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 AGERELATIVE AGE.

1. Law of Superposition

2. Extrusion & Intrusion

3. Faults

4. 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?

First of all, which rock layer is the oldest?What is this called?

INTRUSION

EXTRUSIONF

G

C

D

A

B

E

More rock layers are deposited on top of the extrusion.

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

FaultsThis crack in the rock layers is called a fault.

Faults move over time causing the rock layers to move.

Faults

As more time goes by, more rock layers are added on top. Making aging the rock layers difficult.

Faults

A

A

B

B

C

C

DE

Geologists must then label each of the rock layers. This can be tough because they weren’t around to see the whole process occur.

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. Law of Superposition – on top is younger

2. Intrusions – magma inside Earth.

3. Extrusions – magma on top of Earth.

4. Faults – breaks in the crust

5. Unconformities – signs of erosion.