The Relative Age of RocksPH 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

Time Detectives…Rock layers 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.