusted with sediment. B. Murton/Southampton Oceanography Centre/Photo Researchers, Inc.  View PDF





Figure 5.2 Brittle stars and their tracks on the continental slope off New England. The depth is 1,476 meters (4,842 feet). Charles D. Hollister, WHOI  View PDF

The extraordinary thickness of some layers of marine sediment can be seen in Figure 5.4, a seismic profile of the eastern edge of a seamount in the North Atlantic's Sohm Abyssal Plain south of Nova Scotia. The sediment at the eastern boundary of this profile covers the oceanic crust to a depth of more than 1.8 kilometers (1.1 miles).
The colors of marine sediments are often striking. Sediments of biological origin are white or cream-colored, with deposits high in silica tending toward gray. Some deep-sea clays—though traditionally termed “red clays” from the rusting (oxidation) of iron within the sediments to form iron oxide—can range from tan to chocolate brown. Other clays are shades of green. Nodular sediments are a dark sooty brown or black. Some nearshore sediments contain decomposing organic material and smell of hydrogen sulfide, but most are odorless.
Very few areas of the seabed are altogether free of overlying sediments. The water over these areas is not completely sediment-free, but for some reason, sediment does not collect on the bottom. Strong currents may scour the sediments away; or the seafloor may be too young in these areas for sediments to have had time to accumulate; or hot water percolating upward through a porous seafloor may dissolve the material as fast as it settles.
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Figure 5.3 Ripples on the sediment beneath the swift Antarctic Circumpolar Current in the northern Drake Passage. The depth here is 4,010 meters (13,153 feet). Charles D. Hollister, WHOI  View PDF





Figure 5.4 The deep sediments of the Sohm Abyssal Plain in the North Atlantic south of Nova Scotia have buried the base of this seamount. This seismic profile shows the depth of the sediments above the geologic base of the seamount to be more than 1.8 kilometers (1.1 miles). Note the scour moat—a depression along the boundary of seamount and sediment—caused by a persistent deep boundary current in the area. The vertical exaggeration in this figure is about 12:1. Charles D. Hollister, WHOI  View PDF

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CONCEPT CHECK
1. What is sediment?
2. Why are very few areas of the seabed completely free of sediments?
3. The ocean is over 4 billion years old, yet, marine sediments are rarely older than about 180 million years. Why?
To check your answers, see the book's website. The website address is printed at the end of the chapter.
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5.3: Sediments May Be Classified by Source Pages 135 to 139


Terrigenous Sediments Come from Land
Biogenous Sediments Form from the Remains of Marine Organisms
Hydrogenous Sediments Form Directly from Seawater
Cosmogenous Sediments Come from Space
Marine Sediments Are Usually Combinations of Terrigenous and Biogenous Deposits
Another way to classify marine sediments is by their origin. Such a scheme was first proposed in 1891 by Sir John Murray and A. F. Renard after a thorough study of sediments collected during the Challengerexpedition. A modern modification of their organization is shown in Table 5.2. This scheme separates sediments into four categories by source: terrigenous, biogenous, hydrogenous (or authigenic), and cosmogenous.
Terrigenous Sediments Come from Land
Terrigenous  (terra, “Earth”; generare, “to produce”) sediments are the most abundant. As the name implies, they originate on the continents or islands from erosion, volcanic eruptions, and blown dust.
The rocks of Earth's crust are made up of minerals , inorganic crystalline materials with specific chemical compositions. The texture of igneous rocks—rocks that crystallize from molten material—is determined by how rapidly they cool. Igneous rocks that cool rapidly, such as the basalt that forms the ocean floor at spreading centers or pours from volcanic vents on land, solidify so quickly that obvious crystals do not have a chance




Figure 5.6 The sediment cycle. Over geological time, mountains rise as lithospheric (crustal) plates collide, fuse, and subduct. Water and wind erode the mountains and transport resulting sediment to the sea. The sediments are deposited on the seafloor, where they travel with the plate and are either uplifted or subducted. Thus, the material is made into mountains again.  View PDF

to form. Slower cooling produces the most commonly encountered crystals, those about the size of a grain or rice or the head of a pin. Nearly all terrigenous sediments are derived directly or indirectly from these crystals. You have probably seen the crystals in granite, the most familiar continental igneous rock. Granite is the source of quartz and clay, the two most common components of terrigenous marine sediments.



Rock Cycle  Click here to view
Terrigenous sediments are part of a slow and massive cycle (Figure 5.6). Over the great span of geological
Table 5.2 Classifiication of Marine Sediments by Source of Particles
Sediment Type	Source	Examples	Distribution	Percent of All Ocean Floor Area Covered
Sources: Kennett, Marine Geology, 1982; Weihaupt, Exploration of the Oceans, 1979; Sverdrup, Johnson, and Fleming, The Oceans: Their Physics, Chemistry, and General Biology, 1942
Terrigenous	Erosion of land, volcanic eruptions, blown dust	Quartz sand, clays, estuarine mud	Dominant on continental margins, abyssal plains, polar ocean floors	~45%
Biogenous	Organic; accumulation of hard parts of some marine organisms	Calcareous and siliceous oozes	Dominant on deep-ocean floor (siliceous ooze below about5 km)	~55%
Hydrogenous (authigenic)	Precipitation of dissolved mineral from water, often by bacteria	Manganese nodules, phosphorite deposits	Present with other, more dominant sediments	1%
Cosmogenous	Dust from space, meteorite debris	Tektite spheres, glassy nodules	Mixed in very small proportion with more dominant sediments	1%


 Table 5.2 Classifiication of Marine Sediments by Source of Particles Kennett, Marine Geology, 1982; Weihaupt, Exploration of the Oceans, 1979; Sverdrup, Johnson, and Fleming, The Oceans: Their Physics, Chemistry, and General Biology, 1942  View PDF
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Figure 5.7 Sources of terrigenous sediments. a Rivers are the main source of terrigenous sediments. This photo, taken from space, shows sediment entering the Gulf of Mexico from the Mississippi River. b The wind may transport ash from a volcanic eruption for hundreds of kilometers and deposit it in the ocean. This ash cloud was caused by the summer 1991 eruption of Mount Pinatubo in the Philippines. c Dust from the Gobi Desert blows eastward across the Pacific on 18 March 2002. The particles will fall to the ocean surface and descend slowly to the bottom to end up as terrigenous sediments. Liam Gumley/Space Science and Engineering Center, University of Wisconsin–Madison/MODIS Science Team Department of Geology, University of Delaware NASA/BSFC, ORBIMAGE, SeaWiFS  View PDF

time, mountains rise as plates collide, fuse, and subduct. The mountains erode. The resulting sediments are transported to the sea by wind and water, where they collect on the seafloor. The sediments travel with the plate and are either uplifted or subducted. The material is made into mountains. The cycle begins anew.
Although estimates vary, it appears that about 15 billion metric tons (16.5 billion tons) of terrigenous sediments are transported in rivers to the sea each year, with an additional 100 million metric tons transported annually from land to ocean as fine airborne dust and volcanic ash (Figure 5.7).
Biogenous Sediments Form from the Remains of Marine Organisms
Biogenous  (bio, “life”; generare, “to produce”) sediments are the next most abundant marine sediment. The siliceous (silicon-containing) and calcareous (calcium carbonate-containing) compounds that make up these sediments of biological origin were originally brought to the ocean in solution by rivers or dissolved in the ocean at oceanic ridges. The siliceous and calcareous materials were then extracted from the seawater by the normal activity of tiny plants and animals to build protective shells and skeletons. Some of this sediment derives from larger mollusk shells or from stationary colonial animals such as corals, but most of the organisms that produce biogenous sediments drift free in the water as plankton (about which you'll learn in Chapter 14). After the death of their owners, the hard structures fall to the bottom and accumulate in layers. Biogenous sediments are most abundant where ample nutrients encourage high biological productivity, usually near continental margins and areas of upwelling. Over millions of years, organic molecules within these sediments can form oil and natural gas (see Chapter 17 for details).
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Note in Table 5.2 that biogenous sediments cover a larger percent of the area of the ocean floor than terrigenous sediments do, but the terrigenous sediments dominate in total volume.
Hydrogenous Sediments Form Directly from Seawater
Hydrogenous  (hydro, “water”; generare, “to produce”) sediments are minerals that have precipitated directly from seawater. The sources of the dissolved minerals include submerged rock and sediment, leaching of the fresh crust at oceanic ridges, material issuing from hydrothermal vents, and substances flowing to the ocean in river runoff. As we shall see, the most prominent hydrogenous sediments are manganese nodules, which litter some deep seabeds, and phosphorite nodules, seen along some continental margins. Hydrogenous sediments are also called authigenic  (authis, “in place, on the spot”)sediments because they were formed in the place they now occupy.
Though they usually accumulate very slowly, rapid deposition of hydrogenous sediments is possible—in a rapidly drying lake, for example.
Cosmogenous Sediments Come from Space
Cosmogenous  (cosmos, “universe”; generare, “to produce”) sediments, which are of extraterrestrial origin, are the least abundant. These sediments typically are greatly diluted by other sediment components and rarely constitute more than a few parts per million of the total sediment in any layer. Scientists believe that cosmogenous sediments come from two major sources: interplanetary dust that falls constantly into the top of the atmosphere and rare impacts by large asteroids and comets.
Interplanetary dust consists of silt- and sand-sized micrometeoroids that come from asteroids and comets or from collisions between asteroids. The silt-sized particles settle gently to Earth's surface, but larger, faster moving dust is heated by friction with the atmosphere and melts, sometimes glowing as the meteors we see in a dark night sky. Though much of this material is vaporized, some may persist in the form of iron-rich cosmic spherules. Most of these dissolve in seawater before reaching the ocean floor. About 15,000 to 30,000 metric tons (16,500 to 33,000 tons) of interplanetary dust enters Earth's atmosphere every year.
The highest concentrations of cosmogenous sediments occur when large volumes of extraterrestrial matter arrive all at once. Fortunately, this happens only rarely, when Earth is hit by a large asteroid or comet. Very few examples of this are known, but most geologists believe that an impact like the one described at the beginning of this chapter would have blown vast quantities of debris into space around Earth. Much of it would fall back and be deposited in layers. Cosmogenous components may make up between 10% and 20% of these extraordinary sediments!
Occasionally cosmogenous sediment includes translucent oblong particles of glass known asmicrotektites  (Figure 5.8). Tektites are thought to form from the




Figure 5.8 Microtektites are very rare particles that began a long journey when a large body impacted Earth and ejected material from Earth's crust. Some of this material traveled through space, re-entered Earth's atmosphere, melted, and took on a rounded or teardrop shape. These specimens of sculptured glass range from 0.2 to 0.8 millimeter in length. Glassy dust much f ner in size, as well as nut-size chunks, have also fallen on Earth. Image Courtesy of Michael Daniels  View PDF

violent impact of large meteors or small asteroids on the crust of Earth. The impact melts some of the crustal material and splashes it into space; the material melts again as it rushes through the atmosphere, producing the various raindrop shapes shown in the photo. Tektites do not dissolve easily and usually reach the ocean floor. Most are smaller than 1.5 millimeters (1⁄ 16 inch) long.
Marine Sediments Are Usually Combinations of Terrigenous and Biogenous Deposits
Sediments on the ocean floor only rarely come from a single source; most sediment deposits are a mixture of biogenous and terrigenous particles, with an occasional hydrogenous or cosmogenous supplement. The patterns and composition of sediment layers on the seabed are of great interest to researchers studying conditions in the overlying ocean. Different marine environments have characteristic sediments, and these sediments preserve a record of past and present conditions within those environments.
The sediments on the continental margins are generally different in quantity, character, and composition from those on the deeper basin floors. Continental shelf sediments called neritic sediments  (neritos, “of the coast”) consist primarily of terrigenous material. Deep-ocean floors are covered by finer sediments than those of the continental margins, and a greater proportion of deep-sea sediment is of biogenous
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Figure 5.9 Total sediment thickness of the ocean floor, with the thinnest deposits in dark blue and the thickest in red. Note the abundant deposits along the east and gulf coasts of North America, in the South China Sea, and in the Bay of Bengal east of India. NGDC/NOAA  View PDF

origin. Sediments of the slope, rise, and deep-ocean floor that originate in the ocean are called pelagic sediments (pelagios, “of the sea”).
The average thickness of the marine sediments in each oceanic region is shown in Figure 5.9 and Table 5.3. Note that 72% of the total volume of all marine sediment is associated with continental slopes and rises, which constitute only about 12% of the ocean's area. Figure 5.10 shows the worldwide distribution of marine sediment types. Put a bookmark in this page—you'll want to refer to these images as our discussion continues.
Table 5.3 The Distribution and Average Thickness of Marine Sediments
Region	Percent of Ocean Area	Percent of Total Volume of Marine Sediments	Average Thickness
Sources: Emery in Kennett, Marine Geology, 1982 (Table 11–1); Weihaupt, Exploration of the Oceans, 1979; Sverdrup, Johnson, and Fleming, The Oceans: Their Physics, Chemistry, and General Biology, 1942
Continental shelves	9%	15%	2.5 km (1.6 mi)
Continental slopes	6%	41%	9 km (5.6 mi)
Continental rises	6%	31%	8 km (5 mi)
Deep-ocean floor	78%	13%	0.6 km (0.4 mi)


 Table 5.3 The Distribution and Average Thickness of Marine Sediments Emery in Kennett, Marine Geology, 1982 (Table 11–1); Weihaupt, Exploration of the Oceans, 1979; Sverdrup, Johnson, and Fleming, The Oceans: Their Physics, Chemistry, and General Biology, 1942  View PDF

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CONCEPT CHECK
7. What are the four main types of marine sediments?
8. Which type of sediment is most abundant?
9. Which type of sediment covers the greatest seabed area?
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Figure 5.10 The general pattern of sediments on the ocean floor. Note the dominance of diatom oozes at high latitudes.  View PDF
10. Which type of sediment is rarest? Where does this sediment originate?
11. Do most sediments consist of a single type? That is, are terrigenous deposits made exclusively of terrigenous sediments?
12. How do neritic sediments differ from pelagic ones?
To check your answers, see the book's website. The website address is printed at the end of the chapter.
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5.5: Pelagic Sediments Vary in Composition and Thickness Pages 140 to 147


Turbidites Are Deposited on the Seabed by Turbidity Currents
Clays Are the Finest and Most Easily Transported Terrigenous Sediments
Oozes Form from the Rigid Remains of Living Creatures
Hydrogenous Materials Precipitate Out of Seawater Itself
Evaporites Precipitate as Seawater Evaporates
Oolite Sands Form When Calcium Carbonate Precipitates from Seawater
Researchers Have Mapped the Distribution of Deep-Ocean Sediments
The thickness of pelagic sediments  is highly variable. When averaged, the Atlantic Ocean bottom is covered by sediments to a thickness of about 1 kilometer (3,300 feet), while the Pacific floor has an average sediment thickness of less than 0.5 kilometer (1,650 feet). There are two reasons for this difference. First, the Atlantic Ocean is fed by a greater number of rivers laden with sediment than the Pacific, and the Atlantic is smaller in area; thus, it gets more sediment for its size than the Pacific. Second, in the Pacific Ocean, many oceanic trenches trap sediments moving toward basin centers. Beyond this, the composition and thickness of pelagic sediments also vary with location, being thickest on the abyssal plains and thinnest (or absent) on the oceanic ridges.
Turbidites Are Deposited on the Seabed by Turbidity Currents
Dilute mixtures of sediment and water periodically rush down the continental slope in turbidity currents(Figure 5.12). A turbidity current is not propelled by the water within it, but by gravity (the water suspends the particles, and the mixture is denser than the surrounding seawater). As we have seen, the erosive force of a turbidity current is thought to help cut submarine canyons (see again Figures 4.17 and 4.18). These underwater avalanches of thick muddy fluid can reach the continental rise and often continue moving onto an adjacent abyssal plain before eventually coming to rest. The resulting deposits are called turbidites , graded layers of terrigenous sand interbedded with the finer pelagic sediments typical of the deep-sea floor. Each distinct layer consists of coarse sediment at the bottom with finer sediment above, and each graded layer is the
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Figure 5.12 The formation of turbidites. (a) A turbidity current—a kind of underwater avalanche—can form when wave turbulence or seismic activity dislodges sediment deposited by rivers or other sources. (b) The turbidity current moves quickly down the continental shelf and slope, sometimes encountering (and further eroding) a submarine canyon (c). When the material comes to rest, it sorts into layers with course sediment at the bottom and finer sediment above (d). e Each graded layer is the result of one turbidity current event.Society for Sedimentary Geology/Dr. Lynton Land Society for Sedimentary Geology/Dr. Lanton Land  View PDF

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Figure 5.13 Organisms that contribute to calcareous ooze. (a) A living foraminiferan, an amoeba-like organism. The shell of this beautiful foram, genus Hastigerina, is surrounded by a bubble-like capsule. It is one of the largest of the planktonic species with spines, reaching nearly 5 centimeters (2 inches) in length. (b) The shell of a smaller foraminiferan—the snail-like, planktonic Globigerina—is visible in this visible light micrograph. c Coccoliths, individual plates of coccolithophores, a form of planktonic algae. Because of their tendency to dissolve, calcareous oozes very rarely occur at bottom depths below 4,500 meters (14,800 feet). Note its very small size in this scanning electron micrograph. Howard Spero/University of California, Davis © Wim van Egmond/Visuals Unlimited Dr. Markus Geisen and Dr. Claudia Sprengel. © Markus Geisen  View PDF

result of sediment deposited by one turbidity current event. Figure 5.12 shows this process and its results.
Clays Are the Finest and Most Easily Transported Terrigenous Sediments
About 38% of the deep seabed is covered by clays and other fine terrigenous particles. As we have seen, the finest terrigenous sediments are easily transported by wind and water currents. Microscopic waterborne particles and tiny bits of windborne dust and volcanic ash settle slowly to the deep-ocean floor, forming fine brown, olive-colored, or reddish clays. As Table 5.1 shows, the velocity of particle settling is related to particle size, and clay particles usually fall very slowly indeed. Terrigenous sediment accumulation on the deep-ocean floor is typically about 2 millimeters (1/8 inch) every thousand years.
Oozes Form from the Rigid Remains of Living Creatures
Seafloor samples taken farther from land usually contain a greater proportion of biogenous sediments than those obtained near the continental margins. This is not because biological productivity is higher farther from land (the opposite is usually true), but because there is less terrigenous material far from shore, and thus pelagic deposits contain a greater proportion of biogenous material.
Deep-ocean sediment containing at least 30% biogenous material is called an ooze  (surely one of the most descriptive terms in the marine sciences). Oozes are named after the dominant remnant organism constituting them. The organisms that contribute their remains to deep-sea oozes are small, single-celled, drifting, plantlike organisms and the single-celled animals that feed on them. The hard shells and skeletal remains of these creatures are composed of relatively dense glasslike silica or calcium carbonate. When these organisms die, their shells settle slowly toward the bottom, mingle with fine-grained terrigenous silts and clays, and accumulate as ooze. The silica-rich residues give rise to siliceous ooze , the calcium-containing material to calcareous ooze .
Oozes accumulate slowly, at a rate of about 1 to 6 centimeters (½ to 2½ inches) per thousand years. But they collect more than ten times more quickly than deep-ocean terrigenous clays. The accumulation of any ooze therefore depends on a delicate balance between the abundance of organisms at the surface, the rate at which they dissolve once they reach the bottom, and the rate of accumulation of terrigenous sediment.
Calcareous ooze forms mainly from shells of the amoeba-like foraminifera  (Figure 5.13a and b), small drifting mollusks called pteropods , and tiny algae known as coccolithophores  (Figure 5.13c). When conditions are ideal, these organisms generate prodigious volumes of sediment. The remains of countless
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Figure 5.14 Dover's famous white cliffs are uplifted masses of lithif ed coccolithophores. This chalk-like material was deposited on the seabed around 100 million years ago, overlain by other sediments, and transformed into soft limestone by heat and pressure. © AM Corporation/Alamy  View PDF




Figure 5.15 The dashed line shows the calcium carbonate (CaCO3) compensation depth (CCD). At this depth, usually about 4,500 meters (14,800 feet), the rate at which calcareous sediments accumulate equals the rate at which those sediments dissolve.  View PDF

coccolithophores have been compressed and lithified to form the impressive white cliffs of Dover in southeastern England (Figure 5.14). Though formed at moderate ocean depth about 100 million years ago, tectonic forces have uplifted Dover's chalk cliffs to their present prominent position.
Although foraminiferans and coccolithophores live in nearly all surface ocean water, calcareous ooze does not accumulate everywhere on the ocean floor. Shells are dissolved by seawater at great depths because seawater at depth contains more carbon dioxide than seawater near the surface, and thus becomes slightly acid. This acidity, combined with the increased solubility of calcium carbonate in cold water under pressure, dissolves the shells more rapidly, as you will see in Figures 7.10 and 7.12. At a certain depth, called thecalcium carbonate compensation depth (CCD) , the rate at which calcareous sediments are supplied to the seabed equals the rate at which those sediments dissolve. Below this depth, the tiny skeletons of calcium carbonate dissolve on the seafloor, so no calcareous oozes accumulate. Calcareous sediment dominates the deep-sea floor at depths of less than about 4,500 meters (14,800 feet), the usual calcium carbonate compensation depth. Sometimes a line analogous to a snow line on a terrestrial mountain can be seen on undersea peaks: Above the line the white sprinkling of calcareous ooze is visible; below it, the “snow” is absent (see Figure 5.15). About 48% of the surface of deep-ocean basins is covered by calcareous oozes.
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Figure 5.16 Micrographs of siliceous oozes, which are most common at great depths. a Shells of radiolarians, amoeba-like organisms. Radiolarian oozes are found primarily in the equatorial regions. b A shell of a diatom, a single-celled alga. Diatom oozes are most common at high latitudes. © Wim van Egmond/Visuals Unlimited Greta Fryxell  View PDF

Siliceous (silicon-containing) ooze predominates at greater depths and in colder polar regions. Siliceous ooze is formed from the hard parts of another amoeba-like animal, the beautiful glassy radiolarian  (Figure 5.16a), and from single-celled algae called diatoms  (Figure 5.16b). After a radiolarian or diatom dies, its shell will also dissolve back into the seawater, but this dissolution occurs much more slowly than the dissolution of calcium carbonate. Slow dissolution at all depths, combined with very high diatom productivity in some surface waters, leads to the buildup of siliceous ooze. Diatom ooze is most common in the deep-ocean basins surrounding Antarctica because strong ocean currents and seasonal upwelling in this area support large populations of diatoms. Radiolarian oozes occur in equatorial regions, most notably in the zone of equatorial upwelling west of South America (as was seen in Figure 5.10). About 14% of the surface of the deep-ocean floor is covered by siliceous oozes.
The very small particles that make up most of these pelagic sediments would need between 20 and 50 years to sink to the bottom. By that time, they would have drifted a great lateral distance from their original surface position. But researchers have noted that the composition of pelagic sediments is usually similar to the particle composition in the water directly above. How could such tiny particles fall quickly enough to avoid great horizontal displacement? The answer appears to involve their compression into fecal pellets (Figure 5.17). While still quite small, the fecal pellets of small animals are much larger than the tiny individual skeletons of diatoms, foraminifera, and other plantlike organisms that they consumed, so they fall much faster, reaching the deep-ocean floor in about two weeks.
Some deep-sea oozes have been uplifted by geological processes and are now visible on land. The calcareous chalk White Cliffs of Dover in eastern England are partially lithified deposits composed largely of foraminifera and coccolithophores. Fine-grained siliceous deposits called diatomaceous earth are mined from other deposits. This fossil material is a valued component in f at paints, pool and spa filters, and mildly abrasive car and tooth polishes.
Hydrogenous Materials Precipitate Out of Seawater Itself
Hydrogenous sediments also accumulate on deep-sea f oors. They are associated with terrigenous or biogenous sediments and rarely form sediments by themselves. Most hydrogenous sediments originate from chemical reactions that occur on particles of the dominant sediment.
The most famous hydrogenous sediments are manganese nodules , which were discovered by the hard-working crew of HMS Challenger. The nodules consist primarily of manganese and iron oxides but also contain small amounts of cobalt, nickel, chromium,
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Figure 5.17 A fecal pellet of a small planktonic animal. a The compressed pellet is about 80 micrometers long. bEnlargement of the pellet's surface, magnified about 2000 times. The pellet consists of the indigestible remains of small microscopic plantlike organisms, mostly coccolithophores. Unaided by pellet packing, these remains might take months to reach the seabed, but compressed in this way, they can be added to the ooze in perhaps two weeks. Susumi Honjo/Woods Hole Oceanographic Institute Susumi Honjo/Woods Hole Oceanographic Institute  View PDF





Figure 5.18 Manganese nodules. a A cross section cut through a manganese nodule, showing the concentric layers of manganese and iron oxides. This nodule is about 11 centimeters (4½ inches) long, a typical size. b Lemon-sized manganese nodules littering the abyssal Pacific. Tom Garrison Institute of Oceanographic Sciences/NERC/Photo Researchers, Inc.  View PDF

copper, molybdenum, and zinc. They form in ways not fully understood by marine chemists, “growing” at an average rate of 1 to 10 millimeters (0.04 to 0.4 inch) per million years, one of the slowest chemical reactions in nature. Though most are irregular lumps the size of a potato, some nodules exceed 1 meter (3.3 feet) in diameter. Manganese nodules often form around nuclei such as sharks’ teeth, bits of bone, microscopic alga and animal skeletons, and tiny crystals—as the cross section of a manganese nodule in Figure 5.18ashows. Bacterial activity may play a role in the development of a nodule. Between 20% and 50% of the Pacific Ocean floor may be strewn with nodules (Figure 5.18b).
Why don't these heavy lumps disappear beneath the constant rain of accumulating sediment? Possibly, the continuous churning of the underlying sediment by creatures living there keeps the dense lumps on the surface, or perhaps slow currents in areas of nodule accumulation waft particulate sediments away.
Challenger scientists also discovered nodules of phosphorite. The first of these irregular brown lumps was taken from the continental rise off South Africa, and phosphorite nodule fields have since been found on shallow bank tops, outer continental shelves, and the upper parts of continental slope areas off California, Argentina, and Japan. Phosphorus is an important
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Figure 5.19 Geology students inspect the base of a thick layer of rock gypsum in Colorado. This rock probably formed by the lithification of evaporites left behind as a shallow inland sea dried up. Photo provided by Stan Finney  View PDF

ingredient in fertilizer, and the nodules may someday be collected as a source of agricultural phosphates. Like manganese nodules, phosphorite nodules are found only in areas with low rates of sediment accumulation.
For now, the low market value of the minerals in manganese and phosphorite nodules makes them too expensive to recover. As techniques for deep-sea mining become more advanced and raw material prices increase, however, the nodules’ concentration of valuable materials will almost certainly be exploited.
Powdery deposits of metal sulfides have been found in the vicinity of hydrothermal vents at oceanic ridges. Hot, metal-rich brines blasting from the vents meet cold water, cool rapidly, and lose the heavy metal sulf des by precipitation. Iron sulf des and manganese precipitates fall in thick blankets around the vents. The cobalt crusts of rift zones also seem to be associated with this phenomenon. These areas may one day be mined for their metal content.
Evaporites Precipitate as Seawater Evaporates
Evaporites  are an important group of hydrogenous deposits that include many salts important to humanity. These salts precipitate as water evaporates from isolated arms of the ocean or from landlocked seas or lakes. For thousands of years people have collected sea salts from evaporating pools or deposited beds. Evaporites are forming today in the Gulf of California, the Red Sea, and the Persian Gulf. The first evaporites to precipitate as water's salinity increases are the carbonates, such as calcium carbonate (from which limestone is formed). Calcium sulfate, which gives rise to gypsum, is next. Crystals of sodium chloride (table salt) will form if evaporation continues.
Figure 5.19 shows a thick deposit of rock gypsum within sedimentary rocks in the Rocky Mountains. Deposition of such a thick evaporite layer would have required evaporation of an arm of the ocean over a long period of time.
Oolite Sands Form When Calcium Carbonate Precipitates from Seawater
Not all hydrogenous calcium carbonate deposits are caused by evaporation, however. A small decrease in the acidity of seawater, or an increase in its temperature, can cause calcium carbonate to precipitate from water of normal salinity. In shallow areas of high biological productivity where sunlight heats the water, microscopic plants use up dissolved carbon dioxide, making seawater slightly less acidic (see Figure 7.12). Molecules of calcium carbonate then may precipitate around shell fragments or other particles. These white, rounded grains are called oo-liths (oon, “egg”) because they resemble fish eggs (Figure 5.20). Oolite sands —sands comprised of oolit-hs—are abundant in many warm, shallow waters such as those of the Bahama Banks.
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Figure 5.20 Oolite sand. Note the uniform rounded shape. Tom Garrison Scripps Institution of Oceanography/UCSD  View PDF

Researchers Have Mapped the Distribution of Deep-Ocean Sediments
Look again at the types and distribution of marine sediments in Figures 5.9 and 5.10. Notice especially the lack of radiolarian deposits in much of the deep North Pacific; the strand of siliceous oozes extending west from equatorial South America; and the broad expanses of the Atlantic, South Pacific, and Indian Ocean floors covered by calcareous oozes. The broad, deep, relatively old Pacific contains extensive clay deposits, most delivered in the form of airborne dust. Why? Though some of the world's largest and muddiest rivers empty into the Pacif c, most of their sediments are trapped in the peripheral trenches and cannot reach the mid-basins. And as you might expect, the poorly sorted glacial deposits are found only at high latitudes.

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CONCEPT CHECK
16. Why are Atlantic sediments generally thicker than Pacific sediments?
17. How do turbidity currents distribute sediments? What do these sediments (turbidites) look like?
18. What is the origin of oozes? What are the two types of oozes?
19. What is the CCD? How does it affect ooze deposition at great depths?
20. How do hydrogenous materials form? Give an example of hydrogenous sediment.
21. How do evaporites form?
To check your answers, see the book's website. The website address is printed at the end of the chapter.
Figures 5.9 and 5.10 summarize more than a century of effort by marine scientists. Studies of sediments will continue because of their importance to natural resource development and because of the details of Earth's history that remain locked beneath their muddy surfaces.




Figure 5.21 a Using a clam-shell sampler. On board research vessel Robert Gordon Sproul, a scoop of muddy ocean-bottom sediments collected with a clamshell sampler is dumped onto the deck for study. (b) Before sampling, (c) during sampling, and (d) after the sample has been taken. Note that the sample is relatively undisturbed.Scripps Institution of Oceanography/UCSD  View PDF

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5.6: Scientists Use Sensitive Tools to Study Ocean Sediments Pages 147 to 148


CONCEPT CHECK
Deep-water cameras have enabled researchers to photograph bottom sediments. The first of these cameras was simply lowered on a cable and triggered by a trip wire. Other more elaborate cameras have been taken to the seafloor on towed sleds or deep submersibles.
Actual samples usually provide more information than photographs do. HMS Challenger scientists used weighted, wax-tipped poles and other tools attached to long lines to obtain samples, but today's oceanographers have more sophisticated equipment. Shallow samples may be taken using a clamshell sampler  (named because of its method of operation, not its target; see Figure 5.21). Deeper samples are taken
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Figure 5.22 Using a piston corer A piston corer. (b) The corer is allowed to fall toward the bottom. (c) The corer reaches the bottom and continues, forcing a sample partway into the cylinder. (d) Tension on the cable draws a small piston within the corer toward the top of the cylinder, and the pressure of the surrounding water forces the corer deeper into the sediment. (e) The corer and sample being hauled in.Tom Garrison  View PDF

by a piston corer  (Figure 5.22), a device capable of punching through as much as 25 meters (82 feet) of sediment and returning an intact plug of material. Using a rotary drilling technique similar to that used to drill for oil, the drilling ship JOIDES Resolution (Figure 5.23) returned much longer core segments, some more than 1,100 meters (3,600 feet) long! These cores are stored in core libraries, a valuable scientific resource (Figure 5.24). Analysis of sediments and fossils from the Deep Sea Drilling Project cores helped verify the theory of plate tectonics. It has also shed light on the evolution of life-forms and helped researchers to decipher the history of changes in Earth's climate over the last 100,000 years.

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CONCEPT CHECK
22. What tools are used to study sediments?
23. How have studies of marine sediments advanced our understanding of plate tectonics?
To check your answers, see the book's website. The website address is printed at the end of the chapter.
Powerful new continuous seismic profilers have also been used to determine the thickness and structure of layers of sediment on the continental shelf and slope, and to assist in the search for oil and natural gas (seeFigure 5.25). Typically, in seismic profiling, a moving ship tows a sound transmitter and receiver behind it. Sounds from the transmitter reflect from the sediment layers beneath the bottom surface. Recent improvements in computerized image processing of the echoes returning from the seabed now permit detailed analysis of these deeper layers. Figure 5.4 was made in that way.
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5.7: Sediments Are Historical Records of Ocean Processes Pages 148 to 151


CONCEPT CHECK


   In 1899, the British geologist W. J. Sollas theorized that deep-sea deposits could reveal much of the planet's history. In the era before plate tectonics theory this certainly seemed reasonable—the deep-ocean bottom was thought to be a calm, changeless place where an unbroken accumulation of sediment could be probed to discover the entire history of the ocean. Unfortunately for this promising idea, difficulties began to crop up almost immediately. For one thing, the sediments should have been much thicker than early probes indicated. If Earth's ocean is truly older than a few hundred thousand years, and if life has existed within it for most of that time, the sediment layer should be thicker than
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Figure 5.23 a JOIDES Resolution, the deep-sea drilling ship operated by the Joint Oceanographic Institutions for Deep Earth Sampling. The vessel is 124 meters (407 feet) long, with a displacement of over 16,000 tons. The rig can drill to a depth of 9,150 meters (30,000 feet) below sea level. b The difficulty of deep-sea drilling can be sensed from this scale drawing: The length of the drill ship is 120 meters (394 feet); the depth of water through which the drill string must pass to reach the bottom is 5,500 meters (18,000 feet)! Joint Oceanographic Institutions for Deep Earth Samplings  View PDF





Figure 5.24 Sediment cores in storage. Cores are sectioned longitudinally, placed in trays, and stored in hermetically sealed cold rooms. The Gulf Coast Repository of the Ocean Drilling Program, located at Texas A&M University (pictured here), stores about 75,000 sections taken from more than 80 kilometers (50 miles) of cores recovered from the Pacific and Indian oceans. Smaller core libraries are maintained at the Scripps Institution in California (Pacific and Indian oceans) and at the Lamont–Doherty Earth Observatory in New York State (Atlantic Ocean). Deep Sea Drilling Project, Texas A&M University  View PDF

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Figure 5.25 A typical method of continuous seismic profiling. A moving ship trails a sound transmitter and receiver at a distance sufficient to minimize interference from the ship's noise. Bubbles and turbulence from a burst of compressed air act as a sound source. The sound is reflected from sediment layers beneath the surface and is detected by a sensitive hydrophone for analysis. Figures 4.23b and 4.28 were made in this way.  View PDF

had been observed. Another difficulty lay in the uneven distribution of sediments. Sollas thought that the center of an ocean basin should contain the thickest layers of sediment, yet the ridged mid-Atlantic bottom was nearly naked. There didn't seem to be any difference in the nature of the overlying seawater that could account for the variations in thickness and composition of the sediments across the bottom of the Atlantic. Oozes were especially puzzling: The organisms that form oozes grow well at the surface of the middle Atlantic, yet the mid-Atlantic floor seemed to bear little ooze.
Turn-of-the-century geologists were understandably confused, but today we know the tectonic reasons for these discrepancies. Because the deep-sea sediment record is ultimately destroyed in the subduction process, the ocean's sedimentary “memory” does not start with the ocean's formation as originally reasoned by early marine scientists. But modern studies of deep-sea sediments using sea floor samples, cores obtained by deep drilling, and continuous seismic profiling have demonstrated that these deposits contain a remarkable record of relatively recent (that is, about the last 180 million years) ocean history. However, these same data have also shown that the record is not uninterrupted, as early workers had originally assumed. In fact, some of the gaps in the deep-sea deposits represent erosional events and constitute evidence of changes in deep-sea circulation, and hence are valuable in their own right. The analysis of layered sedimentary deposits, whether in the ocean or on land, represents the discipline of stratigraphy (stratum, “layer”; graph, “a drawing”). Deep-sea stratigraphy utilizes variations in the composition of rocks, microfossils, depositional patterns, geochemical character, and physical character (density, and such) to trace or correlate distinctive sedimentary layers from place to place, establish the age of the deposits, and interpret changes in ocean and atmospheric circulation, productivity, and other aspects of past ocean behavior. In turn, these sorts of studies and the advent of deep-sea drilling have given rise to the emerging science of paleoceanography  (palaios, “ancient”), the study of the ocean's past.
Early attempts to interpret ocean and climate history from evidence in deep-sea sediments occurred in the 1930s through 1950s as cores became available. These initial studies relied primarily on identifying variations in the abundance and distribution of glacial marine sediments, carbonate and siliceous oozes, and temperature-sensitive microfossils in the cores. Modern paleoceanographic studies continue to utilize these same features, but researchers have much greater understanding of their significance and are aided by seismic imaging of the deposits over large areas. In addition, newer and more precise methods of dating deep-sea sediments have enabled them to place events in a proper time context. Finally, scientists now have instruments capable of analyzing very small variations in the relative abundances of the stable isotopes of oxygen preserved within the carbonate shells of microfossils found in deep-sea sediments; these instruments allow them to interpret changes in the temperature of surface and deep water over time. These same data are also used to estimate variations in the volume office stored in continental ice sheets, and thus to track the ice ages. Other geochemical evidence contained in the shells of marine microfossils, including variations in carbon isotopes and trace metals such as cadmium, provide insights into ancient patterns of ocean circulation, productivity of the marine biosphere, and upwelling. These sorts of data have already provided quantitative records of the glacial-interglacial climatic cycles of the past 2 million years. Future drilling and analysis of deep-sea sediments is poised to extend our paleoceano-graphic perspective much further back in time.
Figure 5.26 shows the age of regions of the Pacific Ocean floor using data obtained largely from analyses of the overlying sediment. Note that sediments get older with increasing distance from the East Pacific Rise spreading center.

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CONCEPT CHECK
24. Would you say the “memory” of the sediments is long or short (in geological time)?
25. How might past climate be inferred from studies of marine sediments?
To check your answers, see the book's website. The website address is printed at the end of the chapter.
Earth might not be the only planet where marine sediments have left historical records. As you read in Chapter 2, Mars probably had an ocean between 3.2 and 1.2 billion years ago. In May 1998 Mars Global Surveyor photographed sediments that look suspiciously marine near the edge of an ancient bay (Figure 5.27). One can only wonder what stories their memories hold.
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Figure 5.26 The ages of portions of the Pacific Ocean floor, based on core samples of sediments just above the basalt seabed, in millions of years ago (Ma, mega-annum). The youngest sediments are found near the East Pacific Rise and the oldest close to the eastern side of the trenches. Contrast this figure with Figure 3.31.  View PDF





Figure 5.27 Ancient marine sediments on Mars? This photo taken in late 2004 by NASA's Mars Exploration RoverOpportunity shows an eroded area of Burns Cliff. The walls of Endurance Crater contain clues about Mars’ distant past, and the deeper the layer, the older the clue. The deepest available layers have been carefully analyzed by instrument aboard the Rover, and appear to contain magnesium and sulfur in conf gurations suggestive of the layers’ deposition by water. NASA  View PDF

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5.8: Marine Sediments Are Economically Important Page 152


CONCEPT CHECK
Study of sediments has brought practical benefits. You probably have more daily contact with marine sediment than you think. Components of the building materials for roads and structures, toothpaste, paint, and swimming pool filters come directly from sediments. In 2008 an estimated 38% of the world's crude oil and 33% of its natural gas will be extracted from the sedimentary deposits of continental shelves and continental rises. Offshore hydrocarbons currently generate annual revenues in excess of $200 billion. Deposits within the sediments of continental margins account for about one-third of the world's estimated oil and gas reserves.

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CONCEPT CHECK
26. What percentages of the total production of petroleum and natural gas are extracted from the seabed?
27. What products containing marine sediments have you used today?
28. Other than petroleum and natural gas, what is the most valuable material taken from marine sediments?
To check your answers, see the book's website. The website address is printed at the end of the chapter.
In addition to oil and gas, in 2005 sand and gravel valued at more than $550 million were taken from the ocean. This is about 1% of world needs. Commercial mining of manganese nodules has also been considered. In addition to manganese, these nodules contain substantial amounts of iron and other industrially important chemical elements. The high iron content of these nodules has prompted a proposal to rename themferromanganese nodules. We will investigate these resources in more detail in Chapter 17.
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End of Chapter Review Pages 152 to 153


Sediments: Questions from Students
Sediments: Chapter in Perspective
Sediments: Terms and Concepts to Remember
Sediments: Study Questions
Sediments: Questions from Students
1. The question of sediment age seems to occupy much of sedimentologists’ time. Why?
The dating of sediments has been a central problem in marine science for many years. In 1957, during the International Geophysical Year, sedimentologists designed a coordinated effort to determine sediment age, which included plans for the Glomar Explorer and Glomar Challenger drilling surveys. Their primary interest was to seek evidence of the hypothesis of the then-new idea of seafloor spreading. Cores returned by the Deep Sea Drilling Project in 1968 enabled researchers including J. Tuzo Wilson, Harry Hess, and Maurice Ewing to put the evidence together. Much of the proof for plate tectonics rests on the interpretation of sediment cores.
2. Where are sediments thickest?
Sediments are thickest close to eroding land and beneath biologically productive neritic waters, and thinnest over the fast-spreading oceanic ridges of the eastern South Pacific. The thickest accumulations of sediment may be found along and beneath the continental margins (especially on continental rises). Some are typically more than 1,500 meters (5,000 feet) thick. Remember, much of the rocky material of the Grand Canyon was once marine sediment atop an isostatically depressed ancient seabed. The Grand Canyon is nearly 2 kilometers (1¼ miles) deep, and the uppermost layer of sedimentary rock has already been eroded completely away!
3. What's the relationship between deep-sea animals and the sediments on which they live?
Though microscopic bacteria and benthic foraminifera may be very abundant on the seabed, visible life is not abundant on the bottom of the deep ocean. There are no plants at great depths because there is no light, but animals do live there. Some, like the brittle stars in Figure 5.2, move slowly along the surface searching for bits of organic matter to eat. Others burrow through the muck in search of food particles. Worms eat quantities of sediment to extract any nutrients that may be present and then deposit strings of fecal material as they move forward. The deeps are uninviting places, but life is tenacious and survives even in this hostile environment.
Sediments: Chapter in Perspective
In this chapter you learned that the sediments covering nearly all of the seafloor are parts of the great cycles of formation and destruction assured by Earth's hot interior. Marine sediment is composed of particles from land, from biological activity in the ocean, from chemical processes within water, and even from space. The blanket of seafloor sediment is thickest at the continental margins and thinnest over the active oceanic ridges.
Sediments may be classified by particle size, source, location, or color. Terrigenous sediments, the most abundant, originate on continents or islands. Biogenous
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sediments are composed of the remains of once-living organisms. Hydrogenous sediments are precipitated directly from seawater. Cosmogenous sediments, the ocean's rarest, come to the seabed from space.
The position and nature of sediments provide important clues to Earth's recent history, and valuable resources can sometimes be recovered from them.
In the next chapter you'll learn about our story's main character—water itself. You know something about how water molecules were formed early in the history of the universe, something about the inner workings of our planet, and something about the nature of the ocean's “container.” Now let's fill that container with water and see what happens.
Sediments: Terms and Concepts to Remember
•	authigenic sediment
•	biogenous sediment
•	calcareous ooze
•	calcium carbonate compensation depth (CCD)
•	clamshell sampler
•	clay
•	coccolithophore
•	cosmogenous sediment
•	diatom
•	evaporite
•	foraminiferan
•	hydrogenous sediment
•	lithification
•	microtektite
•	mineral
•	neritic sediment
•	nodule
•	oolite sands
•	ooze
•	paleoceanography
•	pelagic sediment
•	piston corer
•	poorly sorted sediment
•	pteropod
•	radiolarian
•	sand
•	sediment
•	siliceous ooze
•	silt
•	stratigraphy
•	terrigenous sediment
•	turbidite
•	well-sorted sediment
Sediments: Study Questions
Thinking Critically
1.	Is the thickness of ooze always an accurate indication of the biological productivity of surface water in a given area? (Hint: See next question.)
2.	What is the calcium carbonate compensation depth? Is there a compensation depth for the siliceous components of once-living things?
3.	What sediments accumulate most rapidly? Least rapidly?
4.	Can marine sediments tell us about the history of the ocean from the time of its origin? Why?
5.	What problems might arise when working with deep-ocean cores? Imagine the process of taking a core sample, and think of what can go wrong!
Thinking Analytically
1.	Given an average rate of accumulation, how much time would it take to build an average-size manganese nodule?
2.	Assuming an average rate of sediment accumulation and seafloor spreading, how far from the Mid-Atlantic Ridge would one need to travel before encountering a layer of sediment 1,000 meters thick?
3.	Microtektites are often found in “fields”—elongated zones of relative concentration a few hundred kilometers long. Why do you suppose that is?
4.	How much faster do fragments of diatoms fall when they are compacted into fecal pellets than when they are not? (Hint: See Table 5.1)

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Visit the Oceanography Resource Center at www.cengage.com/login for more assets, including animations, videos, audio clips, and more.
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Key Concept Review (Answers to in-text “Concept Checks”)
Chapter 4

1.	The simplest methods involved lowering a weight on a line.  The length of line is measured, and the depth determined.  Sometimes the weight was tipped with wax to retrieve a sample of bottom sediment.  Scientists now use beams of sound to measure depth.
2.	Echo sounders sense the contour of the seafloor by beaming sound waves to the bottom and measuring the time required for the sound waves to bounce back to the ship.  Unlike a simple echo sounder, a multibeam system may have as many as 121 beams radiating from a ship’s hull.
3.	Satellites cannot measure ocean depths directly, but they can measure small variations in the elevation of surface water using radar beams.  This is useful because the pull of gravity varies across Earth’s surface depending on the nearness (or distance away) of massive parts of Earth. An undersea mountain or ridge “pulls” water toward it from the sides, forming a mount of water over itself, and that mount is detected by the orbiting satellite.
4.	Ocean basins are not bathtub-shaped.  The submerged edges of continents form shelves at basin margins, and the center of a basin is often raised by a ridge.
5.	The transition to basalt marks the true edge of the continent and divides ocean floors into two major provinces. The submerged outer edge of a continent is called the continental margin. The deep seafloor beyond the continental margin is properly called the ocean basin.
6.	The continental margin is characterized by thick (and less dense) granitic rock of the continents. Near shore the features of the ocean floor are similar to those of the adjacent continents because they share the same granitic basement.  Relatively thin (and denser) basalt forms the adjacent deep seafloor.
7.	The continental slope, shelf break, continental slope, and continental rise are the main features of continental margins.
8.	Continental margins facing the edges of diverging plates are called passive margins because relatively little earthquake or volcanic activity is now associated with them. Continental margins near the edges of converging plates (or near places where plates are slipping past one another) are called active margins because of their earthquake and volcanic activity.
9.	The width of a shelf is usually determined by its proximity to a plate boundary. The shelf at a passive margin is usually broad, but the shelf at the active margin is often very narrow.
10.	Figures 4.15 and 12.2 provide graphs of sea level through time.  Sea level is high at the moment, and is rising as the ocean warms.
11.	Submarine canyons cut into the continental shelf and slope, often terminating on the deep-sea floor in a fan-shaped wedge of sediment.  Most geologists believe that the canyons have been formed by abrasive turbidity currents plunging down the canyons.
12.	Along passive margins, the oceanic crust at the base of the continental slope is covered by an apron of accumulated sediment called the continental rise. Sediments from the shelf slowly descend to the ocean floor along the whole continental slope, but most of the sediments that form the continental rise are transported to the area by turbidity currents.
13.	Oceanic ridges are Earth’s most remarkable and obvious feature. Other deep-ocean features are abyssal plains, seamounts, fracture zones, and the deep trenches.
14.	Oceanic ridges stretch 65,000 kilometers. Although these features are often called mid-ocean ridges, less than 60% of their length actually exists along the centers of ocean basins.
15.	Check Figures 4.26 and 4.27 after you make your drawing.
16.	Fracture zones extend outward from the ridge axis.  They are seismically inactive areas that show evidence of past transform fault activity. While segments of a lithospheric plate on either side of a transform fault move in opposite directions from each other, the plate segments adjacent to the outward segments of a fracture zone move in the same direction.
17.	Abyssal plains are flat, featureless expanses of sediment-covered ocean floor found on the periphery of all oceans. Abyssal plains are extraordinarily flat.
18.	Abyssal plains are relatively rare in the active Pacific, where peripheral trenches trap most of the sediments flowing from the continents.
19.	Guyots are flat-topped seamounts that once were tall enough to approach or penetrate the sea surface. The flat top suggests that they were eroded by wave action when they were near sea level. Movement of the lithosphere away from spreading centers has carried them outward and downward to their present positions.
20.	A trench is an arc-shaped depression in the deep-ocean floor that forms where a converging oceanic plate is subducted.
21.	Trenches are curved because of the geometry of plate interactions on a sphere. The convex sides of these curves generally face the open ocean. The trench walls on the island side of the depressions are steeper than those on the seaward side, indicating the direction of plate subduction.  Trenches are prevalent in the western Pacific because that is an area of vigorous plate subduction.
22.	Seeing the whole picture at once has its advantages.  Patterns not discernable from looking at individual pieces of the puzzle can jump out.  Look, for instance, at the area south and east of South America (and north and east of the Antarctic’s Palmer Peninsula) in Figure 4.33 – notice the strong suggestion that the moving sea floor has smeared those projections eastward?

CHAPTER IN PERSPECTIVE
Chapter 4: Ocean Basins

In this chapter you learned how difficult it has been to discover the shape of the seabed. Even today, the surface contours of Mars are better known than those of our ocean floor.
We now know that seafloor features result from a combination of tectonic activity and the processes of erosion and deposition. The ocean floor can be divided into two regions: continental margins and deep ocean basins. The continental margin, the relatively shallow ocean floor nearest the shore, consists of the continental shelf and the continental slope. The continental margin shares the structure of the adjacent continents, but the deep-ocean floor away from land has a much different origin and history. Prominent features of the deep-ocean basins include rugged oceanic ridges, fl at abyssal plains, occasional deep trenches, and curving chains of volcanic islands. The processes of plate tectonics, erosion, and sediment deposition have shaped the continental margins and ocean basins.

In the next chapter you will learn that nearly all the ocean floor is blanketed with sediment. Except for the spreading centers themselves, the broad shoulders of the oceanic ridge systems are buried according to their age— the older the seabed, the greater the sediment burden. Some oceanic crust near the trailing edges of plates may be overlain by sediments more than 1,500 meters (5,000 feet) thick. Sediments have been called the “memory of the ocean.” The memory, however, is not a long one. Before continuing, can you imagine why that is so?
Glossary
abyssal hill
Small sediment-covered inactive volcano or intrusion of molten rock less than 200 meters (650 feet) high, thought to be associated with seafloor spreading. Abyssal hills punctuate the otherwise flat abyssal plain.

abyssal plain
Flat, cold, sediment-covered ocean floor between the continental rise and the oceanic ridge at a depth of 3,700 to 5,500 meters (12,000 to 18,000 feet). Abyssal plains are more extensive in the Atlantic and Indian Oceans than in the Pacific.

active margin
The continental margin near an area of lithospheric plate convergence; also called Pacific-type margin.

angle of incidence
In meteorology, the angle of the sun above the horizon.

bathymetry
The discovery and study of submerged contours.

continental margin
The submerged outer edge of a continent, made of granitic crust; includes the continental shelf and continental slope. Compare ocean basin.

continental rise
The wedge of sediment forming the gentle transition from the outer (lower) edge of the continental slope to the abyssal plain; usually associated with passive margins.

continental shelf
The gradually sloping submerged extension of a continent, composed of granitic rock overlain by sediments; has features similar to the edge of the nearby continent.

continental slope
The sloping transition between the granite of the continent and the basalt of the seabed; the true edge of a continent.

epicenter
The point on Earth’s surface directly above the focus of an earthquake.

fracture zone
Area of irregular, seismically inactive topography marking the position of a once-active transform fault.

guyot
A flat-topped, submerged inactive volcano.

hydrothermal vent
A spring of hot, mineral- and gas-rich seawater found on some oceanic ridges in zones of active seafloor spreading.

ice age
One of several periods (lasting several thousand years each) of low temperature during the last million years. Glaciers and polar ice were derived from ocean water, lowering sea level at least 100 meters (328 feet). (See Appendix II, “Geological Time.”)

ice cap
Permanent cover of ice; formally limited to ice atop land, but informally applied also to floating ice in the Arctic Ocean.

iceberg
A large mass of ice floating in the ocean that was formed on or adjacent to land. Tabular icebergs are tablelike or flat; pinnacled icebergs are castellated, or jagged. Southern icebergs are often tabular; northern icebergs are often pinnacled.

island arc
Curving chain of volcanic islands and seamounts almost always found paralleling the concave edge of a trench.

ocean basin
Deep-ocean floor made of basaltic crust. Compare continental margin.

oceanic ridge
Young seabed at the active spreading center of an ocean, often unmasked by sediment, bulging above the abyssal plain. The boundary between diverging plates. Often called a mid-ocean ridge, though less than 60% of the length exists at mid-ocean.

passive margin
The continental margin near an area of lithospheric plate divergence; also called Atlantic-type margin.

polynya
A gap in polar pack ice at which liquid water contacts the atmosphere.

seamount
A circular or elliptical projection from the seafloor, more than 1 kilometer (0.6 mile) in height, with a relatively steep slope of 20° to 25°.

shelf break
The abrupt increase in slope at the junction between continental shelf and continental slope.

submarine canyon
A deep, V-shaped valley running roughly perpendicular to the shoreline and cutting across the edge of the continental shelf and slope.

transform fault
A plane along which rock masses slide horizontally past one another.

trench
An arc-shaped depression in the deep-ocean floor with very steep sides and a flat sediment-filled bottom coinciding with a subduction zone. Most trenches occur in the Pacific.

turbidity current
An underwater “avalanche” of abrasive sediments thought responsible for the deep sculpturing of submarine canyons and a means of transport for sediments accumulating on abyssal plains.

CHAPTER IN PERSPECTIVE
Chapter 5: Sediments

In this chapter you learned that the sediments covering nearly all of the seafloor are parts of the great cycles of formation and destruction assured by Earth’s hot interior. Marine sediment is composed of particles from land, from biological activity in the ocean, from chemical processes within water, and even from space. The blanket of seafloor sediment is thickest at the continental margins and thinnest over the active oceanic ridges.
Sediments may be classified by particle size, source, location, or color. Terrigenous sediments, the most abundant, originate on continents or islands. Biogenous sediments are composed of the remains of once-living organisms. Hydrogenous sediments are precipitated directly from seawater. Cosmogenous sediments, the ocean’s rarest, come to the seabed from space.
The position and nature of sediments provide important clues to Earth’s recent history, and valuable resources can sometimes be recovered from them.

In the next chapter you’ll learn about our story’s main character—water itself. You know something about how water molecules were formed early in the history of the universe, something about the inner workings of our planet, and something about the nature of the ocean’s “container.” Now let’s fi ll that container with water and see what happens.

Glossary
absolute dating
Determining the age of a geological sample by calculating radioactive decay and/or its position in relation to other samples.

authigenic sediment
Sediment formed directly by precipitation from seawater; also called hydrogenous sediment.

biogenous sediment
Sediment of biological origin. Organisms can deposit calcareous (calcium-containing) or siliceous (silicon-containing) residue.

calcareous ooze
Ooze composed mostly of the hard remains of organisms containing calcium carbonate.

calcium carbonate compensation depth
The depth at which the rate of accumulation of calcareous sediments equals the rate of dissolution of those sediments. Below this depth, sediment contains little or no calcium carbonate.

clamshell sampler
A sampling device used to take shallow samples of the ocean bottom.

clay
Sediment particle smaller than 0.004 millimeter in diameter; the smallest sediment size category.

coccolithophore
A very small planktonic alga carrying discs of calcium carbonate, which contributes to biogenous sediments.

cosmogenous sediment
Sediment of extraterrestrial origin.

diatom
Earth’s most abundant, successful, and efficient single-celled phytoplankton. Diatoms possess two interlocking valves made primarily of silica. The valves contribute to biogenous sediments.

evaporite
Deposit formed by the evaporation of ocean water.

foraminiferan
One of a group of planktonic amoeba-like animals with a calcareous shell, which contributes to biogenous sediments.

hydrogenous sediment
A sediment formed directly by precipitation from seawater; also called authigenic sediment.

lithification
Conversion of sediment into sedimentary rock by pressure or by the introduction of a mineral cement.

microtektite
Translucent oblong particles of glass, a component of cosmogenous sediment.

mineral
A naturally occurring inorganic crystalline material with a specific chemical composition and structure.

neritic
Of the shore or coast; refers to continental margins and the water covering them, or to nearshore organisms.

neritic sediment
Continental shelf sediment consisting primarily of terrigenous material.

nodule
Solid mass of hydrogenous sediment, most commonly manganese or ferromanganese nodules and phosphorite nodules.

oolite sand
Hydrogenous sediment formed when calcium carbonate precipitates from warmed seawater as pH rises, forming rounded grains around a shell fragment or other particle.

ooze
Sediment of at least 30% biological origin.

paleoceanography
The study of the ocean’s past.

pelagic
Of the open ocean; refers to the water above the deep-ocean basins, sediments of oceanic origin, or organisms of the open ocean.

piston corer
A seabed-sampling device capable of punching through up to 25 meters (80 feet) of sediment and returning an intact plug of material.

poorly sorted sediment
A sediment in which particles of many sizes are found.

pteropod
A small planktonic mollusk with a calcareous shell, which contributes to biogenous sediments.

radiolarian
One of a group of usually planktonic amoeba-like animals with a siliceous shell, which contributes to biogenous sediments.

relative dating
Determining the age of a geological sample by comparing its position to the positions of other samples.

sand
Sediment particle between 0.062 and 2 millimeters in diameter.

sediment
Particles of organic or inorganic matter that accumulate in a loose, unconsolidated form.

siliceous ooze
Ooze composed mostly of the hard remains of silica-containing organisms.

silt
Sediment particle between 0.004 and 0.062 millimeter in diameter.

stratigraphy
The branch of geology that deals with the definition and description of natural divisions of rocks; specifically, the analysis of relationships of rock strata.

tektite
A small, rounded, glassy component of cosmogenous sediments, usually less than 1.5 millimeters (1/20 inch) in length; thought to have formed from the impact of an asteroid or meteor on the crust of Earth or the moon.

terrigenous sediment
Sediment derived from the land and transported to the ocean by wind and flowing water.

turbidite
A terrigenous sediment deposited by a turbidity current; typically, coarse-grained layers of nearshore origin interleaved with finer sediments.

turbulence
Chaotic fluid flow.

well-sorted sediment
A sediment in which particles are of uniform size.

Key Concept Review (Answers to in-text “Concept Checks”)
Chapter 5


1.	Sediment is particles of organic or inorganic matter that accumulate in a loose, unconsolidated form.
2.	The marine processes that generate sediments are widespread.  Sediment particles may consist of the remains of once-living organisms, bits of windblown dust, volcanic ash, etc., that pervade the marine environment.
3.	As you learned in Chapter 3, tectonic processes form and destroy the seabed over time.  Because of subduction, seabed older than about 180 million years is rare.
4.	Most marine sediments are made of finer particles: sand, silt, and clay.
5.	The smaller the particle, the more easily it can be transported by streams, waves, and currents.
6.	Sediments composed of particles of mostly one size are said to be well-sorted sediments. Sediments with a mixture of sizes are poorly sorted sediments. Sorting is a function of the energy of the environment—the exposure of that area to the action of waves, tides, and currents.
7.	Marine sediments may be terrigenous (from the land), biogenous (of biological origin), hydrogenous (formed in place), or cosmogenous (from space).
8.	Terrigenous sediments are most abundant.  The largest terrigenous deposits form near continental margins.
9.	Biogenous sediments cover the greatest area of seabed, but their total volume is less than terrigenous sediments.
10.	Cosmogenous sediments are very rare.  They originate from interplanetary dust that falls constantly into the top of the atmosphere and rare impacts by large asteroids and comets.
11.	Most sediment deposits are a mixture of biogenous and terrigenous particles, with an occasional hydrogenous or cosmogenous supplement.  The dominant type gives its name to the mixture.
12.	Neritic sediments consist primarily of terrigenous material. Deep-ocean floors are covered by finer sediments than those of the continental margins, and a greater proportion of deep-sea sediment is of biogenous origin. Sediments of the slope, rise, and deep-ocean floor that originate in the ocean are called pelagic sediments.
13.	The bulk of neritic sediments are terrigenous; they are eroded from the land and carried to streams, where they are transported to the ocean.
14.	Neritic sediments can undergo lithification: They are converted into sedimentary rock by pressure-induced compaction or by cementation.
15.	Much of the Colorado Plateau, with its many stacked layers easily visible in the Grand Canyon, was formed by sedimentary deposition and lithification beneath a shallow continental sea beginning about 570 million years ago.
16.	When averaged, the Atlantic Ocean bottom is covered by sediments to a thickness of about 1 kilometer, while the Pacific floor has an average sediment thickness of less than 0.5 kilometer.
17.	A turbidity current is a dilute mixtures of sediment and water that periodically rushes down the continental slope.  The resulting deposits (turbidites) are graded layers of terrigenous sand interbedded with the finer pelagic sediments typical of the deep-sea floor.
18.	The organisms that contribute their remains to deep-sea oozes are small, single-celled, drifting, plantlike organisms and the single-celled animals that feed on them. The silica-rich residues give rise to siliceous ooze, the calcium-containing material to calcareous ooze.
19.	At the calcium carbonate compensation depth (CCD), the rate at which calcareous sediments are supplied to the seabed equals the rate at which those sediments dissolve. Below this depth, the tiny skeletons of calcium carbonate dissolve on the seafloor, so no calcareous oozes accumulate.
20.	Most hydrogenous sediments originate from chemical reactions that occur on particles of the dominant sediment. The most famous hydrogenous sediments are manganese nodules.
21.	Evaporites are hydrogenous deposits that include salts that salts precipitate as water evaporates from isolated arms of the ocean or from landlocked seas or lakes.
22.	Cameras are used to visualize the bottom, and direct samplers (clamshell, piston corers) are used to obtain specimens.  Reflected sound can image strata beneath the surface covering.
23.	The discovery that marine sediments are comparatively young (compared with terrestrial sediments) is a prime proof of the tectonic theory.  Remember what happens at subduction zones!
24.	Because the deep sea sediment record is ultimately destroyed in the subduction process, the ocean's sedimentary "memory" does not start with the ocean’s formation as originally reasoned by early marine scientists.
25.	Scientists now have instruments capable of analyzing very small variations in the relative abundances of the stable isotopes of oxygen preserved within the carbonate shells of microfossils found in deep sea sediments.  These instruments allow them to interpret changes in the temperature of surface and deep water over time.
26.	In 2008 an estimated 38% of the world’s crude oil and 33% of its natural gas will be extracted from the sedimentary deposits of continental shelves and continental rises.
27.	Toothpaste contains finely-ground diatomaceous residue, and gravel for concrete is often of marine origin.  And then there’s gasoline, heating oil, natural gas, and the raw material of plastics.  The list is long.
28.	Sand and gravel are the second most valuable physical marine resource.