Earth Science Unit 2 Vocabulary Practice

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Geologic Time Scale

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A record of Earth’s history from its origin (4.6 billion years ago) to the present.

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Supereon

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The largest unit on the geologic time scale.

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Eon

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The second largest division of Earth’s history, spanning hundreds of millions to billions of years.

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Era

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A major division of geologic time, shorter than an eon but longer than a period.

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Period

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A major subdivision of a geologic era, representing a span of tens to hundreds of millions of years.

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Epoch

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A subdivision of a geologic period, spanning millions of years.

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Giant Impact Theory

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Early in Earth’s history, a Mars-sized object (Theia) collided with Earth, forming the moon.

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Mass Extinction

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Occurs when many groups of organisms disappear from the rock record at about the same time.

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The Cambrian Explosion

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The sudden appearance of a diverse collection of organisms in the Cambrian fossil record.

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Radiometric (Absolute) Dating

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A method of measuring the decay of radioactive isotopes in rocks in order to determine the age of the rock (gives a specific numerical age).