Absolute age is the number of years that have passed since a rock was formed. It is determined using radioactive decay, where unstable particles in the rock break down and form atoms of a different element. By measuring the amount of the original element and the new element, along with the element’s half-life, scientists can calculate the age of the rock.

Relative age, on the other hand, is a rock’s age compared to the ages of other rocks. It is determined by looking at the position of rock layers, the presence of igneous extrusions and intrusions, faults, and fossils. The Law of Superposition states that in undisturbed horizontal sedimentary rocks, the bottom layer is the oldest and the top layer is the youngest. Extrusions and intrusions are younger than the rocks they are found in or surrounded by. Faults are younger than the rocks they cut through. Fossils found in sedimentary rock can also provide clues about the relative age of the rock.

Both relative and absolute age are used to describe rocks because knowing the absolute age of some rocks is impossible. Absolute age dating requires the presence of certain elements and the ability to measure their decay, which may not be possible for all rocks. However, understanding the ages of rocks is important for constructing a record of Earth’s history and understanding how the Earth has changed over time.