Absolute time dating
Gaps in the geologic record, called unconformities, are common where deposition stopped and erosion removed the previously deposited material.
Fortunately, distinctive features such as index fossils can aid in matching, or correlating, rocks and formations from several incomplete areas to create a more complete geologic record for relative dating.
For really old dates we need to use radioactive isotopes.
RADIOACTIVE ISOTOPES ARE UNSTABLE VERSIONS OF NORMAL ELEMENTS.
Faunal Succession: Similar to the law of superposition is the law of faunal succession, which states that groups of fossil animals and plants occur throughout the geologic record in a distinct and identifiable order.
If a geologist claims to be younger than his or her co-worker, that is a relative age.
This section provides access to a number of visualizations and supporting material illustrating the concept of radioactive decay and its central role in radiometric dating.
Visualizations include cross-linked series of diagrams, static illustrations, and photos.
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.