Transcript Lab 08-Geologic Events and Histories

Lab 08
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LESSON
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peruse through this
lesson…
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Geologic Events and
Histories
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Geologists spend a
goodly amount of
time trying to figure
out the history of our
planet…
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To do this, geologists
look at sedimentary
rocks…
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Sedimentary rocks
are tremendously
powerful records of
the history of the
planet…
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Sediment is
deposited in flat,
horizontal planes
called bedding
planes…
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Flat and horizontal…
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Each of these beds represents a
specific event of sedimentary
deposition…
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Bedding planes act
as geologic
markers…they start
out as flat layers…
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Layered sedimentary
rocks provide insight
into which rock layers
came first and last…
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This was first…
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This was second…
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This was last…
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Knowing the order of these rocks…we
can assign relative time to this rock
sequence…
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OLDEST
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YOUNGEST
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With me so far…
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We can use
our old friend
the Geologic
Time Scale…
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Cretaceous
rocks are
younger than
Jurassic
rocks…
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Jurassic
rocks are
older than
Cretaceous
rocks…
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There are several
laws that should be
understood…
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1. Law of Original
Horizontality: sedimentary
layers and lava flows were
originally deposited as relatively
horizontal sheets, like a layer
cake.
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Just as these layers are in horizontal
sheets…
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2. Law of Superposition: the
oldest layer is at the bottom of
the sequence and the youngest
is at the top.
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YOUNGEST
OLDEST
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3. Law of Cross-Cutting
Relationships: Any feature that
cuts across a rock or body of
sediment must be younger than
the rock or sediment that it cuts
across such as fractures, faults
or igneous intrusions
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The igneous intrusion H cuts across the
layers A,B,C etc…so H is younger than all
the rest…
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4. Law of Inclusions: Rocks or
fossils that have become
included in another rock or body
of sediment must be older than
the rock or sediment into
which it has been incorporated.
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Trilobite existed first…then the orange
sediment was deposited later…trilobite
is older than the orange sediment…
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5. Law of Unconformities:
Gaps in the geologic record that
formed wherever layers were not
deposited for a time or else
layers were removed by erosion
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3 types of
unconformities
exist…
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Disconformity: missing layers…
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Layers are missing due to
erosion…
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Angular unconformity …layers on top of
tilted rocks…
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Rocks were tilted by tectonics, eroded and
new layers deposited
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Nonconformity …igneous intrusions
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This is a sequence of stratified rock. A is the oldest because it is
on the bottom of the series of layers. Each layer is distinct. Layer
D is the youngest because it is on top. This is a case of Original
Horizontality and Superposition.
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This is an example of an angular unconformity. Layers A, B, C
and D are tilted. They were originally horizontal until an episode
of tectonics forced these layers into a non horizontal position.
Event E is an erosional surface. It is an example of Cross
Cutting Relationships. F is the topmost layer and so it is the
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youngest. A is at the bottom, so it is the oldest.
Layers A, B, C, and D were originally horizontal until an episode of
tectonics formed these layers into a geologic structure called an
anticline. Erosional event E crosscuts the top of the anticline.
Layers F and G were deposited later, then igneous intrusion H
cross cut all layers, making it the youngest of all the events. 41
Igneous body A existed first, then came
along igneous intrusion B. C cut across
later making it the youngest, while A is the
oldest.
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Layers A, B and C were deposited first.
Fault D cut across A,B,and C. Erosional
surface F cut across Fault D and a layer of
shale E deposited on top of F.
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Color References
to Lab 8
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END
Lab 08
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