Relative Dating

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Relative Age Noun

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[rel-uh-tiv eyj]

Back

Relative Age

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The age of one object or rock layer compared to another, without determining a specific numerical age.

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Example: This cross-section shows rock layers of different ages. Newer layers are on top, and features that cut through layers, like intrusion R, are younger.

Front

Absolute Time Noun

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[ab-suh-loot tahym]

Back

Absolute Time

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The exact numerical age of an object or rock layer, often determined through methods like radioactive dating.

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Example: This chart shows absolute time by assigning specific numerical ages, in millions of years (Ma), to geologic periods and the fossils found within them.

Front

Stratigraphy Noun

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[struh-tig-ruh-fee]

Back

Stratigraphy

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The scientific study of rock layers, known as strata, including their formation, sequence, and correlation between different locations.

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Example: This cross-section shows rock layers (strata) stacked in order. Geologists use stratigraphy to determine that lower layers are older than upper layers.

Front

Superposition Noun

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[soo-per-puh-zish-uhn]

Back

Superposition

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The geological principle that in undisturbed rock layers, the oldest layers are at the bottom and the youngest are at the top.

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Example: In undisturbed rock layers (strata), the oldest layers are at the bottom and the youngest layers are at the top, which helps determine their relative age.

Front

Lateral Continuity Noun

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[lat-er-uhl kon-tin-oo-i-tee]

Back

Lateral Continuity

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The principle that sedimentary layers are deposited in continuous sheets and can be matched across eroded areas like valleys.

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Example: This diagram shows that rock layers (like layer C) on opposite sides of a valley were once connected before erosion separated them.

Front

Original Horizontality Noun

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[uh-rij-uh-nl hor-uh-zon-tal-i-tee]

Back

Original Horizontality

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The principle that layers of sediment are originally deposited in flat, horizontal layers under the action of gravity.

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Example: This diagram shows that layers of sediment, like sand and mud, are originally deposited in flat, horizontal layers due to gravity.

Front

Cross-Cutting Relationships Noun

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[kros-kut-ing ri-ley-shuhn-ships]

Back

Cross-Cutting Relationships

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The geological principle that any feature that cuts across another is younger than the feature it cuts through.

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Example: This diagram shows that a geologic feature is younger than the features it cuts across. The fault (E) is the youngest because it cuts through all other layers and the intrusion (D).

Front

Intrusion Noun

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[in-troo-zhuhn]

Back

Intrusion

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A body of igneous rock that has forced its way into pre-existing rock layers, making it younger than those layers.

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Example: This diagram shows an intrusion, where magma pushes into existing rock layers but cools and hardens underground without ever reaching the surface.

Front

Fault Noun

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[fawlt]

Back

Fault

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A fracture in the Earth's crust where blocks of rock have moved, which is younger than the rocks it displaces.

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Example: A fault is a fracture in a body of rock where one side has moved relative to the other, displacing the rock layers.

Front

Unconformity Noun

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[un-kuhn-fawr-mi-tee]

Back

Unconformity

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A gap in the geological record, representing a period of erosion or non-deposition, separating younger rocks from older rocks.

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Example: This diagram shows an unconformity, a buried surface of erosion representing a gap in time between older, tilted rock layers and younger, flat layers.