Dating Rock Layers

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Dating Rock Layers

• How to tell how old the layers of rock are.

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What is Age?

• There are 2 kinds
• Absolute Age The number of years since the rock
  formed. (150 million years old, 10 thousand
  years old.)
• Relative Age The age compared to the ages of
  other rocks. (older than this rock, younger than
  that rock.)

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Age cont.

• Relative Age Much easier to figure out. Can be
  deduced from clues on the Earth.
• Absolute Age Much harder to figure out. Often
  impossible.

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What are the ways to tell RELATIVE AGE.

• 1. Law of Superposition
• 2. Extrusion Intrusion
• 3. Faults
• 4. Unconformities

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What is 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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Age and Superposition
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What is Extrusion?

• When magma cools on the surface of the Earth its
  called an EXTRUSION.
• Extrusions are always YOUNGER than the rock it is
  sitting on.
• For extrusion think exit or outside.

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What is an Intrusion?

• When magma cools beneath the surface of the Earth
  its called an INTRUSION.
• Intrusions are always YOUNGER than the rocks
  around it.
• For intrusion think inside.

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Heres an Intrusion
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Extrusions? Intrusions?
First of all, which rock layer is the oldest?
What is this called?
EXTRUSION
INTRUSION
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What is a Fault?

• A BREAK in the Earths crust.
• A fault is always YOUNGER than the rock it cuts
  through.
• Faults move over time, making determining the age
  of the layers confusing.

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Heres a Fault
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Faults
Faults move over time causing the rock layers to
move.
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Faults
As more time goes by, more rock layers are added
on top. Making aging the rock layers difficult.
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Faults
Geologists must then label each of the rock
layers. This can be tough because they werent
around to see the whole process occur.
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What is an Unconformity?

• The surface where new rock layers meet a much
  older rock surface beneath them.
• An unconformity shows where some rock layers have
  been lost because of erosion.

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Unconformity
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Summary

• Relative vs. Absolute Age
• Determining Relative Age
• Law of Superposition on top is younger
• Intrusions magma inside Earth.
• Extrusions magma on top of Earth.
• Faults breaks in the crust
• Unconformities signs of erosion.