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Dating sedimentary rock layers
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.Development of the geologic time scale and dating of formations and rocks relies upon two fundamentally different ways of telling time: relative and absolute.
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.
If a geologist claims to be 45 years old, that is an absolute age.
Superposition: The most basic concept used in relative dating is the law of superposition.
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.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.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.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.Relative dating techniques provide geologists abundant evidence of the incredible vastness of geologic time and ancient age of many rocks and formations.However, in order to place absolute dates on the relative time scale, other dating methods must be considered.