Geologic Time Scale

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Geologic Time Scale Noun

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[jee-uh-loj-ik tahym skeyl]

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Geologic Time Scale

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A system of chronological dating that classifies geological strata in time, used to describe events in Earth's history.

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Example: This chart organizes Earth's 4.54 billion-year history into units of time, from largest to smallest: Eons, Eras, Periods, and Epochs.

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Fossil Noun

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[fos-uhl]

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Fossil

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The preserved remains, impressions, or traces of organisms that lived in the past, providing evidence for geologic history.

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Example: This diagram shows that older rock layers at the bottom contain older fossils, while younger layers on top contain younger fossils, a principle called fossil succession.

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Relative Age Noun

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[rel-uh-tiv eyj]

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Relative Age

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The age of a rock layer or fossil determined by comparing its position to that of other layers.

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Example: This diagram shows that lower rock layers (like B) are older than the layers above them. A fault (J) that cuts through layers is younger.

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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 exact numerical age of a rock or fossil, typically expressed in years and determined through radiometric dating.

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Example: This diagram shows how scientists determine a rock's absolute age by comparing the amount of radioactive parent atoms to stable daughter atoms over time.

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Radioactivity Noun

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[rey-dee-oh-ak-tiv-i-tee]

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Radioactivity

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The process by which an unstable atomic nucleus loses energy by radiation, used to determine the absolute age of rocks.

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Example: This graph shows radioactive decay. After each half-life passes on the x-axis, the percentage of the original radioactive substance on the y-axis is reduced by half.

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Eon Noun

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[ee-on]

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Eon

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The largest division of geologic time, spanning hundreds of millions to billions of years and comprising two or more eras.

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Example: This diagram shows that an Eon, like the Phanerozoic, is one of the largest units of geologic time, which is then divided into smaller units called Eras.

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Era Noun

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[er-uh]

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Era

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A major division of geologic time that is a subdivision of an eon and is itself divided into periods.

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Example: This image shows a scene from the Jurassic Period, which is part of the larger Mesozoic Era, helping to explain that an era is a major division of geologic time.

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Period Noun

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[peer-ee-uhd]

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Period

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A basic unit of geologic time that is a subdivision of an era, during which a specific rock system formed.

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Example: This chart of the Geologic Time Scale shows that a 'Period' (like Jurassic) is a major unit of time that is part of a larger Era.

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Epoch Noun

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[ep-uhk]

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Epoch

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A unit of geologic time that is a subdivision of a period, representing a smaller span of time.

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Example: An epoch is a unit of geologic time, smaller than a period, used to mark specific events and stages in Earth's history.

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Phanerozoic Eon Noun

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[fan-er-uh-zoh-ik ee-on]

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Phanerozoic Eon

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The current geologic eon, which began about 570 million years ago, marked by the appearance of abundant complex life.

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Example: This chart shows the Phanerozoic Eon, the current major division of geologic time, which is defined by 'visible life' and subdivided into three eras.