Chapter 13 Section 2 – Relative Ages of Rock

1. Chapter 13 Section 2 – Relative Ages of Rock

2. What you can do after we are finished with Section 2 • Describe methods used to assign relative age to rock layers • Interpret gaps in rock layers • Give example of how rock layers can be correlated with other rock layers.

3. Rock layers • Sediments accumulate and form layers of sedimentary rock • 1st layer on bottom • 2nd layer on top of first • And so forth

4. Principle of Superposition – process of reading undisturbed rock layers. • Oldest rock in the bottom layer • Younger rock in the top layers

6. Relative age • Discuss the example in the book using magazines. • Now write your own example of how relative age works.

7. How old something is in comparison with something else is its relative age. • The age of undisturbed rocks can be determined by examining layer sequences. • The age of disturbed rocks may have to be determined by fossils or other clues. • Relative age does NOT tell you the actual age of the rock. ( you only know it is older or younger than the rock above/below it.)

8. Turned Upside down!!! Sometimes rocks can be folded under by mountain making. It is more difficult to determine the relative age.

9. Unconformities • Gap in rock sequence • Develop from: erosion, running water, glaciers, etc.

10. Unconformities – gaps in rock layers • Angular unconformity - rock layers are tilted and uplifted • younger sediments layers are deposited horizontally on top of the eroded and tilted layers.

11. Unconformities – gaps in rock layers • A layer of horizontal rock exposed and eroded before younger rocks formed over it is called disconformities.

12. Unconformities – gaps in rock layers • Nonconformity - sedimentary rock forms over eroded metamorphic or igneous rock that has been uplifted and eroded

13. The same rock layers can be found in different locations; fossils can be used to correlate those rock layers. • Rocks from different areas may be correlated if they are part of the same layer.