CHAPTER THREE
Igneous Rocks

 Rocks are classified by mineral and chemical composition, by the texture of the constituent particles and by the processes that formed them.
Igneous rocks are formed when molten magma cools and are divided into two main categories: plutonic rock and volcanic. Plutonic or intrusive rocks result when magma cools and crystallizes slowly within the Earth's crust (example granite), while volcanic or extrusive rocks result from magma reaching the surface either as lava or fragmental ejecta (examples pumice and basalt) .

Igneous rocks  (from latin ignis, fire) are rocks formed by solidification of cooled magma (molten rock), with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks. This magma can be derived from partial melts of pre-existing rocks in either the Earth's mantle or crust. Typically, the melting is caused by one or more of the following processes - an increase in temperature, a decrease in pressure, or a change in composition. Over 700 types of igneous rocks have been described, most of them formed beneath the surface of the Earth's crust.

Igneous rocks make up approximately ninety-five percent of the upper part of the Earth's crust, but their great abundance is hidden on the Earth's surface by a relatively thin but widespread layer of sedimentary and metamorphic rocks.

Igneous rocks are geologically important because:
1) their minerals and global chemistry give information about the composition of the mantle, from which some igneous rocks are extracted, and the temperature and pressure conditions that allowed this extraction, and/or of other pre-existing rock that melted;
2) their absolute ages can be obtained from various forms of radiometric dating and thus can be compared to adjacent geological strata, allowing a time sequence of events;
3) their features are usually characteristic of a specific tectonic environment, allowing tectonic reconstitutions in some special circumstances they host important mineral deposits (ores): for example, tungsten, tin, and uranium are commonly associated with granites, whereas ores of chromium and platinum are commonly associated with gabbros.

Morphology and setting

In terms of modes of occurrence, igneous rocks can be either intrusive (plutonic) or extrusive (volcanic).

Intrusive igneous rocks

Intrusive igneous rocks are formed from magma that cools and solidifies within the earth. Surrounded by pre-existing rock (called country rock), the magma cools slowly, and as a result these rocks are coarse grained. The mineral grains in such rocks can generally be identified with the naked eye. Intrusive rocks can also be classified according to the shape and size of the intrusive body and its relation to the other formations into which it intrudes.

Typical intrusive formations are batholiths, stocks, laccoliths, sills and dikes. The extrusive rocks often produce lava flows.

The central cores of major mountain ranges consist of intrusive igneous rocks, usually granite. When exposed by erosion, these cores (called batholiths) may occupy huge areas of the Earth's surface.

Coarse grained intrusive igneous rocks which form at depth within the earth are termed as abyssal; intrusive igneous rocks which form near the surface are termed hypabyssal.

Extrusive igneous rocks

Extrusive igneous rocks are formed at the Earth's surface as a result of the partial melting of rocks within the mantle and crust.

The melt, with or without suspended crystals and gas bubbles, is called magma. Magma rises because it is less dense than the rock from which it was created. When it reaches the surface, magma extruded onto the surface either beneath water or air, is called lava. Eruptions of volcanoes into air are termed subaerial whereas those occurring underneath the ocean are termed submarine. Black smokers and mid-ocean ridge basalt are examples of submarine volcanic activity.

The volume of extrusive rock erupted annually by volcanoes varies with plate tectonic setting. Extrusive rock is produced in the following proportions:

divergent boundary: 73%
convergent boundary (subduction zone): 15%
hotspot: 12%

Magma which erupts from a volcano behaves according to its viscosity, determined by temperature, composition, and crystal content. High-temperature magma, most of which is basaltic in composition, behaves in a manner similar to thick oil and, as it cools, treacle. Long, thin basalt flows with pahoehoe surfaces are common. Intermediate composition magma such as andesite tends to form cinder cones of intermingled ash, tuff and lava, and may have viscosity similar to thick, cold molasses or even rubber when erupted. Felsic magma such as rhyolite is usually erupted at low temperature and is up to 10,000 times as viscous as basalt. Volcanoes with rhyolitic magma commonly erupt explosively, and rhyolitic lava flows typically are of limited extent and have steep margins, because the magma is so viscous.

Felsic and intermediate magmas that erupt often do so violently, with explosions driven by release of dissolved gases - typically water but also carbon dioxide. Explosively erupted pyroclastic material is called tephra and includes tuff, agglomerate and ignimbrite. Fine volcanic ash is also erupted and forms ash tuff deposits which can often cover vast areas.

Because lava cools and crystallizes rapidly, it is fine grained. If the cooling has been so rapid as to prevent the formation of even small crystals after extrusion, the resulting rock may be mostly glass (such as the rock obsidian). If the cooling of the lava happened slowly, the rocks would be coarse-grained.

Because the minerals are mostly fine-grained, it is much more difficult to distinguish between the different types of extrusive igneous rocks than between different types of intrusive igneous rocks. Generally, the mineral constituents of fine-grained extrusive igneous rocks can only be determined by examination of thin sections of the rock under a microscope, so only an approximate classification can usually be made in the field.

Classification

Igneous rock are classified according to mode of occurrence, texture, mineralogy, chemical composition, and the geometry of the igneous body.

The classification of the many types of different igneous rocks can provide us with important information about the conditions under which they formed. Two important variables used for the classification of igneous rocks are particle size, which largely depends upon the cooling history, and the mineral composition of the rock. Feldspars, quartz, olivine, pyroxenes, amphiboles, and micas are all important minerals in the formation of almost all igneous rocks, and they are basic to the classification of these rocks. All other minerals present are regarded as nonessential in almost all igneous rocks and are called accessory minerals.

In a simplified classification, igneous rock types are separated on the basis of the type of feldspar present, the presence or absence of quartz, and in rocks with no feldspar or quartz, the type of iron or magnesium minerals present.


The two most important variables used for the classification of igneous rocks. What are_


Texture

Igneous rocks which have crystals large enough to be seen by the naked eye are called phaneritic; those with crystals too small to be seen are called aphanitic. Generally speaking, phaneritic implies an intrusive origin; aphanitic an extrusive one.

An igneous rock with larger, clearly discernible crystals embedded in a finer-grained matrix is termed porphyry. Porphyritic texture develops when some of the crystals grow to considerable size before the main mass of the magma crystallizes as finer-grained, uniform material.

Texture is an important criterion for the naming of volcanic rocks. The texture of volcanic rocks, including the size, shape, orientation, and distribution of mineral grains and the intergrain relationships, will determine whether the rock is termed a tuff, a pyroclastic lava or a simple lava.

However, the texture is only a subordinate part of classifying volcanic rocks, as most often there needs to be chemical information gleaned from rocks with extremely fine-grained groundmass or from airfall tuffs, which may be formed from volcanic ash.


Composition

Most igneous rocks can be classified according to the following chemical or mineralogical parameters:

1) acid igneous rocks containing a high silica content, greater than 63% SiO2 (examples rhyolite and granite)

2) intermediate igneous rocks containing between 52 - 63% SiO2 (example andesite and diorite)

2) basic igneous rocks have low silica 45 - 52% and typically high iron - magnesium content (example basalt and gabbro)

3) ultrabasic igneous rocks with less than 45% silica. (examples peridotite and komatiite)

Note: the acid-basic terminology is used more broadly in older geological literature. In current literature felsic-mafic roughly substitutes for acid-basic (See next section).

Mineralogic contents:

1) felsic rock, with predominance of quartz, alkali feldspar; the felsic minerals; these rocks (e.g., rhyolite and granite) are usually light coloured, and have low density.

2) mafic rock, with predominance of mafic minerals pyroxenes, olivines and calcic plagioclase; these rocks (example, basalt and gabbro) are usually dark coloured, and have a higher density than felsic rocks.

3) ultramafic rock, with more than 90% of mafic minerals (e.g., peridotite and komatite)

For igneous rocks where all minerals are visible at least via microscope, the mineralogy is used to classify the rock. This usually occurs on ternary diagrams, where the relative proportions of three minerals are used to classify the rock.

The table above is a simple subdivision of igneous rocks according both to their composition and mode of occurrence. For example, granite is an igneous intrusive rock (crystallized at depth), with felsic composition (rich in silica and predominately quartz plus alkali feldspar and phaneritic texture (minerals are visible to the unaided eye).

Magma origination

The Earth's crust averages about 35 kilometers thick under the continents, but averages only some 7-10 kilometers beneath the oceans. The continental crust is composed primarily of sedimentary rocks resting on crystalline basement formed of a great variety of metamorphic and igneous rocks including granulite and granite. Oceanic crust is composed primarily of basalt and gabbro. Both continental and oceanic crust rest on peridotite of the mantle.

Rocks may melt in response to a decrease in pressure, to a change in composition such as an addition of water, to an increase in temperature, or to a combination of these processes.

Other mechanisms, such as melting from impact of a meteorite, are less important today, but impacts during accretion of the Earth led to extensive melting, and the outer several hundred kilometers of our early Earth probably was an ocean of magma. Impacts of large meteorites in last few hundred million years have been proposed as one mechanism responsible for the extensive basalt magmatism of several large igneous provinces.


Decompression Melting

Decompression melting occurs because of a decrease in pressure. rocks (the temperatures below which they are completely solid) increase with increasing pressure in the absence of water. Peridotite at depth in the Earth's mantle may be hotter than its solidus temperature at some shallower level. If such rock rises during the convection of solid mantle, it will cool slightly as it expands in an adiabatic process, but the cooling is only about 0.3�C per kilometer. Experimental studies of appropriate peridotite samples document that the solidus temperatures increase by 3�C to 4�C per kilometer. If the rock rises far enough, it will begin to melt. Melt droplets can coalesce into larger volumes and be intruded upwards. This process of melting from upward movement of solid mantle is critical in the evolution of the earth.

Decompression melting creates the ocean crust at mid-ocean ridges. Decompression melting caused by the rise of mantle plumes is responsible for creating ocean islands like the Hawaiian islands. Plume-related decompression melting also is the most common explanation for flood basalts and oceanic plateaus (two types of large igneous provinces), although other causes such as melting related to meteorite impact have been proposed for some of these huge volumes of igneous rock.

Effects of water and carbon dioxide

The change of rock composition most responsible for creation of magma is the addition of water. Water lowers the solidus temperature of rocks at a given pressure. For example, at a depth of about 100 kilometers, peridotite begins to melt near 800�C in the presence of excess water, but near or above about 1500�C in the absence of water. Water is driven out of the oceanic lithosphere in subduction zones, and it causes melting in the overlying mantle. Hydrous magmas of basalt and andesite composition are produced directly and indirectly as results of dehydration during the subduction process. Such magmas and those derived from them build up island arcs such as those in the Pacific ring of fire. These magmas form rocks of the calc-alkaline series, an important part of continental crust.
The addition of carbon dioxide is relatively a much less important cause of magma formation than addition of water, but genesis of some silica-undersaturated magmas has been attributed to the dominance of carbon dioxide over water in their mantle source regions. In the presence of carbon dioxide, experiments document that the peridotite solidus temperature decreases by about 200�C in a narrow pressure interval at pressures corresponding to a depth of about 70 km. Magmas of rock types such as kimberlite are among those that may be generated following an influx of carbon dioxide into mantle at depths greater than about 70 km.

Temperature increase

Increase of temperature is the most typical mechanism for formation of magma within continental crust. Such temperature increases can occur because of the upward intrusion of magma from the mantle. Temperatures can also exceed the solidus of a crustal rock in continental crust thickened by compression at a plate boundary. The plate boundary between the Indian and Asian continental masses provides a well-studied example, as the Tibetan Plateau just north of the boundary has crust about 80 kilometers thick, roughly twice the thickness of normal continental crust. Granite and rhyolite are types of igneous rock commonly interpreted as products of melting of continental crust because of increases of temperature. Temperature increases also may contribute to the melting of lithosphere dragged down in a subduction zone.

Magma Evolution

Most magmas are only entirely melt for small parts of their histories. More typically, they are mixes of melt and crystals, and sometimes also of gas bubbles. Melt, crystals, and bubbles usually have different densities, and so they can separate as magmas evolve.

Igneous differentiation

As magma cools, minerals typically crystallize from the melt at different temperatures (fractional crystallization). As minerals crystallize, the composition of the residual melt typically changes. If crystals separate from melt, then the residual melt will differ in composition from the parent magma.

For instance, a magma of gabbroic composition can produce a residual melt of granitic composition if early formed crystals are separated from the magma. Gabbro may have a liquidus temperature near 1200�C, and derivative granite-composition melt may have a liquidus temperature as low as about 700�C. Incompatible elements are concentrated in the last residues of magma during fractional crystallization and in the first melts produced during partial melting: either process can form the magma that crystallizes to pegmatite, a rock type commonly enriched in incompatible elements. Bowen's reaction series is important for understanding the idealised sequence of fractional crystallisation of a magma.

Magma composition can be determined by processes other than partial melting and fractional crystallization. For instance, magmas commonly interact with rocks they intrude, both by melting those rocks and by reacting with them. Magmas of different compositions can mix with one another. In rare cases, melts can separate into two immiscible melts of contrasting compositions.



Magma chemisry

There are relatively few minerals that are important in the formation of common igneous rocks, because the magma from which the minerals crystallize is rich in only certain elements: silicon, oxygen, aluminium, sodium, potassium, calcium, iron, and magnesium. These are the elements which combine to form the silicate minerals, which account for over ninety percent of all igneous rocks.

COMMON IGNEOUS ROCKS

Granite

Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granites are usually medium to coarsely crystalline, occasionally with some individual crystals larger than the groundmass forming a rock known as porphyry. Granites can be pink to dark gray or even black, depending on their chemistry and mineralogy. Outcrops of granite tend to form tors, and rounded massifs. Granites sometimes occur in circular depressions surrounded by a range of hills, formed by the metamorphic aureole or hornfels.

Granite is nearly always massive (lacking internal structures), hard and tough, and therefore it has gained widespread use as a construction stone. The average density of granite is 2.75 with a range of 1.74 to 2.80. The word granite comes from the Latin granum, a grain, in reference to the coarse-grained structure of such a crystalline rock.

True granite according to modern petrologic convention contains both plagioclase and alkali feldspars. When a granitoid is devoid or nearly devoid of plagioclase the rock is referred to as alkali granite. When a granitoid contains <10% orthoclase it is called tonalite; pyroxene and amphibole are common in tonalite. A granite containing both muscovite and biotite micas is called a binary or two-mica granite. Two-mica granites are typically high in potassium and low in plagioclase. The volcanic equivalent of plutonic granite is rhyolite.

Granite is currently known only on Earth. It forms a major part of continental crust. Granite occurs as relatively small masses (stocks) and as huge batholiths that are often associated with orogenic mountain ranges. Small dikes of granitic composition called aplites are often associated with the margins of granitic intrusions. In some locations very coarse-grained pegmatite masses occur with granite.

Granite has been intruded into the crust of the Earth during all geologic periods, although much of it is of Precambrian age. Granitic rock is widely distributed throughout the continental crust of the Earth and is the most abundant basement rock that underlies the relatively thin sedimentary veneer of the continents.
Most granite intrusions are emplaced at depth within the crust, usually greater than 1.5 kilometres and up to 50 km depth within thick continental crust. The origin of granite is contentious and has led to varied schemes of classification.
The final mineralogy, texture and chemical composition of a granite is often distinctive as to its origin. For instance, a granite which is formed from melted sediments may have more alkali feldspar, whereas a granite derived from melted basalt may be richer in plagioclase feldspar.

The ascent and emplacement of large volumes of granite within the upper continental crust is a source of much debate amongst geologists. Several mechanisms have been proposed to explain how large batholiths have been emplaced:

1) Stoping, where the granite cracks the wall rocks and pushes upwards as it removes blocks of the overlying crust
2) Assimilation, where the granite melts its way up into the crust and removes overlying material in this way
3) Inflation, where the granite body inflates under pressure and is injected into position

Most geologists today accept that a combination of these phenomena can be used to explain granite intrusions, and that not all granites can be explained by one or another mechanism.

Granite is a normal, geological, source of radiation in the natural environment. Granite contains around 10 to 20 parts per million of uranium. By contrast, more mafic rocks such as tonalite, gabbro or diorite have 1 to 5 ppm uranium, and limestones and sedimentary rocks usually have equally low amounts. Granite could be considered a potential natural radiological hazard as, for instance, villages located over granite may be susceptible to higher doses of radiation than other communites. Cellars and basements sunk into soils formed over or from particularly uraniferous granites can become a trap for radon gas, which is heavier than air. However, in the majority of cases, although granite is a significant source of natural radiation as compared to other rocks it is not often an acute health threat or significant risk factor. Various resources from national geological survey organizations are accessible online to assist in assessing the risk factors in granite country and design rules relating, in particular, to preventing accumulation of radon gas in enclosed basements and dwellings.

Granite has been used since antiquity as a dimension stone and as flooring tiles in public and commercial buildings and monuments. With increasing amounts of acid rain in parts of the world, granite has begun to supplant marble as a monument material, since it is much more durable. Polished granite is also a popular choice for kitchen countertops due to its high durability and aesthetic qualities.


Rhyolite

Rhyolite is an igneous, volcanic (extrusive) rock, of felsic (acidic) composition (typically >69% silica.. It may have any texture from aphanitic to porphyritic. The mineral assemblage is usually quartz, alkali feldspar and plagioclase (in a ratio > 1:2 . Biotite and pyroxene are common accessory minerals.

Rhyolites can be considered as the extrusive equivalent to the plutonic granite rock, due to their high content of silica and low iron and magnesium contents, rhyolites polymerize quickly and form highly viscous lavas. They can also occur as breccias or in volcanic necks and dikes. Rhyolites that cool too quickly to grow crystals form a natural glass or vitrophyre, also called obsidian. Slower cooling forms microscopic crystals in the lava and results in textures such as flow foliations.


Obsidian

Obsidian is a type of naturally-occurring glass formed as an extrusive igneous rock. It is produced when felsic lava erupted from a volcano cools rapidly through the glass transition temperature and freezes without sufficient time for crystal growth. Obsidian is commonly found within the margins of rhyolitic lava flows known as obsidian flows, where cooling of the lava is rapid. Because of the lack of crystal structure, obsidian blade edges can reach almost molecular thinness, leading to its ancient use as projectile points, and its modern use as surgical scalpel blades.

Obsidian is mineral-like, but not a true mineral because as a glass it is not crystalline; in addition, its composition is too complex to comprise a single mineral. It is sometimes classified as a mineraloid. Though obsidian is dark in color similar to mafic rocks such as basalt, obsidian's composition is extremely felsic. Obsidian consists mainly of SiO2 (silicon dioxide), usually 70% or more. Crystalline rocks with obsidian's composition include granite and rhyolite. Because obsidian is metastable at the earth's surface (over time the glass becomes fine-grained mineral crystals), no obsidian has been found that is older than Cretaceous age. This breakdown of obsidian is accelerated by the presence of water. Obsidian has a low water content when fresh, typically less than 1 weight % water, but becomes progressively hydrated when exposed to groundwater, forming perlite.

Pure obsidian is usually dark in appearance, though the color varies depending on the presence of impurities. Iron and magnesium typically give the obsidian a dark green to brown to black color. A very few samples are nearly clear. In some stones, the inclusion of small, white, radially clustered crystals of cristobalite in the black glass produce a blotchy or snowflake pattern (snowflake obsidian). It may contain patterns of gas bubbles remaining from the lava flow, aligned along layers created as the molten rock was flowing before being cooled. These bubbles can produce interesting effects such as a golden (sheen obsidian) or rainbow sheen (rainbow obsidian).

Obsidian can be found in many locations around the world which have experienced rhyolitic eruptions. Among other places, large obsidan flows are found within the calderas of Newberry Volcano and Medicine Lake Volcano in the Cascade Range of western North America, and at Inyo Craters east of the Sierra Nevada in California. Yellowstone National Park has a mountainside containing much obsidian located between Mammoth Hot Springs and the Norris Geyser Basin, and deposits can be found in many other western U. S. states including Arizona, Colorado, Texas, Utah, and Idaho.

Obsidian was highly valued in certain Stone Age cultures because, like flint, it could be fractured to produce sharp blades or arrowheads. Like all glass and some other types of naturally occurring rocks, obsidian breaks with a characteristic conchoidal fracture. It was also polished to create early mirrors. Pre-Columbian Mesoamericans' use of obsidian was extensive and sophisticated, including carved and worked obsidian for tools and decorative objects. Mesoamericans also made a type of sword with obsidian blades mounted in a wooden body. Called a macuahuitl, the weapon was capable of inflicting terrible injuries, combining the sharp cutting edge of an obsidian blade with the ragged cut of a serrated weapon.

Native Americans traded obsidian throughout North America. Each volcano and in some cases each volcanic eruption produces a distinguishable type of obsidian, making it possible for archaeologists to trace the origins of a particular artifact. Similar tracing techniques have allowed obsidian to be identified in Greece also as coming from either Melos, Nisyros or Yiali, islands in the Aegean Sea. Obsidian cores and blades were traded great distances inland from the coast.

Obsidian is used in cardiac surgery, as well-crafted obsidian blades have a cutting edge many times sharper than high-quality steel surgical scalpels, with the edge of the blade being only about 3 nm wide. Even the sharpest metal knife has a jagged, irregular blade when viewed under a strong enough microscope. When examined under an electron microscope an obsidian blade is still smooth and even. One study found that obsidian produced narrower scars, fewer inflammatory cells, and less granulation tissue in a group of rats.

Obsidian is also used for ornamental purposes and as a gemstone, for it possesses the peculiar property of presenting a different appearance according to the manner in which it is cut. When cut in one direction it is a beautiful jet black; when cut across another direction it is glistening gray. "Apache tears" are small rounded obsidian nuggets embedded within a grayish-white perlite matrix.


Pumice

Pumice is a textural term for a volcanic rock that is a solidified glassy foam. It is commonly, but not exclusively of felsic to intermediate in composition but occurrences of basaltic and other compositions are known. Pumice is commonly pale in color, ranging from white, cream or grey, but can be green brown or black. It forms when gases exsolving from viscous magma nucleate bubbles which cannot readily decouple from the viscous magma prior to chilling to glass. Pumice is a common product of explosive eruptions  and commonly forms zones in upper parts of silicic lavas. Pumice has an average porosity of 90%, and initially floats on water.
Scoria differs from pumice in being denser, with larger vesicles and thicker vesicle walls; it sinks rapidly. The difference is the result of the lower viscosity of the magma that formed scoria. Pumice is considered a glass because it has no crystal structure. Pumice varies in density according to the thickness of the solid material between the bubbles; many samples float in water. After the explosion of Krakatoa, rafts of pumice drifted through the Pacific Ocean for up to 20 years, with tree trunks floating among them. In fact, pumice rafts disperse and support several marine species. In 1979, 1984 and 2006, underwater volcanic eruptions near Tonga created large pumice rafts, some as large as 30 km that floated hundreds of miles to Fiji.

Pumice has a very low density. There are two main forms of vesicles. Most pumice contains tubular microvesicles that can impart a silky or fibrous fabric. The elongation of the microvesicles occurs due to ductile elongation in the volcanic conduit or, in the case of pumiceous lavas, during flow. The other form of vesicles are subspherical to spherical and result from high vapour pressure during eruption.

Pumice is widely used to make lightweight concrete or insulative low-density 'breeze-block' type bricks. When used as an additive for cement, a fine-grained version of pumice is mixed with lime to form a light-weight, smooth, plaster-like concrete. This form of concrete was used as far back as Roman times. It is also used as an abrasive, especially in polishes, cosmetics exfoliants, and for stone-washed jeans. "Pumice stones" are often used in salons during the pedicure process to remove dry and excess skin from the bottom of the foot and also calluses. Finely ground pumice is added to some toothpastes and heavy-duty hand cleaners as a mild abrasive. Perhaps the most famous product advertised to contain pumice is Lava soap. It is a heavy-duty hand soap, old in both bar and liquid form, for mechanics and others who get very dirty hands.

Andesite

Andesite is an igneous, volcanic rock, of intermediate composition, with aphanitic to porphyritic texture. The mineral assembly is typically dominated by plagioclase plus pyroxene and/or hornblende.
Andesite can be considered as the extrusive equivalent to plutonic diorite. Andesites are characteristic of subduction tectonic environments in active oceanic margins, such as the western coast of South America. The name andesite is derived from the Andes mountain range.

Andesite is formed at accretionary plate margins. Intermediate volcanic rocks are created via several processes:
1) Dehydration melting of mantle peridotite and fractional crystallization above subduction zones.
2) Melting of subducted slab containing sediments
3) Magma mixing between felsic rhyolitic and mafic basaltic magmas in an intermediate reservoir prior to emplacement or eruption.


Diorite

Diorite is a grey to dark grey intermediate intrusive igneous rock composed principally of plagioclase feldspar, biotite, hornblende, and/or pyroxene. It may contain small amounts of quartz, microcline, and olivine. It can also be black or bluish-grey, and frequently has a greenish cast. The presence of significant quartz makes the rock type quartz-diorite (>5% quartz) or tonalite (>20% quartz), and if orthoclase (potassium feldspar) is present at greater than ten percent the rock type grades into monzodiorite or granodiorite.

Diorites may be associated with either granite or gabbro intrusions, into which they may subtly merge. Diorite results from partial melting of a mafic rock above a subduction zone. It is commonly produced in volcanic arcs, and in cordilleran mountain building such as in the Andes Mountains as large batholiths. The extrusive volcanic equivalent rock type is andesite. .

Diorite is an extremely hard rock, making it difficult to carve and work with. It is so hard that ancient civilizations used diorite balls to work granite. Its hardness, however, also allows it to be worked finely and take a high polish, and to provide a durable finished work. Thus, major works in diorite tend to be important.

One comparatively frequent use of diorite was for inscription, as it is easier to carve in relief than in three-dimensional statuary. Perhaps the most famous diorite work extant is the Code of Hammurabi, inscribed upon a 2 metre (7 ft) pillar of black diorite. The original can be seen today in Paris' Mus�e de Louvre. A few large statues remain, including several statues of King Khafre in the Egyptian Museum. The use of diorite in art was most important among very early Middle Eastern civilizations such as Ancient Egypt, Babylonia, Assyria and Sumer. It was so valued in early times that the first great Mesopotamian empire -- the Empire of Sargon of Akkad -- listed the taking of diorite as a purpose of military expeditions.

Although one can find diorite art from later periods, it became more popular as a structural stone and was frequently used as pavement due to its durability. Diorite was used by both the Inca and Mayan civilizations, but mostly for fortress walls, weaponry, etc. It was especially popular with medieval Islamic builders. In later times, diorite was commonly used as cobblestone; today many diorite cobblestone streets can be found in England, Guernsey and Scotland, and scattered throughout the world in such places as Ecuador and China. Although diorite is rough-textured in nature, its ability to take a polish can be seen in the diorite steps of St. Paul's Cathedral, London, where centuries of foot traffic have polished the steps to a sheen.

Basalt

Basalt is a common gray to black extrusive volcanic rock. It is usually fine-grained due to rapid cooling of lava on the Earth's surface. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or gray.

Basalt magmas have formed by decompression melting of the Earth's mantle. The crustal portions of oceanic tectonic plates are composed predominantly of basalt, produced from upwelling mantle below ocean ridges.

Large masses must cool slowly to form a polygonal fracture pattern. Tholeiitic basalt is relatively poor in silica and poor in sodium. Included in this category are most basalts of the ocean floor, most large oceanic islands, and continental flood basalts such as the Columbia River Plateau.

The mineralogy of basalt is characterized by a preponderance of calcic plagioclase feldspar and pyroxene. Olivine can also be a significant constituent. Accessory minerals present in relatively minor amounts include iron oxides and iron-titanium oxides, such as magnetite, ulvospinel, and ilmenite. Because of the presence of such oxide minerals, basalt can acquire strong magnetic signatures as it cools, and paleomagnetic studies have made extensive use of basalt.

Basalt generally has a composition of 45-55 wt% SiO2. Isotope ratios of elements such as strontium, neodymium, lead, hafnium, and osmium in basalts have been much-studied, so as to learn about evolution of the Earth's mantle. Isotopic ratios of noble gases, such as helium, are also of great value.


The shape, structure and texture of a basalt is diagnostic of how and where it erupted - whether into the sea, in an explosive cinder eruption or as creeping pahoehoe lava flows, the classical image of Hawaiian basalt eruptions.

Basalt which erupts under open air (that is, subaerially) forms three distinct types of lava or volcanic deposits: scoria, ash or cinder; breccia and lava flows.

Basalt in the tops of subaerial lava flows and cinder cones will often be highly vesiculated, imparting a lightweight "frothy" texture to the rock. Basaltic cinders are often red, coloured by oxidised iron from weathered iron-rich minerals such as pyroxene.

The "aa"  types of blocky, cinder and breccia flows of thick, viscous basaltic lava are common in Hawaii. Pahoehoe is a highly fluid, hot form of basalt which tends to form thin aprons of molten lava which fill up hollows and sometimes forms lava lakes. Lava tubes are common features of pahoehoe eruptions.

Basaltic tuff or pyroclastic rocks are rare but not unknown. Usually basalt is too hot and fluid to build up sufficient pressure to form explosive lava eruptions but occasionally this will happen by trapping of the lava within the volcanic throat and build up of volcanic gases. Hawaii's Mauna Loa volcano erupted in this way in the 19th century, as did Mount Tarawera, New Zealand in its violent 1886 eruption..

 Maar volcanoes are typical of small basalt tuffs, formed by explosive eruption of basalt through the crust, forming an apron of mixed basalt and wall rock breccia and a fan of basalt tuff further out from the volcano.

Amygdaloidal structure is common in relict vesicles and beautifully crystallized species of zeolites, quartz or calcite are frequently found.

During the cooling of a thick lava flow, contractional joints or fractures form. If a flow cools relatively rapidly, significant contraction forces build up. While a flow can shrink in the vertical dimension without fracturing, it cannot easily accommodate shrinking in the horizontal direction unless cracks form. The extensive fracture network that develops results in the formation of columns. The topology of the lateral shapes of these columns can broadly be classed as a random cellular network. These structures are often erroneously described as being predominantly hexagonal. In reality, the mean number of sides of all the columns in such a structure is indeed six (by geometrical definition), but polygons with three to twelve or more sides can be observed. Note that the size of the columns depends loosely on the rate of cooling; very rapid cooling may result in very small (<1 cm diameter) columns, and vice versa.

When basalt erupts underwater or flows into the sea, the cold water quenches the surface and the lava forms a distinctive pillow shape, through which the hot lava breaks to form another pillow. This pillow texture is very common in underwater basaltic flows and is diagnostic of an underwater eruption environment when found in ancient rocks. Pillows typically consist of a fine-grained core with a glassy crust and have radial jointing. The size of individual pillows varies from 10 cm up to several metres.

When pahoehoe lava enters the sea it usually forms pillow basalts. However when a'a enters the ocean it forms a littoral cone, a small cone-shaped accumulation of tuffaceous debris formed when the blocky a'a lava enters the water and explodes from built-up steam.

Volcanic glass may be present, particularly as rinds on rapidly chilled surfaces of lava flows, and is commonly (but not exclusively) associated with underwater eruptions.

The lava flows of the Deccan Traps in India, the Siberian Traps in Russia, the Columbia River Plateau of Washington are basalts. Other famous accumulations of basalts include Iceland and the islands of the Hawaii volcanic chain, forming above a mantle plume. Basalt is the rock most typical of large igneous provinces.

Ancient Precambrian basalts are usually only found in fold and thrust belts, and are often heavily metamorphosed. These are known as greenstone belts, because low-grade metamorphism of basalt produces chlorite,  epidote and other green minerals.

The dark areas visible on Earth's moon, the lunar maria, are plains of flood basaltic lava flows. These rocks were sampled by the manned American Apollo program, the robotic Russian Luna program, and are represented among the lunar meteorites.





Gabbro

Gabbro is a dark, coarse-grained, intrusive igneous rock chemically equivalent to basalt. It is a plutonic rock, formed when molten magma is trapped beneath the Earth's surface and cools into a crystalline mass.

The vast majority of the Earth's surface is underlain by gabbro within the oceanic crust, produced by basalt magmatism at mid-ocean ridges.

Gabbro is dense, greenish or dark-colored and contains varied percentages of pyroxene, plagioclase, amphibole, and olivine. Gabbro is generally coarse grained, with crystals in the size range of 1 mm or greater. Finer grained equivalents of gabbro are called diabase, although the vernacular term microgabbro is often used when extra descriptiveness is desired. Gabbro may be extremely coarse grained to pegmatitic, and some pyroxene-plagioclase cumulates are essentially coarse grained gabbro, although these may exhibit acicular crystal habits.

Gabbro can be formed as a massive uniform intrusion or as part of a layered layered intrusions as a cumulate formed by settling of pyroxene and plagioclase.  Gabbro is an essential part of the oceanic crust.

Gabbro often contains valuable amounts of chromium, nickel, cobalt, gold, silver, platinum, and copper sulfides. Varieties of gabbro are often used as ornamental facing stones, paving stones and it is also known by the trade name of 'black granite', which is a popular type of graveyard headstone used in funerary rites. It is even more popular now for use in kitchen and their countertops, also under the misnomer of 'black granite'.

Pyroclastics

Pyroclastic rocks or pyroclastics (derived from the Greek, meaning fire-broken) are clastic rocks composed solely or primarily of volcanic materials. Where the volcanic material has been transported and reworked through mechanical action, such as by wind or water, these rocks are termed volcaniclastic. Commonly associated with explosive volcanic activity - such as plinean or krakatoan eruption styles, or phreatomagmatic eruptions - pyroclastic deposits are commonly formed from airborne ash, lapilli and bombs or blocks ejected from the volcano itself, mixed in with shattered country rock.

Pyroclastic rocks may be composed of a large range of clast sizes; from the largest agglomerates, to very fine ashes and tuffs. Pyroclasts of different sizes are classified as volcanic bombs, lapilli and volcanic ash. Ash is considered to be pyroclastic because it is a fine dust made up of volcanic rock. One of the most spectacular forms of pyroclastic deposit are the ignimbrites, deposits formed by the high-temperature gas and ash mix of a pyroclastic flow event.

Three modes of transport can be distinguished: pyroclastic flow, pyroclastic surge, and pyroclastic fall. During Plinian eruptions, pumice and ash are formed when silicic magma is fragmented in the volcanic conduit, because of decompression and the growth of bubbles. Pyroclasts are then entrained in a buoyant eruption plume which can rise several kilometers into the air and cause aviation hazards. Particles falling from the eruption clouds form layers on the ground (this is pyroclastic fall or tephra). Pyroclastic density currents, which are referred to as 'flows' or 'surges' depending on particle concentration and the level turbulence, are sometimes called glowing avalanches. The deposits of pumice-rich pyroclastic flows can be called ignimbrites.

A pyroclastic eruption entails spitting or "fountaining" lava, where the lava will be thrown into the air along with ash, pyroclastic materials, and other volcanic byproducts. Hawaiian eruptions such as those at Kilauea can eject clots of magma suspended into gas; this is called a 'fire fountain'. The magma clots, if hot enough may coalesce upon landing to form a lava flow.

Pyroclastic deposits consist of pyroclasts which are not cemented together. Pyroclastic rocks (tuff) are pyroclastic deposits which have been lithified.










Plutons

A pluton in geology is an intrusive igneous rock body that crystallized from a magma below the surface of the Earth. Plutons include batholiths, dikes, sills, laccoliths, lopoliths, and other igneous bodies. In practice, "pluton" usually refers to a distinctive mass of igneous rock, typically kilometers in dimension, without a tabular shape like those of dikes and sills. Batholiths commonly are aggregations of plutons. The most common rock types in plutons are granite, granodiorite, tonalite, and quartz diorite.

The term originated from Pluto, the ancient Roman god of the underworld. The use of the name and concept goes back to the beginnings of the science of geology in the late 1700s and the then hotly debated theories of Neptunism, Vulcanism and Plutonism regarding the origin of basalt.

Intrusive rocks exist in a wide range of forms from mountain range sized batholiths to thin vein-like fracture fillings of aplite. Intrusive structures are often classified according to whether or not they are parallel to the bedding planes or foliation of the country rock: if the intrusion is parallel, the body is concordant, while if it cuts across the country rock, it is discordant.

Structural types include:
1) batholith: large irregular intrusions.
2) stock: smaller irregular discordant intrusions
3) dike: a relatively narrow tabular discordant body, often with near-vertical attitude.
4) sill: a relatively thin tabular concordant body intruded along bedding planes
5)  laccolith: concordant body with essentially flat base and dome shaped upper surface, usually has a feeder pipe below.

Batholith

A batholith (from Greek bathos, depth + lithos, rock) is a large emplacement of igneous intrusive (also called plutonic) rock that forms from cooled magma deep in the Earth's crust. Batholiths are almost always made mostly of felsic or intermediate rock-types, such as granite, quartz monzonite, or diorite (see also granite dome).
Although they may appear uniform, batholiths are in fact structures with complex histories and compositions. They are composed of multiple masses, or plutons, bodies of igneous rock of irregular dimensions (typically at least several kilometers) that can be distinguished from adjacent igneous rock by some combination of criteria including age, composition, texture, or mappable structures. Individual plutons are crystallized from magma that traveled toward the surface from a zone of partial melting near the base of the Earth's crust.

Traditionally, these plutons have been considered to form by ascent of relatively buoyant magma in large masses called plutonic diapirs. Because the diapirs are liquefied and very hot, they tend to rise through the surrounding country rock, pushing it aside and partially melting it. Most diapirs do not reach the surface to form volcanoes, but instead slow down, cool and usually solidify 5 to 30 kilometers underground as plutons (hence the use of the word pluton; in reference to the Roman god of the underworld Pluto). It has also been proposed that plutons commonly are formed not by diapiric ascent of large magma diapirs, but rather by aggregation of smaller volumes of magma that ascended as dikes.

A batholith is formed when many plutons converge together to form a huge expanse of granitic rock. Some batholiths are mammoth, paralleling past and present subduction zones and other heat sources for hundreds of kilometers in continental crust. One such batholith is the Sierra Nevada Batholith, which is a continuous granitic formation that forms much of the Sierra Nevada in California. An even larger batholith, found predominantly in the Coast Mountains of western Canada, extends for 1,800 kilometers and reaches into southeastern Alaska, which is called the Coast Plutonic Complex.

There is also an important geographic usage of the term batholith. For a geographer, a batholith is an exposed area of mostly continuous plutonic rock that covers an area larger than 100 square kilometers. Areas smaller than 100 kilometers are called stocks. However, the majority of batholiths visible at the surface (via outcroppings) have areas far greater than 100 square kilometers. These areas are exposed to the surface through the process of erosion accelerated by continental uplift acting over many tens of millions to hundreds of millions of years. This process has removed several tens of kilometers of overlying rock in many areas, exposing the once deeply buried batholiths.

Batholiths exposed at the surface are also subjected to huge pressure differences between their former homes deep in the earth and their new homes at or near the surface. As a result, their crystal structure expands slightly and over time. This manifests itself by a form of mass wasting called exfoliation. This form of erosion causes convex and relatively thin sheets of rock to slough off the exposed surfaces of batholiths (a process accelerated by frost wedging). The result is fairly clean and rounded rock faces. A famous example of the result of this process is Half Dome, which located in the world-famous Yosemite Valley


Dikes

A dike or dyke in geology refers to an intrusive igneous body. The thickness is usually much smaller than the other two dimensions. Thickness can vary from sub-centimeter scale to many meters in thickness and the lateral dimensions can extend over many kilometers. A dike is an intrusion into a cross-cutting fissure, meaning a dike cuts across other pre-existing layers or bodies of rock, this means that a dike is always younger than the rocks that contain it. Dikes are usually high angle to near vertical in orientation, but subsequent tectonic deformation may rotate the sequence of strata through which the dike lies so that the latter becomes horizontal. Near horizontal or conformable intrusions along bedding planes between strata are called intrusive sills. The world's largest dike swarm is the Mackenzie dyke swarm in the Northwest Territories, Canada.[

Dikes often form as either radial or concentric swarms around plutonic intrusives or around volcanic necks or feeder vents in volcanic cones. These are known as "Ring Dike"s.

Dikes can vary in texture and composition from diabase or basaltic to granitic or rhyolitic. Pegmatite dikes are extremely coarsely crystalline granitic rocks often associated with late stage granite intrusions or metamorphic segregations. Aplite dikes are fine grained or sugary textured intrusives of granitic composition.

Sills

In geology, a sill is a tabular pluton that has intruded between older layers of sedimentary rock, beds of volcanic lava or tuff, or even along the direction of foliation in metamorphic rock. The term sill is synonymous with concordant intrusive sheet. This means that the sill does not cut across preexisting rocks, in contrast to dikes, which do cut across older rocks.

Sills are always parallel to beds (layers) of the surrounding country rock. Usually they are in a horizontal orientation, although tectonic processes can cause rotation of sills into near vertical orientations. They can be confused with solidified lava flows; however there are several differences between them. Intruded sills will show partial melting of and incorporation of the surrounding country rock. On both the "upper" and "lower" contact surfaces of the country rock into which the sill has intruded, evidence of heating will be observed (contact metamorphism). Lava flows will show this evidence only on the lower side of the flow. In addition, lava flows will typically show evidence of vesicles (bubbles) where gases escaped into the atmosphere. Because sills generally form at depth (up to many kilometers), the pressure of overlying rock prevents this from happening much, if at all. Lava flows will also typically show evidence of weathering on their upper surface, whereas sills, if still covered by country rock, typically do not.

Laccolith

A laccolith is an igneous intrusion (or concordant pluton) that has been injected between two layers of sedimentary rock. The pressure of the magma is high enough that the overlying strata are forced upward, giving the laccolith a dome or mushroom-like form with a generally planar base.

Laccoliths tend to form at relatively shallow depths and are typically formed by relatively viscous magmas, such as those that crystallize to diorite, granodiorite, and granite. Cooling underground takes place slowly, giving time for larger crystals to form in the cooling magma. The surface rock above laccoliths often erodes away completely, leaving the core mound of igneous rock. The term was first applied as laccolite by Grove Karl Gilbert after his study of intrusions of diorite in the Henry Mountains of Utah in about 1875.