Determining Ages of Rocks

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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 geologic feature when compared to the ages of other rocks or geologic features.

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Example: This diagram shows layers of rock, called strata. The principle of superposition states that lower layers are older, helping scientists determine the relative age of rocks.

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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 number of years that have passed since a rock or geologic feature was formed.

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Example: This chart shows how radioactive parent atoms (U-235) decay into stable daughter atoms (Pb-207). By measuring this ratio, scientists calculate a rock's specific age.

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Geologic Record Noun

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[jee-uh-loj-ik rek-erd]

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Geologic Record

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The history of Earth as documented by the evidence found within its rock layers and formations.

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Example: This cross-section shows how Earth's history is recorded in rock layers (strata), with older layers at the bottom and younger geologic events.

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Law of Superposition Noun

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[law uv soo-per-puh-zish-uhn]

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Law of Superposition

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A principle stating that in undisturbed rock layers, the oldest layer is at the bottom and the youngest is at the top.

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Example: This image shows layers of rock and soil (strata), where the bottom layers are the oldest and the top layers are the youngest.

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Strata Noun

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[strah-tuh]

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Strata

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The distinct horizontal layers of sedimentary rock, with each layer representing a different period of deposition.

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Example: This cross-section shows strata, which are the different layers of sediment and rock that have built up over a long period of time.

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Extrusion Noun

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[ik-stroo-zhuhn]

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Extrusion

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An igneous rock layer that is formed when lava flows onto the Earth's surface and subsequently hardens.

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Example: This diagram shows extrusion, where a force from a ram pushes a material (billet) through a shaped opening (die) to create a new shape.

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Intrusion Noun

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[in-troo-zhuhn]

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Intrusion

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An igneous rock mass formed when magma pushes into existing rock layers below the surface and then hardens.

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Example: This image shows a large body of molten rock, a magma chamber, which has formed beneath layers of existing rock, representing a large-scale intrusion.

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Fault Noun

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[fawlt]

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Fault

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A break or fracture in the Earth's crust along which significant movement of rock has occurred.

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Example: This diagram shows a normal fault, a crack in the Earth's crust where one block of rock (hanging wall) slides down past another (footwall).

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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 a living organism from a past geological age.

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Example: This image shows an ammonite, the preserved spiral shell of an ancient sea creature, which is a classic example of a fossil.

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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 from a widespread organism that lived for a short geologic time, used for dating rock layers.

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Example: This image shows an ammonite fossil, a type of index fossil, preserved in rock, which helps scientists date different rock layers.