1. The Relative Age of Rocks
Chapter 7
Section 2

2. Two ways to express the age of a rock:
Relative Age – age compared to the age of other rocks
Absolute Age – number of years that have passed since the rock was formed.
Geologists often use a combination of both.
How Old Are Rock Layers??

3. Fossils are often found in layers of sedimentary rock
Fossils are often found in layers of sedimentary rock. Geologists use law of superposition to determine the relative age. The oldest layer is at the bottom
Rock Layers

4. Where is the oldest layer of rock located?
Questions:
Where is the oldest layer of rock located?
A. At the bottom
What is the relative age of the highest layer of rock?
A. It is younger than the rock layers below it
Why is it important that the rock layers be undisturbed?
A. If they are undisturbed, you know the oldest are at the bottom

5. Clues From Igneous Rock
See figures on p 248
Extrusion- Lava that hardens on the surface and forms igneous rock is called an extrusion. Extrusions are always younger than the rocks below it.
Intrusion- When magma pushes into the rock below the surface, then cools and hardens into a mass of igneous rock it is called intrusion. An intrusion is always younger than the rock around it.
Clues From Faults
A fault is a break in Earth’s crust. A fault is always younger than the rock it cuts through.

6. Geologists can find the relative age of a layer of rock at one location and match layers at other locations to that layer.
Index fossils help geologist do this.
An index fossil is a fossil that is widely distributed and existed for a short period of time.
How Do Fossils Show Age?

7. How Can Rock Layers Change?
The record of sedimentary rock layers is not complete.
Most have been lost to erosion.
When rock layers erode away, an older rock surface may be exposed. Then deposition begins again, building new rock layers. This area is called an unconformity.
Forces inside Earth fold rock layers so much that the layers are turned over completely. The youngest rock layers may end up on bottom.
How Can Rock Layers Change?

8. Questions: 1. What process could have made the sedimentary layers?
A- Deposition from water, wind, waves or glaciers
2. What could have caused the folding of the layers?
A- Forces from inside Earth can cause movement
3. What could have caused the wearing away of the surface?
A- Chemical or mechanical weathering, and then erosion by water, wind, waves or glaciers
Questions: