Geologic Time Notes

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Geologic Time
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Why is this statement false The caveman had
dinosaur for breakfast.?

• man and dinosaurs
• never lived at the same time

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Laws of geologic history

• SuperpositionOldest rocks on the bottom of an
  undisturbed sequence of rock
• Law of original horizontalityAll rocks are
  initially deposited in flat (horizontal) rows
• Cross-cutting relationshipsObjects which
  cross-cut rocks are younger than the rocks
  themselves (ex fault or intrusion)

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Superposition
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Using superposition, label the strata below from
oldest to youngest.
youngest
oldest
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Horizontality
All rock layers are originally deposited in flat
(horizontal) layers
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Faults are always (older, younger) than the
rocks they cut through.
Cross-cutting relationships
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(No Transcript)
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Which is older F or S How do you know?
F cuts through S S must have been there first
(it's older)
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Uniformitarianism
These laws are based on the assumption that the
forces at work today are the same forces at work
in history uniformitarianism.
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If a geologist finds an igneous sill, how can she
determine if the sill is an intrusion or an
extrusion?
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(No Transcript)
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Is H an intrusion or extrusion?How can you
tell?
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H is an intrusion.Contact metamorphism on top
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On to the next question...
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What is an unconformity? How does it complicate
the relative dating of rock layers?

• a buried erosional surface

• a part of the rock record
• is missing

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Using the diagram to the left, identify where the
unconformity is located by drawing an arrow and
writing the word unconformity next to it.
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What are
Index fossils?
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What characteristics must fossils have in order
to be good index fossils?

• lived over a large geographic area (large
  horizontal distribution)
• lived for a short period of time (small vertical
  distribution)

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In the diagram below, a geologist has matched up
rock layers based on index fossils.
In geology, this is referred to as CORRELATION
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On to the next question...
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Absolute Age
Why are radioactive isotopes useful in
determining the absolute age of a rock?
their half-lives are constant
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Absolute Age

• What can be done to change the
• half-life of a radioactive isotope?
• Why is this important?

NOTHING!
it is reliable to calculate age
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Draw the generic graph for the half-life of a
radioactive isotope.
stable atoms
unstable atoms
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Absolute Age

• What radioactive isotope could be
• used to determine the absolute
• age of material that was recently living?
• carbon-14

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Please answer the two questions on page 7 of
your Notes
What is the half life of uranium-238? 4.5 x
109 4,500,000,000 4.5 billion years choice (c)

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If there is a 100g sample of C14, how many grams
of C14 would remain after three half-lives?
How long would this take? Show all work.

?
?
?
100g
50g
25g
12.5g
3 half-lives x (5.7 x 103) 1.71 x 104
17,100 years