Gaps in the geologic record, called unconformities, are common where deposition stopped and erosion removed the previously deposited material.
Geologists generally know the age of a rock by determining the age of the group of rocks, or formation, that it is found in.
The age of formations is marked on a geologic calendar known as the geologic time scale.
Inclusions: Inclusions, which are fragments of older rock within a younger igneous rock or coarse-grained sedimentary rock, also facilitate relative dating.
Inclusions are useful at contacts with igneous rock bodies where magma moving upward through the crust has dislodged and engulfed pieces of the older surrounding rock.
For example, shells, wood, and other material found in the shoreline deposits of Utah’s prehistoric Lake Bonneville have yielded absolute dates using this method.
These distinct shorelines also make excellent relative dating tools.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.The nuclear decay of radioactive isotopes is a process that behaves in a clock-like fashion and is thus a useful tool for determining the absolute age of rocks.Radioactive decay is the process by which a “parent” isotope changes into a “daughter” isotope.If a geologist claims to be 45 years old, that is an absolute age.