Geological Concepts and Terms

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Fossil Record

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A collection of fossils that document the history of life on Earth, arranged in chronological order from oldest to youngest.

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Index Fossil

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A fossil that is distinctive, easily recognizable, plentiful, has a wide geographic distribution, and a short range through time.

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K-Pg boundary

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A thin layer of sediment containing large quantities of iridium, marking the end of the Mesozoic era and the beginning of the Paleogene Period.

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Law of Superposition

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The principle that sediment layers are formed in order of formation, with the oldest at the bottom and the youngest at the top.

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Absolute Dating

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Determining the actual age of a rock or fossil using radioactive decay.

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Relative Dating

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Determining the sequence of events or objects, such as which rock or fossil came first, second, etc.

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Intrusion

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Magma that cools and becomes solid under the Earth's surface.

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Extrusion

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Magma that cools and becomes solid above the Earth's surface.

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Fault

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A fracture or break in the Earth's crust.

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Mass Extinction

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An event where at least half of all species die out in a relatively short time.