Relative age and relative dating
Particularly useful are index fossils, geographically widespread fossils that evolved rapidly through time.Crosscutting Relationships: Relative ages of rocks and events may also be determined using the law of crosscutting relationships, which states that geologic features such as igneous intrusions or faults are younger than the units they cut across.
Relative dating places events or rocks in their chronologic sequence or order of occurrence.
Most ancient sedimentary rocks cannot be dated radiometrically, but the laws of superposition and crosscutting relationships can be used to place absolute time limits on layers of sedimentary rocks crosscut or bounded by radiometrically dated igneous rocks.
Sediments less than about 50,000 years old that contain organic material can be dated based on the radioactive decay of the isotope Carbon 14.
Rates of radioactive decay are constant and measured in terms of half-life, the time it takes half of a parent isotope to decay into a stable daughter isotope.
Some rock-forming minerals contain naturally occurring radioactive isotopes with very long half-lives unaffected by chemical or physical conditions that exist after the rock is formed.
Half-lives of these isotopes and the parent-to-daughter ratio in a given rock sample can be measured, then a relatively simple calculation yields the absolute (radiometric) date at which the parent began to decay, i.e., the age of the rock.