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Determining the Age of Rocks

Determining the Age of Rocks. Relative and Absolute Dating Methods. - The Relative Age of Rocks. The Position of Rock Layers. According to the law of superposition, in horizontal sedimentary rock layers, the oldest layer is at the bottom. Each higher layer is younger than the layers below it.

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Determining the Age of Rocks

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  1. Determining the Age of Rocks Relative and Absolute Dating Methods

  2. - The Relative Age of Rocks The Position of Rock Layers • According to the law of superposition, in horizontal sedimentary rock layers, the oldest layer is at the bottom. Each higher layer is younger than the layers below it.

  3. The Law of Superposition • The geologic principle that states that in horizontal layers of sedimentary rock, each layer is older than the layer above it and younger than the layer below it. • relative age-The age of a rock compared to the ages of rock layers. • absolute age- The age of a rock given as the number of years since the rock formed.

  4. - The Relative Age of Rocks Determining Relative Age • To determine relative age, geologists also study extrusions and intrusions of igneous rock, faults, and gaps in the geologic record.

  5. Intrusions and extrusions • intrusion- An igneous rock layer formed when magma hardens beneath Earth’s surface. • extrusion- An igneous rock layer formed when lava flows onto Earth’s surface and hardens • fault- a break in the crust caused by earthquakes

  6. - The Relative Age of Rocks Determining Relative Age • An unconformity occurs where erosion wears away layers of sedimentary rock. Other rock layers then form on top.

  7. - The Relative Age of Rocks Using Fossils to Date Rocks • Index fossils are useful because they tell the relative ages of the rock layers in which they occur.

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