Dating Rock Layers
Relative vs. Absolute Age
• What is Relative Age?
– The age of an object or event in
comparison to another object or event.
• What is Absolute Age?
– The actual age, in years, of a rock or fossil.
• Tells how many years ago a rock or fossil was
formed.
Relative Age
• Methods of determining
Relative Age:
1.
2.
3.
4.
Law of Superposition
Extrusion & Intrusion
Faults
Unconformities
What is the Law of
Superposition?
• The Law of Superposition- in
undisturbed SEDIMENTARY
rock layers, the older layers
are at the bottom and
younger layers are
above it.
What is Extrusion?
• Extrusion is when magma
cools on the surface of the
earth.
–Forms a YOUNGER layer than
the rock below.
Here’s an Extrusion
Another Extrusion
What is Intrusion?
• Intrusion is when magma is
forced into older rock layers
and cools.
–Forms a YOUNGER layer within
the layers of rock.
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?
• Faults are a BREAK in the Earth’s
crust due to tectonic plates colliding and
rock layers being squeezed together.
– Faults are YOUNGER than the rock they
cut through.
– Faults move over time, which causes
rock layers to move.
• This makes it difficult to determine the age
or the surrounding rock.
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 Unconformity
• Unconformity is when new rock
layers meet a much older rock
surface beneath.
–Shows where
some rock
layers have
been lost due
to erosion.