Geologic Time

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Aim How do Scientists read Rocks?
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Vocabulary
Outcrop a section of rock exposed on the
surface that shows a geologic sequence.
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I. Discovering Earths History

• Uniformitarianism - the processes that we observe
  today are most likely the same processes that
  occurred millions of years ago.

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• Relative dating - tells us the sequence in which
  events occurred.

Does not tell us the actual age or use numbers
to express time.
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II. Three principles for Relative Dating

• A. Law of Superposition
• Within an outcrop of undisturbed sedimentary
  rock, the oldest layers are at the bottom.

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• B. Law of Original Horizontality
• Most sediments are deposited as horizontal beds
  (layers) and remain that way unless disturbed.

Which picture shows the Law of Original
Horizontality
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Disturbed Rock Layers
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• C. Law of Cross-Cutting Relationships
• A disrupted rock is older than the rock
  disrupting it.

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Types of Cross-Cutting Relationships
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III. Important Features

• Volcanic Eruption occur at one specific time
  and leaves a layers of ash which marks the time
  eruption.

Do not copy
K-T ash Boundary which marks the end of the
Cretaceous Period and the beginning of the
Paleogene Period. This layer is 65 million years
old and below this layer, scientists find fossils
of dinosaurs. Above this layer, there are no
dinosaur fossils.
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• Unconformity (UNC)-
• a gap in the
• sedimentary rock
• record. Usually
• Caused by some
• type of erosion.

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• Contact Metamorphism occurs as an igneous
  intrusion touches and metamorphosis's the
  surrounding rock.
• Uses the law of cross-cutting relationships to
  describe the relative sequence.

Can you cause a metamorphosis of layer that does
not exist yet? Which comes first, the rock being
metamorphosized or the magma?
NO
The rock being metamorphosized
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Closure 1 Place the letters in relative order
using the notes as a guideline. Which letter
comes first, second, third

• Answer P, K, M, S, Intrusion R, erosion A (UNC),
  B, J, F.

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• Closure 2
• Relatively sequence the letters in the outcrop
  below. List which layer comes first, second,
  third
• Answers
• D
• C
• B
• F
• A
• E