Geologic Time

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Geologic Time Noun

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[jee-uh-loj-ik tahym]

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Geologic Time

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The vast time scale, spanning billions of years, over which Earth's geologic processes operate and shape the planet.

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Example: This diagram shows rock layers (strata) with fossils. Older layers are at the bottom and younger layers are at the top, illustrating relative dating.

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Trilobite Noun

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[trahy-luh-bahyt]

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Trilobite

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An extinct marine arthropod that was abundant for nearly 300 million years and is often used as an index fossil.

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Example: This diagram shows the main body parts of a trilobite fossil: the cephalon (head), segmented thorax, and pygidium (tail), as well as its three distinct lobes.

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Supercontinent Noun

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[soo-per-kon-tuh-nuhnt]

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Supercontinent

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A large landmass formed by the convergence and assembly of multiple continents, such as the ancient landmass Pangaea.

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Example: This map shows the supercontinent Pangaea, where all of Earth's continents were once joined together in a single giant landmass millions of years ago.

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Uniformitarianism Noun

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[yoo-nuh-fawr-mi-tair-ee-uh-niz-uhm]

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Uniformitarianism

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The principle that Earth's features were formed by slow, gradual, and continuous processes that are still at work today.

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Example: This diagram shows the rock cycle, where processes like erosion and uplift continuously shape the Earth. Uniformitarianism is the idea that these same slow processes have always been at work over geologic time.

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Catastrophism Noun

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[kuh-tas-truh-fiz-uhm]

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Catastrophism

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The geological principle that Earth's features are primarily the result of sudden, short-lived, and violent global-scale events.

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Example: This image shows massive, sudden volcanic eruptions changing the Earth's surface, illustrating the theory of catastrophism where geologic features form from violent events.

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Relative Time Noun

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[rel-uh-tiv tahym]

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Relative Time

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The sequencing of geologic events or objects as older or younger than others, without determining their exact numerical age.

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Example: This diagram shows how geologists determine the order of events. Older rock layers (like C) are at the bottom, and younger features (like fault E) cut across existing layers.

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Absolute Age Noun

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[ab-suh-loot eyj]

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Absolute Age

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The specific numerical age of a rock, fossil, or geologic event, typically determined through radiometric dating methods.

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Example: A radioactive 'parent' atom breaks down into a stable 'daughter' atom, releasing a particle. Scientists use this predictable process to determine a rock's absolute age.

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Radiometric Dating Noun

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[rey-dee-oh-me-trik dey-ting]

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Radiometric Dating

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A technique used to determine the absolute age of materials by measuring the decay of their radioactive isotopes.

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Example: This graph shows that after each 'half-life' passes, the amount of a radioactive substance decreases by half, a key principle for dating rocks.

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Principle of Horizontality Noun

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[prin-suh-puhl ov hawr-uh-zon-tal-i-tee]

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Principle of Horizontality

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The geological principle stating that layers of sediment are originally deposited in horizontal, laterally continuous sheets under gravity.

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Example: This diagram shows that sediments, like sand and mud, settle in flat, horizontal layers due to gravity, especially when deposited in water.

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Principle of Superposition Noun

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[prin-suh-puhl ov soo-per-puh-zish-uhn]

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Principle of Superposition

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The principle that in an undisturbed sequence of rock layers, the youngest layers are on top and the oldest are at the bottom.

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Example: This diagram shows layers of rock, called strata. The Principle of Superposition states that lower layers are older than the upper layers above them.