Before the advent of absolute dating methods in the twentieth century, nearly all dating was relative.
The main relative dating method is stratigraphy (pronounced stra-TI-gra-fee), which is the study of layers of rocks or the objects embedded within those layers.
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.
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.
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.
Dating techniques are procedures used by scientists to determine the age of an object or a series of events.
The two main types of dating methods are relative and absolute.
Relative dating methods are used to determine only if one sample is older or younger than another.
Absolute dating methods are used to determine an actual date in years for the age of an object.
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.
Similarly, pollen grains released by seed-bearing plants became fossilized in rock layers.
If a certain kind of pollen is found in an archaeological site, scientists can check when the plant that produced that pollen lived to determine the relative age of the site.
Absolute dating methods are carried out in a laboratory.