1. Discovering Earth’s History

2. Two Methods to Date Rock Layers
Absolute
dating

3. Relative Dating
By studying the layers of rock exposed at the surface, geologists can infer the order in which the layers formed.
Places rocks in chronological order
Can determine the sequence of events by comparing rocks, but cannot tell us how long ago the events occurred.
Follows different “rules” to interpret what happened to the exposed rocks

4. The Rules to Follow
Law of Superposition - older rocks are found under younger rocks
Principle of Original Horizontality – sediment is deposited in flat, horizontal layers
Intrusions – pieces of rock that are contained within another rock layer

5. Rules Continued
Principle of Cross-cutting Relationships – A fault or intrusion must be younger than any other layer that it cuts through
Unconformities – A break in the rock record. 3 Types
Disconformity
Nonconformity
Angular Unconformity

6. Disconformity Nonconformity
Only with Sedimentary Rock - Erosion occurs before new layers are formed
Difficult to see since rock layers are very similar
Erosion occurs to metamorphic or igneous rocks and
then younger sedimentary layers are formed
Nonconformity

7. Angular Unconformity
New layers of sedimentary rock are deposited over older layers that have been tilted or folded

8. Practice – Which type of Unconformity?

10. Online Interactives

11. Practice

12. Correlation
Scientists compare rock layers at different locations to get a more complete picture of the rock record
The rock layers they compare formed at the same time and by the same processes.
Usually fossils are also used to help when areas being compared are on different continents