Stratigraphy

Stratigraphy
stratigraphy, in geology, the study of rock layers, or strata, particularly their ages, compositions, and relationships to other rock layers. stratigraphy provides geologists with clues about the earth`s past. stratigraphy also allows geologists to predict what types of rocks lie below the ground and to understand geologic processes. consequently, most geologists regularly use basic elements of stratigraphy in their work.

Principles of stratigraphy
Stratigraphy relies on four simple principles
Stratigraphy relies on four simple principles to unveil geologic history. the principle of original horizontality states that the sediments that form sedimentary rocks are usually deposited in approximately horizontal sheets. this principle is invaluable for reconstructing the earth`s history because it allows geologists to distinguish between undisturbed and deformed rock layers. the term deformation generally refers to faults ( fractures where one side has moved relative to the other ) or folds that form in rocks, typically during mountain-building events. for example, rocks 250 million years old near the top of the grand canyon lie in horizontal sheets ; similar rocks near las vegas, nevada, are tilted on end. therefore, geologists infer that rock layers in the grand canyon have not been strongly deformed in the 250 million years since they were deposited. by contrast, the las vegas area has experienced strong deformation since that time.


The principle of superposition states
The principle of superposition states that, in an undeformed sequence of strata, younger strata lie on top of older strata. strata are sheets of sedimentary rock that, as a group, are visibly distinct from those above or below ( see bed ). this principle results from the simple observation that new sediment settles on top of previously deposited sediment. consequently, strata are deposited sequentially, sheet after sheet. by using this principle, geologists can reconstruct geologic history as recorded in the sedimentary rocks. for example, where marine-deposited rock lies above river-deposited rock, geologists reason that the site evolved from a river environment to a marine environment during the period when the sediments were deposited. either the land sank, or the sea rose.

The principle of cross-cutting relations
The principle of cross-cutting relations states that if a layer of rock is cut, for example, by a fault or an intruding rock, then the cut rock must be older than the event or intrusion that cut it. for instance, if a granite intrusion cuts across a layer of sandstone, then the sandstone must be older than the granite intrusion.

The principle of faunal and floral succession
The principle of faunal and floral succession states that because animals and plants evolve into new species, sedimentary rocks of different ages will contain fossils of different species. knowing the age of a fossil helps to date the rock in which it is found. index fossils, which form from species that only exist for a short time, are especially valuable in determining a rock`s age. two rocks from different locations containing the same index fossil must be approximately the same age. this principle is especially useful because it allows geologists to show that different rock layers from different areas were deposited at roughly the same time.

These four principles of stratigraphy can
these four principles of stratigraphy can be used to unravel the geologic history of a given area. geologists may study a cliff face, for example, having tilted sedimentary rocks at its bottom that were originally deposited in a shallow sea. granite intrusions have cut through the sedimentary rocks. both the sedimentary rocks and the granite intrusions are faulted. other sedimentary rocks, which are horizontal and were originally deposited in a riverbed, lie on top of the tilted and faulted rocks. no faults or intrusions have cut the horizontal rocks.

Applying the principles of stratigraphy
Applying the principles of stratigraphy, the oldest feature must be the tilted marine sedimentary rocks because, according to the principle of superposition, they lie beneath the younger horizontal sedimentary rocks, and, using the principle of cross-cutting relations, they are cut by the younger granite intrusions and faults. the second oldest feature must be the granite intrusions because, using the principle of cross-cutting relations, they are cut by the younger faults, and, applying the principle of superposition, they lie below the younger horizontal sedimentary rocks. the third oldest feature must be the faults because, using the principle of cross-cutting relations, they fail to cut the younger upper rocks. finally, the youngest feature is the flat-lying sedimentary rock on top.

Given this chronology
Given this chronology, the geologic history of the cliff face is as follows. initially, the region was a shallow sea in which sediments accumulated. additional sediments ( now gone ) accumulated over them, providing conditions that turned them into sedimentary rock ( see rock cycle ). granite then intruded these rocks. subsequently, a period of uplift tilted and faulted the rocks. a period of erosion removed the overlying sediment. after erosion, a river system established itself and deposited the younger sediments. these sediments were eventually buried and turned into rock.
relative versus absolute ages

The above example gives only the sequence
The above example gives only the sequence of geologic events for a particular place ; it provides no information as to how long ago the events occurred. the example illustrates the use of relative ages by showing the occurrence of events with respect to each other. in contrast, absolute ages specify, in years, when a rock formed. absolute ages rely on a variety of geologic dating methods. the most important technique, radiometric dating, involves measuring minute amounts of radioactive elements in a rock to calculate the age of the rock.

When combined
When combined, absolute and relative ages give a great deal of information. for example, if the granite intrusion in the cliff face had an absolute age of 20 million years, then the tilted sedimentary rocks must be older than 20 million years. furthermore, the faulting and the horizontal sedimentary rocks must be younger than 20 million years.



By using geologic information gleaned from
By using geologic information gleaned from rocks in other locations, geologists might be able to draw more conclusions about the ages of different rocks or events. for example, if some of the horizontal rocks contained fossils known from other rocks to have an absolute age of 10 million years, then this information would allow geologists to bracket the age of the faulting to between 20 and 10 million years ago.

Historic importance of stratigraphy

Prior to the advent of stratigraphy
Prior to the advent of stratigraphy, the earth was generally considered to be significantly younger than a million years. in fact, the dominant estimate, by irish archbishop james ussher in the 17th century, held that the earth was younger than 6000 years. ussher`s estimate was based on his chronology of the old testament.

Stratigraphy developed in england during
Stratigraphy developed in england during the early 1800s with the work of a land surveyor named william smith. using stratigraphic concepts, 19th-century geologists clearly demonstrated that the earth was far older than a million years. for example, sedimentation rates of marine limestone not deposited on reefs typically range from about 2 to 20 cm ( about 0. 8 to 8 in ) per thousand years. at these rates, about 6000 m ( 20, 000 ft ) of marine limestone exposed as tilted beds in southern nevada required at least 30 to 300 million years of continuous deposition. additionally, these rocks overlie rock units that show a complex history of metamorphism, intrusion, faulting, and uplift prior to deposition of the limestone.

The knowledge that the earth is ancient
The knowledge that the earth is ancient rather than young has several important implications for our society. from a historical perspective, this recognition led, in the 19th century, to a broadening of the scientific mind beyond the confines of religious dogma and opened the way for darwin`s theory of natural selection ( see darwin, charles : theory of natural selection. it also demonstrated that humanity has only existed for a small period of the earth`s history. an ancient earth points to the urgency for resource conservation. because natural resources form on a geologic time scale, the rate at which they are naturally replenished is far slower than the rate at which they are currently being depleted. finally, realizing the earth`s age makes habitat preservation and other environmental issues particularly relevant. like resource depletion, environmental degradation occurs on a human time scale, but generally requires a geologic time scale for repair.