the relative age of rocks ph 272-277. essential question how does the law of superpostion help...
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The Relative Age of RocksPH 272-277
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Essential Question
• How does the law of Superpostion help geologists date rocks and what evidence do they use?
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Previously, in life science…
You learned the process of rock formation is very slow. Now, you will learn to compare the ages of rocks.
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To relative age
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Santa Ana Mountains (your backyard)…
…and yes, there are marine fossils up here!
(how did they get up here?)
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1. What are two ways geologists determine the ages of rocks?
• Relative age and absolute age
Time Detectives…Rock layers Atomic clocks
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2. What is absolute age?
• the number of years since the rock formed
• An actual age of the rock in years.
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3. What is relative age?
• the age of a rock compared to the ages of other rocks.
• A comparison only…older or younger
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4. What is the law of superposition?• In horizontal layers of sedimentary rock, the
oldest layer is on the bottom…each higher layer is younger than the layer below it.
Diagram:
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Why do sedimentary rocks have layers?
• The sediment that forms sedimentary rock is deposited in layers!
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In which layer would you find the oldest rocks?
• The bottom layer, because they had to be there first!
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5. What are other ways to find the relative ages of rocks?
• Geologists use extrusions, intrusions, faults, gaps, and inclusions to determine relative age.
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Diagram of an extrusion:
• Lava that hardens on the surface
• Always younger than the layers below it.
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Diagram of an intrusion:
• Lava that hardens beneath the surface
• Always younger than the layers around it.
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Diagram of a fault:
• A break in the Earth’s crust
• Always younger than the rock it cuts through.
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Diagram of an unconformity:
• A gap in the geological record
• Rock layers may be lost due to erosion
Diagram 5
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6. What is an index fossil?
• Fossils that help match rock layers
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7. Why are index fossils useful?
• Because they tell the relative ages of the rock layers in which they occur.
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8. What fossils make good index fossils?
• Exist in many areas
• But should exist only briefly in time.
(found in lots of places, but in only one layer)
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What characteristics must a fossil have to be a useful index fossil?
• It must be widely distributed, and it must have existed for only a brief period of time.