1. The Rock Record Chapter 8

2. “the present is the key to the past”
How is the Earth shaped?
Uniformitarianism – the belief that Earth has changed gradually and continuously over time.
“the present is the key to the past”
Catastrophism – All geologic changes occur suddenly.

3. Which one is true?
Modern geology is based on the idea that GRADUAL geologic change is interrupted by CATASTROPHIES.
Gradual processes such as erosion and weathering
Sudden changes like a meteor impact or a volcanic eruption

4. How can we tell how old a rock is?
There are two way to determine the age of a rock or rock layer…
Absolute Age
Relative Age

5. What is Absolute Age?
Absolute Age The age of a rock given as the number of years since the rock formed.
example: I am 15 and my sister is 19.

6. How is Absolute Age determined?
How do we determine exactly how old a rock is?
Radiometric Dating
Parent isotopes are compared to daughter isotopes to determine the rate of radioactive decay

7. Absolute Age and Radiometric Dating

8. What is Relative Age?
Relative Age The age of a rock compared to the ages of rock layers.
example: I am younger than my sister.

9. How is Relative Age determined?
Law (principle) of Superposition: The law states that the younger/newer rocks are on top of or above older rocks.

10. Everyday Examples
Pile of laundry. Bricks used to build a house. Your school folder. Stack of magazines.

11. Disturbing Forces
Rock layers will remain in horizontal layers if left undisturbed. However, movements in Earth’s crust and tectonic forces can move layers out of their horizontal positions.

12. How are rock layers disturbed?
Rock layers can be disturbed by forces from within the Earth.
rocks can intrude (magma)
rocks can tilt (shift in plates)
rocks can fold
faults form (earthquakes)
Because of these disturbances the sequence of the layers may be changed.

13. How do you determine when disturbances occurred?
Principle of Crosscutting Relationships
The principle of cross-cutting relationships states that a fault or intrusion is younger than the rocks that it cuts through. 

15. List in order from youngest to oldest.

16. Did you get the correct answer?
Youngest E D A B C Oldest

17. Geologic Time Brainpop: http://www. brainpop
Geologic Time Brainpop: Geologic Time Song & Video: