geologic history the rules of the game © copyright 2005 - 2006 m. j. krech. all rights reserved
TRANSCRIPT
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GEOLOGIC HISTORY
THE RULES OF THE GAME
© Copyright 2005 - 2006 M. J. Krech. All rights reserved.
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Time
Whatis
time?
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Time is…
Either relative or absolute
Time is: a measured period during which an event, process, or condition exists or continues.
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One Year =
the time it takes the Earth to complete one orbit around the Sun.
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Geologic Time
• Based on the same time units
• Generally much greater periods of time
• 100,000 years• 1,000,000
years
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Dating Techniques
One ways in which geologists look at time: Relative Age
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Relative Age of a rock…
…is its age compared to the ages of other rocks.
What happened first?What happened next?
Example:
My niece, Sara, is older than my other niece, Mariam.
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Absolute Age of a rock…
…is the number of years since the rock formed.
Example:
My niece, Sara, is 3 years old and my niece, Mariam, is 3 months old.
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Relative and Absolute Age
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Law of Superposition
• The youngest rocks are on the top.
• The oldest rocks are on the bottom.
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Principle of Superposition
•Which rocks are older?
•Which rocks are younger?
Q#1
Younger Rocks
Older Rocks
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Principle of Cross-Cutting
• Fault is a break or cut in the rock.
• You can't cut something until that something exists, therefore the cut has to be younger.
• So faults are younger than the rock it cuts through.Volcanic vents cutting across
sedimentary rock layers -Which is youngest?
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Principle of Cross-Cutting
•What is younger?
•The rocks or the fault line?
Q#2
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Intrusions
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Principle of Inclusions
The inclusion is always younger than the rock layers around and beneath it.
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Principle of Inclusions
Which is older? The rock layer or the rocks included in the layer?
Q#3
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Principle of Unconformities
Unconformities are surfaces that represent gaps in the geologic record.
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Example of Unconformity
• http://www.classzone.com/books/earth_science/terc/content/visualizations/es2902/es2902page01.cfm?chapter_no=visualization
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Principle of Unconformities
missing rock layer = missing time
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Where is the youngest rock?Q#4
How do you know?
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Where is the unconformity?Q#5
How do you know?
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What rock layer is the oldest?Q#6
How do you know?
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Which is younger? The fault or the rock layers?
Q#7
How do you know?
FAULT
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Which is younger? The rock layer or the
inclusion?
Q#8
How do you know?
inclusion
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Using Fossils to Date Rocks
• Give relative age to a layer of rock• Match layers of rock at other locations• Use index fossils to match rock layers• Index fossils are distributed in
different areas & only occur in one area
• Ex. pg. 117