Transcript Relative Age 2013

The Relative Age of Rocks
PH 272-277
Essential Question
• How does the law of Superpostion help
geologists date rocks and what evidence do
they use?
Previously, in life
science…
You learned the process of
rock formation is very
slow. Now, you will
learn to compare the
ages of rocks.
To relative age
Santa Ana Mountains (your backyard)…
…and yes, there are marine fossils up here!
(how did they get up here?)
1. What are two ways geologists
determine the ages of rocks?
• Relative age and absolute age
Rock layers
Time
Detectives…
Atomic clocks
2. What is absolute age?
• the number of years since the rock formed
• An actual age of the rock in years.
3. What is relative age?
• the age of a rock compared to the ages of
other rocks.
• A comparison only…older or younger
4. What is the law of superposition?
• In horizontal layers of sedimentary rock, the
oldest layer is on the bottom…each higher
layer is younger than the layer below it.
Diagram:
Why do sedimentary rocks have
layers?
• The sediment that forms sedimentary rock
is deposited in layers!
In which layer would you find the
oldest rocks?
• The bottom layer, because they had to be
there first!
5. What are other ways to find the
relative ages of rocks?
• Geologists use extrusions, intrusions, faults,
gaps, and inclusions to determine relative
age.
Diagram of an extrusion:
• Lava that hardens on
the surface
• Always younger than
the layers below it.
Diagram of an intrusion:
• Lava that
hardens beneath
the surface
• Always
younger than
the layers
around it.
Diagram of a fault:
• A break in the
Earth’s crust
• Always younger
than the rock it
cuts through.
Diagram of an unconformity:
• A gap in the
geological record
• Rock layers may
be lost due to
erosion
Diagram 5
6. What is an index fossil?
• Fossils that help match rock layers
7. Why are index fossils useful?
• Because they tell the relative ages of the
rock layers in which they occur.
8. What fossils make good index
fossils?
•
•
Exist in many areas
But should exist only
briefly in time.
(found in lots of places, but
in only one layer)
What characteristics must a fossil
have to be a useful index fossil?
• It must be widely distributed, and it must
have existed for only a brief period of time.