The Relative Age of Rocks

1. The Relative Age of Rocks Section 4.2

2. How old are rock layers? • The relative age of a rock is its age compared to the ages of other rocks. • Ex. Mrs. Herrscher is older than her sister and brother. • The absolute age of a rock is the number of years that have passed since the rock formed. • Ex. Mrs. Herrscher is 33 years old. • It may be impossible to know a rock’s absolute age exactly, so geologists often use both absolute and relative ages.

3. How old are rock layers? • According to the law of superposition, in undisturbed horizontal sedimentary rock layers, the oldest layer is at the bottom. • Each higher layer is younger than the layers below it. • The principle of original horizontality states that most sedimentary layers of rock are deposited in a horizontal position

4. Clues from Igneous Rock • Lava that hardens on the surface and forms igneous rock is called an extrusion. • An extrusion is always younger than the rocks below it. • When magma cools and hardens into a mass of igneous rock beneath the surface, it is called an intrusion. • An intrusion is always younger than the rock layers around and beneath it.

5. Clues from Fossils • A fault is a break in the Earth’s crust. • A fault is always younger than the rock it cuts through. Why? • An index fossil is a fossil that is widely distributed and represent an organism that only existed for a short period of time. • Index fossils are useful because they tell the relative ages of the rock layers in which they occur.

6. Clues with Fossils

7. How can rock layers change? • Gaps in the geologic record and folding can change the position in which rock layers appear. • An unconformity is a gap in the geologic record. It shows where rock layers have been lost doe to erosion.