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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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relative and absolute ages
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.

  • Sometimes geologists can determine a rock's absolute age to within a certain number of years!
how do we determine the relative age of a rock
How do we determine the relative age of a rock?

3. Law of Superposition – younger rock layers are generally found above older rock layers.

  • 4. The Law of Super-position helps determine the of relative age of fossils and rock layers.
clues from igneous rock
Clues From Igneous Rock

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.

gaps in the geologic record
Gaps in the Geologic Record

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.

how do we determine the relative age of a rock7
How do we determine the relative age of a rock?

11. Law or Original Horizontality – as sediments settle out of water, they are deposited in horizontal layers.

  • This law shows where the Earth has shifted and changed the placement of rock layers.

12. Law of Uniformitarianism – events occurring today also occurred in the past.

  • All the process we see today, such as volcanic activity, earthquakes, and mountain building were also occurring millions of years ago
using fossils to date rocks
Using Fossils to Date Rocks

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.