Correlation (Geology) - The Nature Of Sedimentary Strata, Physical Correlation, Interpreting Earth History Within A Stratum, Fossil Correlation
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In geology, the term correlation refers to the methods by which the age relationship between various strata of Earth's crust is established. Such relationships can be established, in general, in one of two ways: by comparing the physical characteristics of strata with each other (physical correlation); and by comparing the type of fossils found in various strata (fossil correlation).
Correlation is an important geological technique because it provides information with regard to changes that have taken place at various times in Earth history. It also provides clues as to the times at which such changes have occurred. One result of correlational studies has been the development of a geologic time scale that separates Earth history into a number of discrete time blocks known as eras, periods, and epochs.
Additional Topics
Sedimentary rocks provide information about Earth history that is generally not available from igneous or metamorphic rocks. To understand why this is so, imagine a region in which sediments have been laid down for millions of years. For example, suppose that for many millions of years a river has emptied into an ocean, laying down, or depositing, sediments eroded from the land. During that period…
Using sedimentary rock strata it should be possible, at least in theory, to write the geological history of the continents for the last billion or so years. Some important practical problems, however, prevent the full realization of this goal. For example, in many areas, erosion has removed much or most of the sedimentary rock that once existed there. In other places, strata are not clearly expose…
Imagine that geologists a million years from now began studying Earth's surface within a 3 mi (5 kilometer) radius of your home. What would they find? They would probably discover considerable variation in the sediment deposits that are accumulating in your region today. They might find the remains of a river bed, a swamp, a lake, and other features. Geologists living today who study strata…
One of the most important discoveries in the science of correlation was made by the English surveyor William Smith in the 1810s. One of Smith's jobs involved the excavation of land for canals being constructed outside of London. As sedimentary rocks were exposed during this work, Smith found that any given stratum always contained the same set of fossils. Even if the stratum were physically…
The correlational studies described so far allow scientists to estimate the relative ages of strata. If stratum B lies above stratum A, B is the younger of the two. However determining the actual, or absolute, age of strata (for example, 3.5 million years old) is often difficult since the age of a fossil cannot be determined directly. The most useful tool in dating strata is radiometric dating of …
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