CHAPTER EIGHT
Geochronology

 Geologic Time Scale

The geological time scale is used by geologists and other scientists to describe the timing and relationships between events that have occurred during the history of Earth.

Evidence from radiometric dating indicates that the Earth is about 4.570 billion years old. The geological or deep time of Earth's past has been organized into various units according to events which took place in each period. Different spans of time on the time scale are usually delimited by major geological or paleontological events, such as mass extinctions. For example, the boundary between the Cretaceous period and the Paleogene period is defined by the extinction event, known as the Cretaceous-Tertiary extinction event, that marked the demise of the dinosaurs and of many marine species. Older periods which predate the reliable fossil record are defined by absolute age.

Terminology

The largest defined unit of time is the Eon. The four eons beginning with the oldest are Hadean, Archean, Proterozoic, and Phanerozoic. The first three are commonly combined in a "supereon" called the Precambrian. Eons are divided into Eras, which are in turn divided into Periods, Epochs and Stages. At the same time paleontologists define a system of faunal stages, of varying lengths, based on changes in the observed fossil assemblages. In many cases, such faunal stages have been adopted in building the geological nomenclature, though in general there are far more recognized faunal stages than defined geological time units.

Geologists tend to talk in terms of Upper/Late, Lower/Early and Middle parts of periods and other units , such as "Upper Jurassic", and "Middle Cambrian". Upper, Middle, and Lower are terms applied to the rocks themselves, as in "Upper Jurassic sandstone," while Late, Middle, and Early are applied to time, as in "Early Jurassic deposition" or "fossils of Early Jurassic age." The adjectives are capitalized when the subdivision is formally recognized, and lower case when not Because geologic units occurring at the same time but from different parts of the world can often look different and contain different fossils, there are many examples where the same period was historically given different names in different locales. A key aspect of the work of the International Commission on Stratigraphy is to reconcile this conflicting terminology and define universal horizons that can be used around the world.

Match left with right:

___Permian			A. Eon
___Phanerozoic			B. Era
___Proterozoic			C. Period
___Cambrian
___Carboniferous
___Jurassic
___Mesozoic
___Paleogene
___Cretaceous
___Triassic
___Devonian
___Ordovician
___Cenozoic
___Silurian
___Neogene


History of the time scale

The principles underlying geologic (geological) time scales were laid down by Nicholas Steno in the late 17th century. Steno argued that rock layers (or strata) are laid down in succession, and that each represents a "slice" of time. He also formulated the principle of superposition, which states that any given stratum is probably older than those above it and younger than those below it. While Steno's principles were simple, applying them to real rocks proved complex. Over the course of the 18th century geologists realized that:

1) Sequences of strata were often eroded, distorted, tilted, or even inverted after deposition;
2) Strata laid down at the same time in different areas could have entirely different appearances;
3) The strata of any given area represented only part of the Earth's long history.

The first serious attempts to formulate a geological time scale that could be applied anywhere on Earth took place in the late 18th century. The most influential of those early attempts (championed by Abraham Werner, among others) divided the rocks of the Earth's crust into four types: Primary, Secondary, Tertiary, and Quaternary. Each type of rock, according to the theory, formed during a specific period in Earth history.

In opposition to the then-popular Neptunist theories expounded by Werner (that all rocks had precipitated out of a single enormous flood), a major shift in thinking came with the reading by James Hutton of his Theory of the Earth. What Hutton proposed was that the interior of the Earth was hot, and that this heat was the engine which drove the creation of new rock: land was eroded by air and water and deposited as layers in the sea; heat then consolidated the sediment into stone, and uplifted it into new lands. This theory was dubbed "Plutonist" in contrast to the flood-oriented theory.

The identification of strata by the fossils they contained, pioneered by William Smith and Georges Cuvier, in the early 19th century, enabled geologists to divide Earth history more precisely. It also enabled them to correlate strata across national (or even continental) boundaries. If two strata (however distant in space or different in composition) contained the same fossils, chances were good that they had been laid down at the same time. Detailed studies between 1820 and 1850 of the strata and fossils of Europe produced the sequence of geological periods still used today.

The process was dominated by British geologists, and the names of the periods reflect that dominance. The "Cambrian," and the "Ordovician," and "Silurian",named after ancient Welsh tribes, were periods defined using stratigraphic sequences from Wales. The "Devonian" was named for the English county of Devon, and the name "Carboniferous" was simply an adaptation of "the Coal Measures," the old British geologists' term for the same set of strata. The "Permian" was named after Perm, Russia, because it was defined using strata in that region by a Scottish geologis. However, some periods were defined by geologists from other countries. The "Triassic" was named in 1834 by a German geologist Friedrich Von Alberti from the three distinct layers -red beds, capped by chalk, followed by black shales- that are found throughout Germany and Northwest Europe, called the 'Trias'. The "Jurassic" was named by a French geologist for the extensive marine limestone exposures of the Jura Mountains. The "Cretaceous"  as a separate period was first defined by a Belgian geologist using strata in the Paris basin and named for the extensive beds of chalk.

British geologists were also responsible for the grouping of periods into Eras and the subdivision of the Tertiary and Quaternary periods into epochs.

When William Smith and Sir Charles Lyell first recognized that rock strata represented successive time periods, time scales could be estimated only very imprecisely since various kinds of rates of change used in estimation were highly variable. While creationists had been proposing dates of around six or seven thousand years for the age of the Earth based on the Bible, early geologists were suggesting millions of years for geologic periods with some even suggesting a virtually infinite age for the Earth. Geologists and paleontologists constructed the geologic table based on the relative positions of different strata and fossils, and estimated the time scales based on studying rates of various kinds of weathering, erosion, sedimentation, and lithification. Until the discovery of radioactivity in 1896 and the development of its geological applications through radiometric dating during the first half of the 20th century which allowed for more precise absolute dating of rocks, the ages of various rock strata and the age of the Earth were the subject of considerable debate.


Write down the names of the geological periods (youngest first) for each of the following eras:

Cenozoic		______________
			______________

Mesozoic		______________
			______________
			______________

Paleozoic		______________
			______________
			______________
			______________
			______________
			______________







Match left with right

___cross-cutting relation		A. Steno
___original horizontality		B. Wm Smith
___uniformitarianism		C. James Hutton
___lateral continuity
___fossil succesion
___superposition

Relative dating

Before the advent of absolute dating in the 20th century, archaeologists and geologists were largely limited to the use of Relative Dating techniques. It estimates the order of prehistoric and geological events were determined by using basic stratigraphic rules, and by observing where fossil organisms lay in the geological record, stratified bands of rocks present throughout the world.

Though relative dating can determine the order in which a series of events occurred, not when they occurred, it is in no way inferior to radiometric dating; in fact, relative dating by biostratigraphy is the preferred method in paleontology, and is in some respects more accurate.

The principles of Relative dating use a combination of fossil study and structural interpretation to learn about the geological history of an area. The rules of relative dating for continuous stratigraphic sequences were worked out long ago, by scientists such as Nicolas Steno (1638-86):

The Law of superposition states that in any undisturbed geologic sequence, older beds lie below younger beds.

The principle of original horizontality states that whether rock beds are deformed or not, they were originally deposited on a horizontal surface.

The principle of lateral continuity states that a given rock bed will extend horizontally for some distance, until it thins out or reaches the edge of a depositional basin.

The principle of cross-cutting relationships states that any geologic feature that off-sets or intersects a series of beds (such as a fault, dike, or sill) must be younger than the beds it cuts across.

The principle of intrusion states that if a body of rock contains intrusions of another type of rock, it must be younger than the source rock.

Principle of fossil succession states that if an index fossil of a known age is found in a rock formation, that formation is of the same approximate age as the index fossil.

These principles are highly useful in interpreting geologic history.


For instance, an angular unconformity such as shown above with horizontal beds above it indicates a sequence in which:
1) The original strata were deposited horizontally.
2) The strata were tilted and deformed.
3) Much of the strata were eroded away.
4) Deposition later began anew above the unconformity.

Unconformities

An unconformity is a buried erosion surface or surface of nondeposition separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval of time before deposition of the younger, but the term is used to describe any break in the sedimentary geologic record. The phenomenon of angular unconformities was discovered by James Hutton.

The rocks above an unconformity are younger than the rocks beneath (unless the sequence has been overturned). An unconformity represents time during which no sediments were deposited and the local record for that time interval is missing and geologists must use other clues to discover that part of the geologic history of that area. The interval of geologic time not represented is called a hiatus. There are four types of unconformities: disconformity, nonconformity, angular unconformity and paraconformity.

A disconformity is an unconformity between parallel layers of sedimentary rocks which represents a period of erosion or non-deposition. A paraconformity is a type of disconformity in which the separation is a simple bedding plane; i.e., there is no obvious buried erosional surface.

A nonconformity exists between sedimentary rocks and crystalline rocks when the sedimentary rock lies above and was deposited on the pre-existing and eroded metamorphic or igneous rock.


An angular unconformity is an unconformity where horizontally parallel strata of sedimentary rock are deposited on tilted and eroded layers that may be either vertical or at an angle to the overlying horizontal layers. The whole sequence may later be deformed and tilted by further orogenic activity.

Determine the type of unconformity being described in each of the following geologic histories:

Deposition, folding, erosion, deposition

Intrusion, erosion, deposition

Metamorphism, erosion, deposition

Deposition, erosion, deposition

Deposition, weathering, deposition

Deposition, no deposition, deposition


Biostratigraphy

Biostratigraphic methods are usually used in tandem with structural ones. For instance, the principle of faunal succession was probably the most important factor behind the elaboration of the geologic time scale, which was more or less complete long before an absolute time scale was available. Beds with a particular fauna can be correlated with others that share it (often globally), and also distinguished from upper and lower beds without them.

Rock units that contain a distinct assemblage of fossils are biostratigraphic units, and are based on the "range", or vertical interval in which a taxon is found.
A taxon is a name designating an organism or group of organisms.  A zone, or biozone is the most basic biostratigraphic unit, one bound on its upper and lower boundaries by the ranges of given species; these can be zones where certain species coexist, or which are defined by the earliest appearance or latest disappearance of taxa in neighboring zones.

Index fossils are invaluable for biostratigraphy. The best index fossils are:
1) Abundant.
2) Distinct from other flora/fauna.
3) Geographically widespread.
4) Found in many kinds of rocks.
5) Narrow in stratigraphic range.

A name designating an organism or group of organisms.


Absolute Dating

Radiometric dating is a technique used to date materials, based on a comparison between the observed abundance of particular naturally occurring radioactive isotopes and their known decay rates. It is the principal source of information about the absolute age of rocks and other geological features, including the age of the Earth itself. Among the best-known techniques are potassium-argon dating and uranium-lead dating. By allowing the establishment of geological timescales, it provides a significant source of information about the dates of fossils and the deduced rates of evolutionary change.

The different techniques of radiometric dating vary in the timescale over which they are accurate, and their use may also be limited by cost.

All ordinary matter is made up of combinations of chemical elements, each with its own atomic number, indicating the number of protons in the atomic nucleus. Additionally, elements may exist in different isotopes, with each isotope of an element differing only in the number of neutrons in the nucleus. A particular isotope of a particular element is called a nuclide. Some nuclides are inherently unstable. That is, at some random point in time, an atom of such a nuclide will be transformed into a different nuclide by the process known as radioactive decay. This transformation is accomplished by the emission of particles such as electrons (known as beta decay) or alpha particles.

While the moment in time at which a particular nucleus decays is random, a collection of atoms of a radioactive nuclide decays exponentially at a rate described by a parameter known as the half-life, usually given in units of years when discussing dating techniques. After one half-life has elapsed, one half of the atoms of the substance in question will have decayed. Many radioactive substances decay from one nuclide into a final, stable decay product (or "daughter") through a series of steps known as a decay chain. In this case, usually the half-life reported is the dominant (longest) for the entire chain, rather than just one step in the chain. Nuclides useful for radiometric dating have half-lives ranging from a few thousand to a few billion years.

In most cases, the half-life of a nuclide depends solely on its nuclear properties; it is not affected by external factors such as temperature, chemical environment, or presence of a magnetic or electric field. Therefore, in any material containing a radioactive nuclide, the proportion of the original nuclide to its decay product(s) changes in a predictable way as the original nuclide decays. This predictability allows the relative abundances of related nuclides to be used as a clock that measures the time from the incorporation of the original nuclide(s) into a material to the present.

Considering that radioactive parent elements decay to stable daughter elements, the mathematical expression that relates radioactive decay to geologic time, called the age equation, is:


where
t = age of the sample
D = number of atoms of the daughter isotope in the sample
P = number of atoms of the parent isotope in the sample
? = decay constant of the parent isotope
ln = natural logarithm

The precision of a dating method depends in part on the half-life of the radioactive isotope involved. For instance, carbon-14 has a half-life of about 6000 years as it decays to nitrogen-14. After an organism has been dead for 60,000 years, so little carbon-14 is left in it that accurate dating becomes impossible. On the other hand, the concentration of carbon-14 falls off so steeply that the age of relatively young remains can be determined precisely to within a few decades. The isotope used in uranium-thorium dating has a longer half-life, but other factors make it more accurate than radiocarbon dating.

The uranium-lead radiometric dating scheme is one of the oldest available, as well as one of the most highly respected. It has been refined to the point that the error in dates of rocks about three billion years old is no more than two million years.

One of its great advantages is that any sample provides two clocks, one based on uranium-235's decay to lead-207 with a half-life of about 700 million years, and one based on uranium-238's decay to lead-206 with a half-life of about 4.5 billion years, providing a built-in crosscheck that allows accurate determination of the age of the sample even if some of the lead has been lost.

Two other radiometric techniques are used for long-term dating. Potassium-argon dating involves electron capture or positron decay of potassium-40 to argon-40. Potassium-40 has a half-life of 1.3 billion years, and so this method is applicable to the oldest rocks. Radioactive potassium-40 is common in micas, feldspars, and  hornblendes, though the blocking temperature is fairly low in these materials, about 125�C (mica) to 450�C (hornblende).

Rubidium-strontium dating is based on the beta decay of rubidium-87 to strontium-87, with a half-life of 50 billion years. This scheme is used to date old igneous and metamorphic rocks, and has also been used to date lunar samples. Blocking temperatures are so high that they are not a concern. Rubidium-strontium dating is not as precise as the uranium-lead method, with errors of 30 to 50 million years for a 3-billion-year-old sample.

Match radioactive parent  to daughter product

___Potassium			A. lead
___Rubidium			B. strontium
___Uranium			C. argon
___Thorium			D. nitrogen
___Carbon

Match left with right:

___mica			A. absolute dating
___dike			B. relative dating
___fault
___strata
___fossils
___zircon
___charcoal
___hornblende