Finding Relative Age of Rocks. Relative and Absolute Ages. 1. The relative age of a rock is its age compared to the ages of other rocks. 2. The absolute age of a rock is the number of years since the rock formed.
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1. The relative age of a rock is its age compared to the ages of other rocks.
2. The absolute age of a rock is the number of years since the rock formed.
3. Law of Superposition – younger rock layers are generally found above older rock layers.
5. Lava that hardens on the surface is called an extrusion.
6. The rock layers below an extrusion are always older than the extrusion.
7. Beneath the surface magma cools and hardens into a mass of igneous rock called an intrusion.
8. An intrusion is always younger than the rock layers around and beneath it.
9. The surface where new rock layers meet a much older rock surface beneath them is called an unconformity.
10. An unconformity is a gap in the geologic record.
11. Law or Original Horizontality – as sediments settle out of water, they are deposited in horizontal layers.
12. Law of Uniformitarianism – events occurring today also occurred in the past.
13. Geologists look for index fossils in layers of rock.
14. Index fossils are useful because they tell the relative ages of the rock layers in which they occur.