geological time the present is the key to the past
TRANSCRIPT
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Geological TimeThe Present is the Key to the Past
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Interpreting Geologic History
• The Earth was thought to have formed about 4.6 billion years ago.
• Geologists try to put the pieces of the Geologic past back together using several methods.
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Interpreting Geologic History
• Uniformitarianism• Original
Horizontality• Law of
Superposition• Intrusions and
Extrusions• Folds and Faults• Unconformities
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Interpreting Geologic History
• Uniformitarianism – The processes we see today are similar to those that have been taking place throughout geologic time.
“The present is the key to the past.”• Original Horizontality – Rock is
always older than the process that changed it. Ex. Tilting, folding, faulting. Layers were always flat first.
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Law of Superposition
• Rocks found at the bottom of an undisturbed exposure are usually the oldest.
D, K, A, G
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Intrusions and Extrusions
• Melted Rock can often rise and cut through sedimentary layers.
• If the magma solidifies in the earth it will form an igneous intrusion surrounded by contact metamorphism.
• If the magma reaches the surface (Lava) It can form an igneous extrusion.
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Intrusions and Extrusions
Intrusion
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Intrusions and Extrusions
R, Y, C, Z, J
An Intrusion is probably going to be Granite or Gabbro
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Intrusions and Extrusions
Extrusion
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Intrusions and Extrusions
R, Y, C, Z, J
An Extrusion is probably going to be Basalt or Rhyolite
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Intrusions and Extrusions
R, Y, C, Z, J, M
A Buried Extrusion Is Still An Extrusion
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Intrusions and Extrusions
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Faults and Folds
• Faults and Folds are a result of the Earth’s shifting tectonic plates.
• Folds are Bends in the rock layers.• Folds are the EXCEPTION to the law
of superposition.• Faults are cracks where layers are
offset from each other.• REMEMBER: rocks must be older then
the process that alters them.
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Folds
Lets add some heat and pressure
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Folds
Lets Convert the colors to Symbols
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Folds
Lets add some more heat and pressure
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Folds
If a Fold is so extreme that it bends back over itself it is called an Overturn
Lets take a closer look
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Folds
Young
old
Young
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Folds
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Folds
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Faults
R, Y, C, Z, J, M, K
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Faults
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Faults
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Unconformities
• An Unconformity is a buried erosional surface.
• They happen when an area of bedrock is weathered and eroded and then buried again by another layer of sedimentary rock.
• This can happen to any kind of rock; sedimentary, igneous, metamorphic, folded, tilted or faulted.
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Unconformities
Erosional Agent
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Unconformities
Unconformity
Remember!! An unconformity is not an unconformity until it is buried!!
R, Y, C, Z, J, H, P
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Let’s Try Some Relative Age Dating
____ Young____________________________________________________ Old
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Geologic History
• The Geologic Timeline is divided up into different units of time.
• The largest unit is the Eon, often made up of billions of years.
• Each Eon is divided up into Eras.• Each Era is made up of Periods.• Each Period is made up of Epochs.
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Geologic History
• In your ESRT you have a chart that has a timeline of geologic history.
• This chart primarily focuses on New York State.
• It is very important that we understand how to read this chart.
• It includes information about rock type, life-forms, fossils, geologic events and continental layouts throughout geologic history.
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Geologic History
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Geologic History
Let’s try a sample question
During what Period did the earliest birds appear?
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Geologic History
Jurassic
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Radioactive Decay
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Radioactive Decay
• Radioactive Decay refers to the breakdown of unstable elements into stable elements.
Unstable element
Stable Element
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Radioactive Decay
• The rate at which these unstable (radioactive) elements decay is predictable and constant.
• This rate can enable us to perform Absolute Dating.
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Absolute Dating
• Where Relative Age Dating can only give us the age of material as it relates to other material… (which came first, last, before etc…)
• Absolute Dating can give us a measurable value for the age of material.
• This is because of that predictable decay rate called Half-Life.
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Half-Life
• Half-Life is defined as the amount of time it takes for ½ of an unstable (radioactive) element to decay into a stable (daughter/decay) element.
• Only half of the radioactive material will decay per each Half-Life.
• This means that there can never be 0% radioactive material remaining or 100% decay material created.
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Half-Life
Radioactive
Stable
Here we start with 100% Radioactive Material
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Half-Life
Radioactive
Stable
After 1 half-life 50% of the radioactive material has decayed into stable material
Let’s see what happens after the second Half-Life
50% 50%
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Half-Life
Radioactive
Stable
After 2 Half-Life’s the remaining radioactive material has now decayed half way. Only 25% of
the original is remaining.
25%
75%
…and a third
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Half-Life
Radioactive
Stable
After a 3rd Half-Life there is only 12.5% of the original radioactive material remaining.
12.5%
87.5%
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Half-Life
The chart for any Radioactive Material will look like this.