HOW CAN WE
APPLY THE
PRINCIPLES OF
SUPERPOSITION?
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Applying the Principles of
Superposition
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The principle of superposition must be used
with care because certain events can disturb
the positions of rock layers.
Forces within the earth may tilt, fold, or fault
rock layers.
Older layers may be pushed on top of younger
ones.
In such cases, it is necessary to work out the
original positions of the rock layers before
applying the principle of superposition.
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In general, a rock layer is
older than any joint, fault, or
fold that appears in it; the
rock had to already exist in
order to be folded or faulted.
By unfolding or unfaulting
the rock layers, one can
determine their positions
before they were disturbed.
The principle of
superposition can then be
applied to determine relative
ages.
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If we were to look only at present
position, we would conclude that
layer C is the youngest and A is
the oldest because C is on top and
A is on the bottom.
If we look at the entire rock
structure, however, we see that
three layers of rock have been
folded into a syncline.
Since the fold is a syncline, we
can assume that the sides have
been pushed upward.
By straightening the limbs of the
fold, we place the layers in their
original positions and see that
layer A is the youngest while C is
the oldest.
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The same is true of faults, that is,
fractures along which the rocks on
either side have moved.
If rock layers fracture and then
move along that fracture, they are
displaced.
Any rock displaced by a fault is
older than the fault.
During faulting, underlying rock
layers may be pushed up so that
they are found on top of younger
rock
Again, to obtain true relative ages,
one must work backwards to
determine the positions the rock
layers were in before the fault
offset them.
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Igneous intrusions or extrusions are
often found in association with other
types of rock.
Igneous intrusions form when
molten rock forces its way into
preexisting rock, cools, and hardens.
Thus, an intrusion is younger than
any rock it cuts through.
Extrusions form during volcanic
eruptions when molten rock flows
out onto the Earth's surface as lava
and hardens, or is blown into the
atmosphere and settles on the
ground, forming a blanket of
volcanic rock particles.
Thus, extrusions are younger than
the rocks beneath them, but older
than any layer that may form over
them.
Therefore, if an igneous body is
found in rock, one must first
determine whether it is an intru-sion
or an extrusion before relative ages
can be established.
Unconformities
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Sometimes movements of the earth's crust lift up rock layers that were buried and expose these
layers to erosion.
Later, if the eroded surface is lowered or the sea level rises, sediments will again be deposited,
forming new rock layers.
The missing rock layers create a break in the geologic record, just like when pages are missing
from a book.
This break in the geologic record is called an unconformity.
An unconformity indicates that for a period of time deposition stopped, rock was removed by
erosion, and then deposition resumed.
Nonconformity
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Sedimentary rock is stratified, or deposited in layers.
Metamorphic and igneous rocks are usually unstratified.
An unconformity in which stratified rock rests upon
unstratified rock is called a nonconformity.
For example, unstratified rock such as granite forms deep
within the earth.
The granite may be lifted to the earth's surface by crustal
movements.
Once exposed, the granite begins eroding.
Sediments may then be deposited on the eroded surface.
The boundary between the sandstone and the granite layers is a
nonconformity.
It represents an unknown period of time during which the
granite was eroded.
Angular Unconformity
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Another type of unconformity results when rocks
deposited in horizontal layers are folded or tilted and
then eroded.
When erosion stops, a new horizontal layer is
deposited on a tilted layer.
The boundary between the tilted layers and the
horizontal layer is called an angular unconformity.
The bedding planes of the older rock layers are not
parallel to those of the younger rock layers deposited
above the angular unconformity.
Disconformities
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Sometimes layers of sediments on the ocean floor are
lifted above sea level without folding or tilting.
Exposed to wind and running water, the surface
layers are eroded.
Eventually, the area again falls below sea level and
deposition resumes.
The boundary between the older, eroded surface and
the younger, overlying layers is nearly horizontal and
is called a disconformity.
Although the rock layers look as if they were
deposited continuously, a large time gap exists where
the upper and lower layers meet at the unconformity.
Crosscutting Relationships
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When tectonic activity has disturbed rock layers, determining
relative age using only the law of superposition may be
difficult.
In such cases, scientists may also apply the law of crosscutting
relationships.
The law of crosscutting relationships states that a fault or an
intrusion is always younger than the rock layers it cuts
through.
A fault is a break or crack in the earth's crust along which
rocks shift their position.
An intrusion is a mass of igneous rock formed when magma is
injected into rock and then cools and solidifies.
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Crosscutting relationships can be extremely complex, and
careful ob-servation is necessary to accurately date rocks.
As you can see, on the previous slide, an intrusion cuts across
layers A, B, and C. According to the law of crosscutting
relationships, the intrusion is younger than layers A, B, and C.
Look at the fault.
What is the relative age of the fault compared with that of the
rock layers?
Both the law of superposition and the law of crosscutting
relationships can be applied to unconformities.
According to the law of superposition, all rocks beneath an
unconformity are older than those rocks above the
unconformity.
If a fault or intrusion cuts through the unconformity, the fault
or intrusion is younger than all the rocks it cuts through above
and below the unconformity.