Geologic Principles
and
Dating Rock Layers
Part 1
How old is Earth?
• The Earth is about 4.6 billion years old.
• Much of Earth’s history is recorded in the rock.
• There are two main areas of research that help
scientists date the materials they find in the Earth:
i. Relative Dating
ii. Absolute Dating
1.
Relative Dating
Absolute Dating
• Provides an order of
items/events in
history.
• Uses several
principles to create
an order of events.
• Provides a numerical
age of items/events
in history.
• Uses the physical and
chemical
composition of a
material to calculate
the age.
2.
Uniformitarianism
• An all encompassing law that states: all the
geological processes that occur today also occurred
in the past in some ways.
• These geologic processes include:
•
•
•
•
•
•
Weathering
Erosion
Deposition
Plate Movement
Volcanism
Earthquakes
3.
Uniformitarianism
• Example #1
i.
The same geologic process, erosion, that has
created the Grand Canyon over the past 40 million
years, is still in effect today, evidenced by the Big
Darby Creek.
4.
Uniformitarianism
• Example #2
i.
It is possible to see lava from volcanoes in Hawaii
cooling to form a black rock known as basalt. This
black rock is also found at Mt. Gambier, although
no volcanoes are present. We therefore conclude
that volcanic activity once occurred in the Mt.
Gambier area.
5.
Relative Dating
6.
Principles of Relative Dating
• Superposition
• Original Horizontality
• Lateral Continuity
• Intrusive/Extrusive
Relationships
• Cross Cutting Relationships
• Fossil Succession
7.
8.
Superposition
• This geologic principle states: the rock layers on the
bottom are the oldest and the rock layers on the top
are the youngest.
i.
Unless the rock layers are overturned or folded.
• Think of Superposition like a laundry basket full of
dirty clothes. Video Analogy
Layer A
Which layer is the youngest?
Which layer is the oldest?
Layer B
Layer C
9.
Superposition
A.
B.
C.
D.
E.
F.
1. Which rock layer is
the oldest?
2. Which rock layer is
the youngest?
3. Two similar fossils
were found in rock
layer C and E. Which
fossil is older? How
do you know?
10.
Superposition
1. Layer F
2. Layer A.
3. The fossil found in
Layer E is older than the
fossil found in Layer C.
A.
B.
C.
D.
E.
F.
Original Horizontality
• This geologic principle states: sediments are
deposited in flat lying or horizontal layers that are
parallel to the surface on which they were deposited.
• This means that if rocks are tilted or folded, they
have been deformed after they were deposited.
11.
Original Horizontality
• Original Horizontality Animation
12.
Lateral Continuity
• This geologic principle states: rock layers or
sediments that are separated by a valley or other
erosional feature were, at one time continuous.
13.
Lateral Continuity
14.
Review #1
• Describe the history of these rock layers in
terms of the geological principles we have just
covered.
15.
Review #1: Example Answer A
• The rock layers above formed in the following
order from earliest to most recent: A, B, C, D, E,
F, and G. After rock layer E formed, erosion
occurred, which caused a valley to form.
Review #1: Example Answer B
• The Principle of Superposition suggests that rock layers
on the bottom are older than the rock layers on top, so
the correct order from earliest to most recent is A, B, C,
D, E, F, and G. A portion of layers C, D, and E were
removed due to erosion, but the Principle of Lateral
Continuity states that these layers were once
continuous, which means the layers across the valley
were formed at the same time.