Transcript ESc Relative Dating

Geologic Time
Chapter 34
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Dating rocks

Relative dating
 Compare
events and put them in
order based on their sequence of
formation, oldest to youngest.

Absolute dating
 Using
radioactive decay to
determine the exact age of rocks
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The Geologic Time Scale
p. 182
Cenozoic
Phanerozoic
Mesozoic
Paleozoic
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Geologic Time Scale


Geologists have divided the earth’s
history into various eras, periods
and epochs using fossil evidence
(which life forms lived when)
The chart has been updated many
times as more fossil evidence is
located
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Principles of relative dating

Law of superposition
 In an undisturbed sequence of
sedimentary rocks the oldest rocks are on
the bottom, youngest on top
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Law of superposition
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Principles of relative dating

Principle of original horizontality
 Layers of sediment are
generally deposited in a
horizontal, flat-lying position
 Rock layers that are flat have
not been disturbed
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Principles of relative dating

Principle of cross-cutting
relationships
 Younger features cut
across older feature
 A fault or dike that
cuts across
sedimentary layers is
younger than the
layers.
Igneous dikes cutting
across metamorphic rock
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Principles of relative dating

Included Fragments
 An inclusion is a piece of rock
that is enclosed within another
rock (2 examples: xenolith and
pebble in conglomerate)
 Rock containing the inclusion
is younger, the inclusion is
older
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Principles of relative dating

Unconformity
 An unconformity is a break in
the rock record produced by
erosion and/or nondeposition
of rock layers
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Sketch of an unconformity:
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Geological events of an unconformity:
1. Sedimentation / deposition
and lithification
2. Deformation / folding
and erosion
3. Subsidence, water is covering
4. More sedimentation happened
once covered with water
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Unconformity in rocks:
Angular unconformity at Siccar Point, Scotland
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