Transcript Relative Time - Cloudfront.net

Relative Time
Dr. David Steer
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?

How do you normally
remember the order of events?
Place these events in some logical order
A) MTV
B) Persian Gulf
C) JFK
Founded
War
Assassinated
D) World Trade
Center Destroyed
E) Man lands
F) Challenger
On Moon
Explodes
Oldest to Youngest
1
2
3
C) JFK
E) Man lands
A) MTV
Assassinated
On Moon
Founded
4
5
6
F) Challenger
B) Persian Gulf
Explodes
War
D) World Trade
Center Destroyed
Three principles used to determine
relative order of geological events

Principle of Superposition
 Principle of Cross-cutting Relationships
 Principle of Original Horizontality
Superposition
Layers (beds) at the bottom of a sequence
of sedimentary rocks are oldest
 Rocks at the top are youngest
Youngest
Oldest
Which layer is oldest?
1)
2)
3)
4)
Layer 1
Layer 2
Layer 3
All same
Layer 2
Layer 1
Layer 3
From http://www.geo.cornell.edu/geology/classes/geol326/photos.html
Cross-cutting Relationships

Older rocks may be cut by younger
geological features
Might be an igneous intrusion
 Might be filling of an eroded feature

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?
Cross-cutting Relationships
Layered rock
formed first
Igneous
rock cuts
earlier layering
Which rock is the youngest?
1)
2)
3)
4)
A
B
C
D
A
B
C
D
Which statement is false?
1)
2)
3)
4)
D crosscuts B
D crosscuts C
C crosscuts D
B was deposited
before A
A
B
C
D
Now tilt and bend the stack
Original horizontality
Sedimentary rocks are deposited in
horizontal layers
 Tilted or folded layers indicate
deformation following deposition

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

Index Fossils
Best correlations
use index fossils
that represent a
short time period
but are found
across a large
region
Which statement is true?
1)
2)
3)
4)
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/geol326/photos.html
Place these units in order from oldest to youngest
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.
Student Examples