Earth History

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Earth History
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All sedimentary rock layers were originally
deposited horizontally
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Law of Continuity of the Layers
Ex. Layer C is the same rock unit separated by
erosion
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Layer X is an intrusion and is younger than any
other rock unit. This is an example of the Law of
Cross Cutting
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This fault is also younger than any other rock
unit and is Another example of the Law of Cross
Cutting
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There are a few exceptions to the Law of
Superposition
1. Overturned folds puts older rock on top of
younger rock
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Overturned fold
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A second exception is a thrust fault
Here older rock is on top of younger rock
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Thrust fault
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contact metamorphism on all sides
As seen previously igneous intrusions are
really exceptions to the law of superposition
The igneous rock is younger than the rock unit
on top of it
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contact metamorphism
Igneous intrusion is the youngest rock unit here
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No contact metamorphism
here
This is not an exception to the law of
superposition, why?
Yes, there is no contact metamorphism on the top
of the basalt, Therefore this is a buried lava
flow and not an intrusion
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Later deposition may cover this lava
Lava flow seen here covering a road in Hawaii
becomes part of the rock layers and follows the
law of superposition
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Volcanic ash
Layers of volcanic ash also become part of
the geologic record, and serve as excellent time
markers
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Every rock layer represents a period of time in
the Earths long history. Think of each layer as
a chapter in a book. the oldest layer on the
bottom is chapter 1. The final chapter in The
book is the top rock layer.
However, sometimes chapters in the history of
the Earth are missing. This is due to erosion
removing rock layers When they are exposed at the
earths surface.
When new layers of rock are deposited on top of
the old Eroded surface, an unconformity exists.
Unconformity a gap in the geologic rock record
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Note- during deposition the region is submerged
underwater
unconformity
During erosion periods the region is dry land
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4 types of unconformities
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Can you find the angular unconformity here?
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Angular unconformity
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Putting it all together
9. Faulting offsets all layers
8.Submergence and new Deposition of layers F- I
Erosion
7. Erosion
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5. Uplift of layers B-E
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6. Intrusion and contact metamorphism
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See if you can list all the geologic events in
the order in which they occurred
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The Grand Canyon has a complex geology