Geologic Principles andDating Rock Layers Part 2

Unconformities 16.

• Refer to “missing pieces” in the dating of rock layers.• Potential causes:• Erosion• Deposition• Tilting• Folding

• Video

Intrusive & Extrusive Relations

17.

• This geologic principle states: that invading igneous rock is always younger than the rock layers that it passes through.• Two types:

i. Igneous Intrusionsii. Igneous Extrusions

What are Intrusions?• Intrusion = when

magma breaks through rock layers above, but DOES NOT reach the Earth’s surface. It cools below ground.

• Intrusions are always younger than the rock around it.

• For intrusion, think inside.

18.

What are Extrusions?• Extrusion = when

magma breaks through rock layers above and DOES reach and cool at the Earth’s surface.

• Extrusions are always younger than the rock it is sitting on.

• For extrusion, think exterior.

19.

Intrusions vs. Extrusions

First of all, which rock layer is the oldest?What is this called?

INTRUSION

EXTRUSIONFG

C

D

A

B

E

More rock layers are deposited on top of the extrusion.

20.

Cross Cutting Relations• This geologic principle states: if something

cuts through the rock layers, the rock layers themselves must be older than the thing cutting through them.

21.

Faults• Fault = a fracture, or

break, in a layer of rock.

• A fault is always younger than the rock it cuts through.

• Faults move over time, making determining the age of the layers confusing.

22.

FaultsThis crack in the rock layers is called a fault.

Faults move over time causing the rock layers to move.

23.

Faults

As more time goes by, more rock layers are added on top. Making aging the rock layers difficult.

23.

Faults

A

A

B

B

C

C

DE

Geologists must then label each of the rock layers. This can be tough because they weren’t around to see the whole process occur.

23.

Faults• Types of Faults:

i. Normal Fault = occur when rocks are pulled apart and the crust is stretched (caused by tensional stress)

ii. Reverse Fault = occur when the crust is squashed and shortened (caused by compressional stress)

iii. Strike-Slip Fault = occur when forces push parts of the crust past one another (caused by shearing stress)

24.

Normal Fault

Reverse Fault

Strike-Slip Fault

(View of Strike-Slip Fault as seen from above)

Fossil Succession• This geological principle states: the species of

animals and plants found as fossils change through time. When we find the same kinds of fossils in rocks from different places, we know that the rocks are the same age.

25.

Review #2 26.

• Describe the history of these rock layers in terms of all the geological principles we covered.