Dating Rock Layers
How to tell how old the layers of
rock are.
What is Age?
• There are 2 kinds:
– Absolute Age – The number of years since
the rock formed. (150 million years old, 10
thousand years old.)
– Relative Age – The age compared to the
ages of other rocks. (older than this rock,
younger than that rock.)
Age cont.
• Relative Age – Much easier to figure out.
Can be deduced from clues on the Earth.
• Absolute Age – Much harder to figure
out. Often impossible.
What are the ways to tell
RELATIVE AGE.
1.
2.
3.
4.
Law of Superposition
Extrusion & Intrusion
Faults
Unconformities
What is the Law of Superposition?
In horizontal sedimentary rock layers, the
oldest layer is at the bottom. Each higher
layer is younger than the layers below it.
Age and Superposition
What is Extrusion?
• When magma cools on the surface of the
Earth it’s called an EXTRUSION.
• Extrusions are always YOUNGER than the
rock it is sitting on.
• For extrusion think exit or outside.
What is an Intrusion?
• When magma cools beneath the surface
of the Earth it’s called an INTRUSION.
• Intrusions are always YOUNGER than the
rocks around it.
• For intrusion think inside.
Here’s an Intrusion
Extrusions? Intrusions?
G
First of all, What
whichisrock
this layer
called?
is the oldest?
F
EXTRUSION
E
D
More rock layers are deposited on
top of the extrusion.
INTRUSION
C
B
A
What is a Fault?
• A BREAK in the Earth’s crust.
• A fault is always YOUNGER than the rock
it cuts through.
• Faults move over time, making
determining the age of the layers
confusing.
Here’s a Fault
Faults
Faults
move
timelayers
causing
the
This crack
in over
the rock
is called
rock
layers to move.
a fault.
Faults
As more time goes by, more rock
layers are added on top. Making
aging the rock layers difficult.
Faults
E
D
Geologists must then label each of
C the rock layers. This can be tough
because they weren’t around to see
the whole process occur.
B
A
C
B
A
What is an Unconformity?
• The surface where new rock layers meet a
much older rock surface beneath them.
• An unconformity shows where some rock
layers have been lost because of erosion.
Unconformity
Summary
•
•
Relative vs. Absolute Age
Determining Relative Age:
1.
2.
3.
4.
5.
Law of Superposition – on top is younger
Intrusions – magma inside Earth.
Extrusions – magma on top of Earth.
Faults – breaks in the crust
Unconformities – signs of erosion.