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 record of geologic events and the evolution of life forms as shown in the rock and fossil records.

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Example: This image shows the geologic time scale with periods and epochs, illustrating the evolution of life and major extinction events.

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Rock Strata Noun

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[rok strah-tuh]

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Rock Strata

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Layers of sedimentary rock that provide evidence for the relative order of geologic events and Earth's long history.

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Example: The image shows layers of rock strata, each representing different geologic time periods, helping to understand Earth's history.

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Fossil Noun

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[fos-uhl]

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Fossil

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The preserved remains or traces of an organism that lived in the past, often found within layers of rock.

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Example: This image shows a fossil of a dinosaur, illustrating how remains are preserved in rock layers over time.

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Index Fossil Noun

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[in-deks fos-uhl]

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Index Fossil

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A fossil of a widespread organism that lived for a short time, used to date the rock layers it's in.

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Example: The image shows how index fossils help date rock layers by depicting fossil succession from oldest to youngest.

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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 or fossil compared to others, without assigning a specific number of years to it.

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Example: The image shows rock layers with fossils labeled as older or younger, explaining relative age.

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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 age of a rock or fossil given as the specific number of years since it was originally formed.

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Example: Rock layers with fossils are labeled with their absolute ages, showing how scientists determine the age of rocks.

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Unconformity Noun

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[un-kuhn-fawr-mi-tee]

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Unconformity

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A gap in the geologic record where rock layers have been lost due to erosion, representing missing geologic time.

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Example: The image shows an unconformity where rock layers are missing, illustrating a gap in geologic time.

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Precambrian Time Noun

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[pree-kam-bree-uhn tahym]

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Precambrian Time

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The longest span of geologic time, covering about 88 percent of Earth's history before complex life appeared.

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Example: This timeline shows the Precambrian Time, highlighting major events like the origin of eukaryotes and the oldest fossils.

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Era Noun

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[eer-uh]

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Era

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One of the three major divisions of geologic time between the Precambrian and the present, such as the Mesozoic Era.

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Example: The image shows the geologic time scale, highlighting eras like the Mesozoic, which helps explain what an 'era' is.

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Period Noun

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[peer-ee-uhd]

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Period

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A unit of geologic time that subdivides an era, typically defined by specific events in Earth's history.

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Example: The image shows how periods fit within eras on the geologic time scale, helping students understand the sequence and duration of Earth's history.