Geologist are often trying to learn more about Earth and its history.
It is widely accepted that the Earth is about 4.5 billion years old. For example, the Colorado river has eroded away parts of the Columbia plateau.
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.
- are the preserved remains or traces of living things that lived millions of years ago.
Fossils can help geologists determine the age of geologic strata.
The successive layers of rock represent successive intervals of time.
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.
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.