How does stratigraphic dating work

Stratigraphy Please include a link to this page if you have found this material useful for research or writing a related article. Content on this website is from high-quality, licensed material originally published in print form. You can always be sure you're reading unbiased, factual, and accurate information. Paste the link into your website, email, or any other HTML document.

Your email address will be altered so spam harvesting bots can't read it easily. Hide my email completely instead? Additional Topics Dating Techniques - Stratigraphy. Dating Techniques - Seriation Seriation is the ordering of objects according to their age. Dating Techniques - Faunal Dating. Dating Techniques - Pollen Dating palynology Each year seed-bearing plants release large numbers of pollen grains. Dating Techniques - Amino Acid Racimization This dating technique was first conducted by Hare and Mitterer in , and was popular in the s.

Dating Techniques - Cation-ratio Dating Although cation-ratio dating has been widely used, recent studies suggest it has many problems. Dating Techniques - Thermoluminescence Dating Thermoluminescence dating is very useful for determining the age of pottery. Dating Techniques - Tree-ring Dating.

Dating Techniques - Potassium-argon Dating. Dating Techniques - Radiocarbon Dating. Dating Techniques - Uranium Series Dating Uranium series dating techniques rely on the fact that radioactive uranium and thorium isotopes decay into a series of unstable, radioactive "daughter" isotopes; this process continues until a stable non-radioactive lead isotope is formed. Dating Techniques - Fission Track Dating. Citing this material Please include a link to this page if you have found this material useful for research or writing a related article.

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Name Email Your email address will be altered so spam harvesting bots can't read it easily. These eras, in turn, are divided into 11 periods, or systems, whose names except for Tertiary and Quaternary refer to the locations in which the respective stratigraphic systems were first observed. The names of these systems, along with their dates in millions of years before the present and the origin of their names, are as follows from the most distant to the most recent:. Within the more recent Cenozoic era , or era them, names of epochs or "series" in stratigraphic terminology become important.

They are all derived from Greek words, whose meanings are given below:. The geologist studying the stratigraphic record is a sort of detective, looking for clues. Just as detectives have their methods for solving crimes, geologists rely on correlation, or methods of establishing age relationships between various strata. There are two basic types of correlation: Actually, chronostratigraphic work is very similar some of the toughest cases confronted by police detectives, because more often than not the geologic detective has little evidence on which to operate.

First of all, as noted earlier, only sedimentary rock can be used in making such determinations: Even when the rock is sedimentary, there is still plenty of room for error. The layers may be many feet or less than an inch deep, and it is up to the geologist to determine whether the stratum has been affected by such geologic forces as erosion. If erosion has occurred, it can cause a disturbance, or unconformity discussed later , which tends to render inaccurate any reading of the stratigraphic record.

Another possible source of disturbance is an earthquake, which could cause one part of Earth's crust to shift over an adjacent section, making the stratigraphic record difficult, if not impossible, to read. Under the best of conditions, after all, the strata are hardly neat, easily defined lines.

If one observes a horizontal section, there is likely to be a change in thickness, because as the stratum extends outward, it merges with the edges of adjacent deposits. Yet another potential pitfall in stratigraphic correlation involves one of the most useful tools available to a geologist attempting to find an absolute age for the materials he or she is studying: Though this method can provide accurate absolute dates, it is quite possible that the age thus determined will be the age of the parent rock from which a sample is taken, not the age of the sample itself.

The grains of sand in a piece of sandstone, for instance, are much older than the larger unit of sandstone, and for this reason, radiometric dating is useful only in specific circumstances. Given all these challenges, it is a wonder that geologists manage to correlate strata successfully, yet they do. Physical correlations are achieved on the basis of several criteria, including color, the size of grains, and the varieties of minerals found within a stratum. By such means, it is sometimes possible to correlate widely separated strata.

Particularly impressive feats of correlation can result from the study of fossils, whose stratigraphic implications, as we have noted, were first discovered by William Smith. Smith hit upon the idea of biostratigraphy while excavating land for a set of canals near London. As he discovered, any given stratum contains the same types of fossils, and strata in two different areas thus can be correlated. Long before his countryman Charles Darwin developed the theory of evolution, Smith conceived his own law of faunal succession , which hints at the idea that species developed and disappeared over given phases in Earth's past.

Stratigraphy (archaeology)

According to the law of faunal succession , all samples of any given fossil species were deposited on Earth, regardless of location, at more or less the same time. As a result, if a geologist finds a stratum in one area that contains a particular fossil and another in a distant area containing the same fossil, it is possible to conclude that the strata are the same. In discussing the many challenges facing a geologist studying stratigraphic data, the role of erosion was noted. Let us return to that subject, because erosion is a source of what are known as unconformities, or gaps in the rock record.

Unconformities are of three types: Angular unconformities involve a tilting of the layers, such that an upper layer does not lie perfectly parallel to a lower one. Disconformities are more deceptive, because the layers are parallel, yet there is still an unconformity between them, and only a study of the fossil record can reveal the unconformity. Finally, a nonconformity arises when sedimentary rocks are divided from a type of igneous rock known as intrusive meaning "cooled within Earth". Angular unconformities emerge as a by-product of the dramatic shifts and collisions that take place in plate tectonics see Plate Tectonics.

Sediment accumulates and then, as a result of plate movement, is moved about and eventually experiences weathering and erosion. Layers are tilted and then flattened by more erosion, and as the solid earth rises or sinks, they are shifted further. Such is the case, for instance, along the Colorado River at the Grand Canyon , where angular unconformities reveal a series of movements over the years. Another famous angular unconformity can be found at Siccar Point in Scotland , where nearly horizontal deposits of sandstone rest atop nearly vertical ones of graywacke, another sedimentary rock.

Observations of this unconformity led the great geologist James Hutton to the realization that Earth is much, much older than the 6, years claimed by theologians in his day see Historical Geology. Cambridge Guide to Minerals, Rocks, and Fossils. Cambridge University Press, Boggy's Links to Stratigraphy and Geochronology Web site.

The Incredible Journey to the Beginning of Time. Peter Bedrick Books, Lamb, Simon, and David Sington. The Shaping of Our World. Princeton University Press, Cosmos, Earth, and Mankind. Spickert, Diane Nelson, and Marianne D. A Rock's Journey Through Time. University of Georgia Stratigraphy Lab Web site. The absolute age of a geologic phenomenon is its age in Earthyears.

Compare with relative age. An area of stratigraphy involving the study of fossilized plants and animals in order to establish dates for and correlations between stratigraphic layers. A subdiscipline of stratigraphy devoted to studying the relative ages of rocks. A method of establishing age relationships between various rock strata.

Any effort directed toward finding the age of a particular item or phenomenon. Methods of geologic dating are either relative i. The latter, based on such methods as the study of radioactive isotopes, usually is given in terms of actual years or millions of years. The longest phase of geologic time, equivalent to an eonothem in the stratigraphic time scale. Earth's history has consisted of four eons, the Hadean or Priscoan, Archaean, Proterozoic, and Phanerozoic.

The next-smallest subdivision of geologic time is the era. The fourth-longest phase of geologic time, shorter than an era and longer than an age and a chron. An epoch is equivalent to a series in the stratigraphictime scale. The current epoch is the Holocene, which began about 0. The second-longest phase of geologic time, after an eon, and equivalent to an era them in the stratigraphic time scale.

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Equally as important as the contents of the layers themselves, however, are the borders between them. This approach helps to order events chronologically but it does not provide the absolute age of an object expressed in years. If we know the date of context 1 and context 9 we can deduce that context 7, the backfilling of pit 8, occurred sometime after the date for 9 but before the date for 1, and if we recover an assemblage of artifacts from context 7 that occur nowhere else in the sequence, we have isolated them with a reasonable degree of certainty to a discrete range of time. Patterns in the very layering of sediments, such as ripple marks and flumes, can introduce discontinuities. Dating Techniques - Amino Acid Racimization This dating technique was first conducted by Hare and Mitterer in , and was popular in the s. Dating Techniques - Cation-ratio Dating Although cation-ratio dating has been widely used, recent studies suggest it has many problems.

The current eon, the Phanerozoic, has had three eras, the Paleozoic, Mesozoic, and Cenozoic, which is the current era. The next-smallest subdivision of geologic time is the period. The movement of soil and rock due to forces produced by water, wind, glaciers, gravity, and other influences. An area of stratigraphy devoted to determining absolute dates and time intervals. A map showing the rocks beneath Earth's surface, including their distribution according to type as well as their ages, relationships, and structural features. The vast stretch of time over which Earth's geologic development has occurred.

This span about 4. Much smaller still is the span of human civilization, only about 5, years. The study of Earth's physical history. Historical geology is one of two principal branches of geology, the other being physical geology. Atoms that have an equal number of protons, and hence are of the same element, but differ in their number of neutrons. This results in a difference ofmass. An isotope may be either stable or radioactive.

Stratigraphic Dating

The principle that all samples of any given fossil species were deposited on Earth, regardless of location, at more or less the same time. This makes it possible to correlate widely separated strata. Theprinciple that strata are deposited in a sequence such that the deeper the layer, the older the rock. This is applicable only or sedimentary rock, as opposed to igneous or metamorphic rock. An area of stratigraphy devoted to the study and description but not the dating of rock layers. An abbreviation used by earth scientists, meaning "million years" or "megayears.

The third-longest phase of geologic time, after an era; it is equivalent to a system in the stratigraphic time scale. The current eon, the Phanerozoic, has had 11 periods, and the current era, the Cenozoic, has consisted of three periods, of which the most recent is the Quaternary. The next-smallest subdivision of geologic time is the epoch. A term that refers to the first three of four eons in Earth's history, which lasted from about4, Ma to about Ma ago.

A term describing a phenomenon whereby certain materials are subject to a form of decay brought about by the emission of high-energy particles or radiation. Forms of particles or energy include alpha particles positively charged helium nuclei , beta particles either electrons or subatomic particles called positrons , or gamma rays , which occupy the highest energy level in the electromagnetic spectrum.

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A method of absolute dating using ratios between "parent" isotopes and "daughter" isotopes, which are formed by the radioactive decay of parent isotopes. The relative age of a geologic phenomenon is its age compared with the ages of other geologic phenomena, particularly the stratigraphic record of rock layers.

Compare with absolute age.

Material deposited at or near Earth's surface from a number of sources, most notably preexisting rock. Rock formed by compression and deposition i. Sedimentary rock is one of the three major types of rock, along with igneous and metamorphic. The succession of rock strata laid down over the course of time, each of which correlates to specific junctures in Earth's geologic history. The breakdown of rocks and minerals at or near the surface of Earth due to physical or chemical processes, or both.

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Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia. Stratigraphy is that subarea of geology that treats the description, correlation , and interpretation of stratified Earth materials. Typically, geologists consider stratified Earth materials as layers of sediment or sedimentary rock. This definition, however, clearly encompasses other materials such as volcanic lava , ash flows, ash-fall layers, meteoritic impact ejecta layers, and soils. In fact, using this definition, any material that obeys the law of superposition during its formation could be placed in the domain of stratigraphy.

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Stratigraphy refers to layers of sediment, debris, rock, and other materials that form or accumulate as the result of natural processes, human activity, or both. Stratigraphy - How it works EARLY WORK IN STRATIGRAPHY. It is concerned primarily with relative dating, whereas geochronometry includes the.

Generally, internal layers within Earth crust , mantle, and core are not considered the type of layers studied by stratigraphers because they formed by Earth's internal differentiation processes. Some geologists give a broader definition to the term stratigraphy.


Planetary geologists sometimes view stratigraphy as if it were the study of the sequence of events on a planet or moon's surface. In addition, stratigraphy has been broadly used by some geologists who study mountain building and plate tectonics to mean the study of order of emplacement of rock units of various types, including igneous and metamorphic rocks , to which the law of superposition does not apply.

In some cases, stratigraphy is used to define the study of geologic history of an area or country, but it is more correct to say that stratigraphy is the practical foundation for historical geology. See Geomagnetism as well as the discussion of paleomagnetism in Plate Tectonics. Chronostratigraphy is devoted to studying the ages of rocks and what they reveal about geologic time, or the vast stretch of history approximately 4.

It is concerned primarily with relative dating, whereas geochronometry includes the determination of absolute dates and time intervals. This typically calls for the use of radiometric dating. The stratigraphic column is the succession of rock strata laid down over the course of time, each of which correlates to specific phases in Earth's geologic history. The record provided by the stratigraphic column is most reliable for studying the Phanerozoic, the current eon of geologic history, as opposed to the Precambrian, which constituted the first three eons and hence the vast majority of Earth's geologic history.

The relatively brief span of time since the Phanerozoic began about million years, or Ma has seen by far the most dramatic changes in plant and animal life. It was in this eon that the fossil record emerged, giving us far more detailed information about comparatively recent events than about a much longer span of time in the more distant past.

Precambrian time is so designated because it precedes the Cambrian period, one of 11 periods in the Phanerozoic eon. The Cambrian period extended for about 50 million years, from approximately Ma to Ma ago. This statement in terms of years, however inexact, is an example of absolute age. By contrast, if we say that the Cambrian period occurred at the beginning of the Paleozoic era, after the end of the Proterozoic eon and before the beginning of the Ordovician period, this is a statement of relative age. Both statements are true, and though it is obviously preferable to measure time in absolute terms, sometimes relative terms are the only ones available.

Dating, in scientific terms, is any effort directed toward finding the age of a particular item or phenomenon. Relative dating methods assign an age relative to that of other items, whereas absolute dating determines age in actual years or millions of years. When geologists first embarked on stratigraphic studies, the only means of dating available to them were relative. Using Steno's law of superposition, they reasoned that a deeper layer of sedimentary rock was necessarily older than a shallower layer.

Advances in our understanding of atomic structure during the twentieth century, however, made possible a particularly useful absolute form of dating through the study of radioactive decay. Radiometric dating, which is explained in more detail in Geologic Time, uses ratios between "parent" and "daughter" isotopes. Radioactive isotopes decay, or emit particles, until they become stable, and as this takes place, parent isotopes spawn daughters.

The amount of time that it takes for half the isotopes in a sample to stabilize is termed a half-life. Elements such as uranium, which has isotopes with half-lives that extend into the billions of years, make possible the determination of absolute dates for extremely old geologic materials. Geologic time is divided into named groupings according to six basic units, which are in order of size from longest to shortest eon, era, period, epoch, age, and chron. There is no absolute standard for the length of any unit; rather, it takes at least two ages to make an epoch, at least two epochs to compose a period, and so on.

The dates for specific eons, eras, periods, and so on are usually given in relative terms, however; an example is the designation of the Cambrian period given earlier. Chronostratigraphy also uses six time units: