Relative age dating of sedimentary rocks

Some of the isotopes used for this purpose are uranium-238, uranium-235 and potassium-40, each of which has a half-life of more than a million years.Unfortunately, these elements don't exist in dinosaur fossils themselves.Photo from Glen Canyon National Recreation Area, Utah.Relative dating is the science of determining the relative order of past events (i.e., the age of an object in comparison to another), without necessarily determining their absolute age (i.e. In geology, rock or superficial deposits, fossils and lithologies can be used to correlate one stratigraphic column with another.The most widely known form of radiometric dating is carbon-14 dating.This is what archaeologists use to determine the age of human-made artifacts. The half-life of carbon-14 is only 5,730 years, so carbon-14 dating is only effective on samples that are less than 50,000 years old.

So to determine the age of sedimentary rock layers, researchers first have to find neighboring layers of Earth that include igneous rock, such as volcanic ash.

The regular order of the occurrence of fossils in rock layers was discovered around 1800 by William Smith.

While digging the Somerset Coal Canal in southwest England, he found that fossils were always in the same order in the rock layers.

These layers are like bookends -- they give a beginning and an end to the period of time when the sedimentary rock formed.

By using radiometric dating to determine the age of igneous brackets, researchers can accurately determine the age of the sedimentary layers between them.

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