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8.2 The Relative Age of Rocks

8.2 The Relative Age of Rocks. 7.4.c Students know that the rock cycle includes the formation of new sediment and rocks and that rocks are often found in layers, with the oldest generally on the bottom. The Position of Rock Layers.

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8.2 The Relative Age of Rocks

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  1. 8.2 The Relative Age of Rocks 7.4.c Students know that the rock cycle includes the formation of new sediment and rocks and that rocks are often found in layers, with the oldest generally on the bottom.

  2. The Position of Rock Layers • Relative age- the age of a rock compared to the age of other rocks • Absolute age- the number of years since the rock was formed • Sedimentary rocks form when sediment is deposited in layers

  3. The Position of Rock Layers • Key Concept: 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. • Scientists can’t determine absolute age using the law of superposition but they can get relate age

  4. Determining Relative Age • Extrusion- lava that hardens on the surface • Intrusion- when magma cools and hardens beneath the surface • Formation of Igneous Rock • Fault- a break in the Earth’s crust

  5. Key concept: To determine relative age, geologists also study extrusions and intrusions of igneous rock, faults, and gaps in the geologic record, and inclusions. Determining Relative Age

  6. Determining Relative Age • Geologists use extrusions, intrusions, or faults to date rocks using the principle of cross-cutting • Principle of cross-cutting states that when something cuts across a body of rock, it is younger than the rock it cuts across • Geological record of sedimentary rock is not complete • Deposition slowly builds layers but some layer erode away exposing older layers, deposition forms new on top of those older layers

  7. Unconformity-the surface where new rock layers meet a much older rock surface Determining Relative Age

  8. Unconformity shows a gap in the geologic record because of erosion Determining Relative Age

  9. An unconformity occurs where erosion wears away layers of sedimentary rock Determining Relative Age

  10. Other rock layers then form on top. Determining Relative Age

  11. Inclusions • Helps date rocks when there is an unconformity • Definition- a piece of rock that is contained in another rock • Starts as a solid rock piece that breaks off an existing rock and becomes part of another rock • The rock containing an inclusion is younger than the rock the inclusion came from

  12. Index fossils- a fossil that is widely distributedand represents a type of organism that existed briefly Key concept: Index fossils are useful because they tell the relative ages of the rock layers in which they occur. Index fossils are used to match up rock layers at locations that may be far apart Using Fossils to Date Rocks • Ammonites in layer C are index fossils because they: • only occur in one layer (existed briefly) • are in different locations (widely distributed)

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