Relative DatingRelative Dating(No, not your cousin)(No, not your cousin)

Finding the Relative Age of RocksFinding the Relative Age of Rocks

Relative Dating and Relative Age

1. Scientists use relative dating to determine whether a rock is older or younger than another rock.

2. The relative age of a rock is its age compared to the ages of other rocks around it.

How do we determine the relative age of a rock?

3. Law or Original Horizontality – states that 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.

4. Law of Uniformitarianism – states that events occurring today also occurred in the past.

– All the processes we see today, such as volcanic activity, earthquakes, and mountain building were also occurring millions of years ago.

How do we determine the relative age of a rock?

5. The Law of Superposition – states that younger rock layers will be found above older rock layers as long as the layers have not been disturbed.

6. The Law of Super-position helps determine the of relative age of fossils and rock layers.

The Geologic Column - The geologic column (stratigraphic column) is a detailed sequence of rock layers that contains all the known fossils & rock formations on Earth, ordered from oldest to youngest.

- Scientists use the geologic column to help them interpret rock sequences especially if the rock layers have been disturbed.

- Geologists created the geologic column by combining information from all over the world.

Disturbed Rock7. Gravity deposits sediment in flat, horizontal layers. If the layers are NOT horizontal they have been disturbed.

8. Folding and tilting of rock layers is caused by pressure and forces within the earth.

Disturbed Rock9. Earth may shift along faults which causes rock layers to not match up.

10. The faults are always younger than the rock layers.

* Igneous rock can give many clues to the relative age of rock

Clues From Igneous Rock

11. Lava that hardens on the surface is called an extrusion. (extrusive igneous rock)

12. The rock layers below an extrusion are always older than the extrusion.

13. Beneath the surface magma cools and hardens into a mass of igneous rock called an intrusion. (intrusive igneous rock)

14. An intrusion is always younger than the rock layers around and beneath it.

Gaps in the Geologic Record

15. The surface where new rock layers meet a much older rock surface beneath them is called an unconformity.

16. An unconformity is a gap in the geologic record.

Gaps in the Geologic Record17. Three types of Unconformities:

a. Disconformities

b. Nonconformity

c. Angular Unconformity

18. All Unconformities are formed in one of two ways:

1. A period of no sediment deposit.2. A period of erosion which removed

sediment .

Unconformity (disconformity)

Unconformity (nonconformity)

Unconformity (angular unconformity)

Date That Rock Layer!

Date That Rock Layer!

Date That Rock Layer!

• Geologists must be able to read all of these and much more difficult rock sequences to determine relative age

• Since the earth has been changing for billions of years this gets very tricky!