Radiometric dating rock strata

To determine the relative age of different rocks, geologists start with the assumption that unless something has happened, in a sequence of sedimentary rock layers, the newer rock layers will be on top of older ones. This rule is common sense, but it serves as a powerful reference point.Geologists draw on it and other basic principles ( to determine the relative ages of rocks or features such as faults.

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Each original isotope, called the parent, gradually decays to form a new isotope, called the daughter.

Each isotope is identified with what is called a ‘mass number’.

Geologists often need to know the age of material that they find.

They use absolute dating methods, sometimes called numerical dating, to give rocks an actual date, or date range, in number of years.

As an example of how they are used, radiometric dates from geologically simple, fossiliferous Cretaceous rocks in western North America are compared to the geological time scale.

To get to that point, there is also a historical discussion and description of non-radiometric dating methods.

When ‘parent’ uranium-238 decays, for example, it produces subatomic particles, energy and ‘daughter’ lead-206.

Isotopes are important to geologists because each radioactive element decays at a constant rate, which is unique to that element.

There are two main methods determining a fossils age, relative dating and absolute dating.

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