5.6: Chapter Summary and Key Terms

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Summary

Relative dating techniques allow a way to place geologic events in sequential order, but do not indicate the amount of time each event took.

Units deposited in horizontal layers at the earth’s surface can be organized according to the Principles of Original Horizontality, Superposition, and Lateral Continuity.
Units that are not emplaced as layers, such as igneous intrusive rocks, dikes or metamorphic rocks can be placed in a relative order using the Principle of Cross-cutting Relationships and Principle of Inclusions. The Principles of Baked Contacts and Chilled Margins are variations on the Principles of Cross-Cutting Relationships and Inclusions.
The Principle of Faunal Succession relies on fossil assemblages to correlate units across large distances. .
The Grand Canyon is a good example of all of these concepts in one place.

Unconformities are indicators of missing time due to erosion or nondeposition.
Three different types of unconformity can be recognized based on geological relationships around them.
An angular unconformity is indicated when tilted sedimentary layers or volcanic flows are truncated by overlying horizontal sedimentary layers or volcanic flows.
A disconformity is apparent when horizontal sedimentary or volcanic layers are separated by horizontal or subhorizontal erosional surfaces.
A nonconformity is developed when crystalline rocks (igneous intrusive or metamorphic rocks) are capped by horizontal or subhorizontal volcanic flows or sedimentary layers.

Fossil assemblages represent distinct periods in earth history.
Index fossils are organisms that lived for relatively short time periods; if they were distributed over a wide geographic area, they can be used to compare the relative ages of rocks from different regions.
Biozone fossils are groups of organisms that contain genera and families that lived over a long time, and species that lived for a relatively short time. They can be easily distinguished from others on the basis of specific features.
Fossil studies must be combined with numerical dating to provide absolute ages for rock units.

Numerical or absolute ages for rocks and minerals in them can be determined using radioactive decay of elements contained in them.
Radio-isotopes are elements that are unstable and decay to form stable daughter isotopes. The rate of day decay is called the “half-life”.
Dating requires determining the ratio of parent and daughter isotopes in target minerals.
The age that’s obtained for a particular rock or mineral has different significance depending on the isotope system and material being analyzed.
Isotopic systems with a range of half lives can be used to learn different things about geological materials including their crystallization ages, metamorphic ages, and time of death (for organic materials).

The geologic time scale was initially constructed based on correlations of units across the globe, based on fossil assemblages.
Absolute ages were subsequently added to the time scale as they were obtained.
The time scale is structured based on Eons, Eras, Periods and Epochs, which are identified on the basis of extinctions.