1. (7th) Chapter 8-2 Cornell Notes
The Relative Age of Rocks

2. Chapter 8-2 Key Questions
1. What is the law of superposition?
2. How do geologists determine the relative age of rocks?

3. Chapter 8-2 Key Terms relative age- absolute age-
law of superposition-
extrusion-
intrusion-
fault-
unconformity-
inclusion-
index fossil-

4. Chapter Paragraph 1
relative age: rock’s age compared to ages of other rocks;
absolute age: number of years since the rock is formed;
Example: sedimentary rock layers provide record of geologic history.

5. Chapter 8-2 Paragraph 2 absolute age difficult to measure;
relative age uses law of superposition;
law of superposition: lower the sedimentary rock layer the older it is.

6. Chapter 8-2 Paragraph 3 relative age methods:
extrusion: lava hardens on the surface, rests on older rock;
intrusion: magma pushes into rock, younger than surrounding rock;
fault: break in earth’s crust, younger than rock it cuts through.

7. Chapter 8-2 Paragraph 4
unconformity: new rock layers meet much older rock surface beneath them;
gap in geologic record;
possibly caused through erosion;
inclusion: present with unconformity; rock within a rock but older than surrounding rock.

8. Chapter Paragraph 5
rock layers dated through relative aging at different locations;
index fossils used;
index fossils: widely distributed fossils but relatively short life span;
examples: ammonites, trilobites, and brachiopods.