1. 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)
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3. 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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4. 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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5. 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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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).
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7. 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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Layers 1-4 are positioned
oldest to youngest
(superposition) 4 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
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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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10. 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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11. 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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15. 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
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Folds
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anticline
syncline
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18. 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
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REVERSE FAULT

24. Strike-Slip or Transform Faults
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TRANSFORM FAULTS – aerial view
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Fault Animations
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Can you date the features from oldest to youngest?
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