Reading
the Rock
Record
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Rock Layers
Sediments and lava
flows form distinct
layers as they are
deposited
 A sedimentary layer
that can be
distinguished from
the layer above and
below is called a
stratum (pl. strata)

http://www.joshushund.com/terragen_01-06/wall_of_strata.jpg
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What Information do Strata Provide
to a Geologist?
To answer this question we need to look
at some geologic principles.
#1 Processes that occurred many millions
of years ago probably happened the
same way as they do today (because the
science principles are the same).
Uniformitarianism
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What Information do Strata Provide
to a Geologist?
#2 Sedimentary deposits or lava flows
generally form horizontal layers.
Principle of Original Horizontality
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What Information do Strata Provide
to a Geologist?
#3 Unless disturbed by an unusual
process, younger layers will be on top of
older layers.
Principle of Superposition
youngest
oldest
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http://www.gc.maricopa.edu/appliedscience/gjc-nsf/images/grndcynxsect.cv.jpg
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Here are the layers
in the Grand Canyon
shown graphically.
The Bright Angel
shale is younger
than the Tapeats
Sandstone but older
than the Muav
Limestone.
The Vishnu Schist
is the oldest
formation (made of
metamorphic rocks).
What Information do Strata Provide
to a Geologist?
#4 When an igneous intrusion or fault
cuts across other rocks of any type the
intrusion is younger than the layers that
are cut across.
Principle of Cross-Cutting Relationships
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4
Layers 1-4 are positioned
oldest to youngest
(superposition)
5
Formation 5 is
younger
than 1-4 since it
cuts through them
3
2
6
1
Fault 6 is youngest
because it cuts
through 5
http://www.winona.edu/geology/MRW/mrwimages/Earth%20History/cross_section.jpg
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The fault and dike
are called
non-conformities.
Notice that the dike
has incorporated
pieces of other
layers.
#6 When a feature contains pieces of other layers it has
to be younger than the layers it contains.
Principle of Inclusion
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What Information do Strata Provide
to a Geologist?
#5 Fossils can be used to determine the
approximate age of a stratum since specific
living organisms lived over identified time
periods. If a particular fossil is present in a
layer, the layer must be the same age as other
layers containing the same fossil, no matter
where the layers occur.
Principle of Faunal Succession
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How Can We Determine Age
More Accurately?
 Radioactive
elements are unstable and
their nuclei will release particles or
energy to stabilize.
 Each radioactive element has a half-life
– the time it takes for one half of the
atoms in a sample to disintegrate
 Some half-lives are fractions of a
second while some are as much as 4.5
million years.
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Radiometric Dating
After a 2nd
half-life has
passed half of
the remaining atoms
have also changed
After 1
half-life has
passed, half
the atoms
have changed
or emitted
energy.
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Determining Rock Ages
We can use radioactive elements in the
rocks to help determine age.
If the rock contains a radioactive element
like potassium-40, we can measure how
much of this element is present.
Knowing how much should be there we
can determine the rock age.
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Determining Rock Ages
Suppose a rock sample has 1/64th of the
expected amount of radioactive material.
1  1/2  1/4  1/8  1/16  1/32  1/64
This means that the sample has been through
6 half-lives.
If each half-life is 100,000 years then the
rock layer is 600,000 years old.
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Why Aren’t All Strata Straight?
Geologic processes generate pressures that
cause layers to become deformed.
 The first type of deformation is folding.

Click on image
to begin
animation
http://www2.wwnorton.com/college/geo/egeo/flash/9_1.swf
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Folds
http://www.glossary.oilfield.slb.com/DisplayImage.cfm?ID=10
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anticline
syncline
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Why Aren’t All Strata Straight?
Geologic processes generate pressures that
cause layers to become deformed.
 The second type of deformation is faulting.

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Faults
 Faults
occur when sections of Earth’s
surface shift position with respect to
another
 One side of the fault is designated as
a footwall block and the other a
hanging wall block
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Normal Faults
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Hanging wall moves down
Hanging wall moves down
NORMAL FAULT
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Reverse Faults
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Hanging wall moves upward
REVERSE FAULT
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Strike-Slip or Transform
Faults
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TRANSFORM FAULTS – aerial view
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Fault Animations
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http://www.winona.edu/geology/MRW/mrwimages/Earth%20History/cross_section.jpg
Can you date the features from oldest to youngest?
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