Absolute dating geology
Development of the geologic time scale and dating of formations and rocks relies upon two fundamentally different ways of telling time: relative and absolute.
Relative dating places events or rocks in their chronologic sequence or order of occurrence.
Metamorphic rocks may also be radiometrically dated.
However, radiometric dating generally yields the age of metamorphism, not the age of the original rock.
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.
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.
These distinct shorelines also make excellent relative dating tools.Absolute dating places events or rocks at a specific time.If a geologist claims to be younger than his or her co-worker, that is a relative age.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.Simply stated, each bed in a sequence of sedimentary rocks (or layered volcanic rocks) is younger than the bed below it and older than the bed above it.This law follows two basic assumptions: (1) the beds were originally deposited near horizontal, and (2) the beds were not overturned after their deposition.Another method of determining absolute age is by looking at varves.VARVES ARE LAYERS OF LIGHT AND DARK SEDIMENT THAT SHOW YEARLY CYCLES. Each spring or summer when the glacier was melting the glacier deposits a ton of sediment it was carrying into strems of water that are melting off of it.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.Following this law, sedimentary rocks can be “dated” by their characteristic fossil content.