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

2. Chapter 8-2 Key Questions What is the law of superposition? How do geologists determine the relative age of rocks? How are index fossils useful to geologists?

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

4. Chapter 8-2 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 8-2 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.