Since certain species of animals existed on Earth at specific times in history, the fossils or remains of such animals embedded within those successive layers of rock also help scientists determine the age of the layers.
Absolute dating methods are carried out in a laboratory.
Absolute dates must agree with dates from other relative methods in order to be valid.
The most widely used and accepted form of absolute dating is radioactive decay dating. Radioactive decay refers to the process in which a radioactive form of an element is converted into a nonradioactive product at a regular rate.
This method is based on the assumption (which nearly always holds true) that deeper layers of rock were deposited earlier in Earth's history, and thus are older than more shallow layers.
The successive layers of rock represent successive intervals of time.
The nucleus of every radioactive element (such as radium and uranium) spontaneously disintegrates over time, transforming itself into the nucleus of an atom of a different element.