Relative Time

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Relative Time

• Dr. David Steer

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Introduction

• Relative Time - How do scientists determine the
  sequence of geological events?
• Absolute Time - How do geologists figure out the
  ages of rocks, and hence the age of Earth?

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How do you normally remember the order of events?
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Place these events in some logical order
A) MTV Founded
B) Persian Gulf War
C) JFK Assassinated
F) Challenger Explodes
E) Man lands On Moon
D) World Trade Center Destroyed
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Oldest to Youngest
1
2
3
E) Man lands On Moon
A) MTV Founded
C) JFK Assassinated
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F) Challenger Explodes
B) Persian Gulf War
D) World Trade Center Destroyed
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Three principles used to determine relative order
of geological events

• Principle of Superposition
• Principle of Cross-cutting Relationships
• Principle of Original Horizontality

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Superposition

• Layers (beds) at the bottom of a sequence of
  sedimentary rocks are oldest
• Rocks at the top are youngest

Youngest
Oldest
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Which layer is oldest?

1. Layer 1
2. Layer 2
3. Layer 3
4. All same

Layer 1
Layer 2
Layer 3
From http//www.geo.cornell.edu/geology/classes/g
eol326/photos.html
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Cross-cutting Relationships

• Older rocks may be cut by younger geological
  features
• Might be an igneous intrusion
• Might be filling of an eroded feature

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If I use a pen to poke a hole in the stack
What is the youngest event?
What does the pen do to the other units?
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Cross-cutting Relationships
Igneous rock cuts earlier layering
Layered rock formed first
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Which rock is the youngest?
A

1. A
2. B
3. C
4. D

B
C
D
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Which statement is false?

• D crosscuts B
• D crosscuts C
• C crosscuts D
• B was deposited
• before A

A
B
C
D
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Now tilt and bend the stack
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Original horizontality

• Sedimentary rocks are deposited in horizontal
  layers
• Tilted or folded layers indicate deformation
  following deposition

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How do we match sequences across large areas?

• Correlation used to determine relative ages of
  rocks across continents
• Correlation matches fossils and physical
  characteristics in sedimentary rocks

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Index Fossils
Best correlations use index fossils that
represent a short time period but are found
across a large region
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Which statement is true?

• Layer 1 is older than layer 3
• Layer 2 is the same age as layer 3
• Layer 3 is younger than layer 2
• Layer 2 is older than layer 1

Layer 1
Layer 2
Layer 3
From http//www.geo.cornell.edu/geology/classes/g
eol326/photos.html
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Place these units in order from oldest to youngest
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Draw this cross section
Unit B Basalt crosscuts all older units and
covers a shale unit. Event F Stream cuts
partially into limestone unit. Unit L Chemical
sedimentary rock that was deposited directly on
top of low silica extrusive unit. Unit M Oldest
unit is plutonic rock that was partly
eroded. Units S Sandstone that formed over the
non-shell bearing limestone rock unit. Unit R
Light colored extrusive rock that completely
crosscuts and covers all other units. Unit C
Small grained clastic rock deposited directly
over granite.
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Student Examples