1. Folds, Faults, and Mountain Building
Chapter 9

3. Stress!!! Stress—a force exerted on an object Rocks react to stress by
Deforming plastically
Deforming elastically
Breaking by brittle fracture
Question of the hour (#1)
How will a particular rock react to a particular stress?

4. It’s all in the Nature… Rock composition
Ex: Hitting a chunk of concrete with a sledgehammer VS. hitting a giant rubber tire with a sledgehammer

5. I Can’t Take the Pressure!!
Pressure and Temp. increase with depth
Temp vs. pressure
Burial promotes plastic deformation

6. If you can’t stand the heat…
Higher temperatures favor plastic deformation
Ex. Bending a steel nail
Need to be super strong
Or just heat the thing up…

7. It’s About Time… Stress applied slowly favors plastic behavior
Stress applied suddenly favors brittle behavior

8. May the Tectonic Force be With You….
Plate tectonic forces
Tensional
Stretch and pull formations apart
Compressive
Squeeze and shorten rock formations
Shearing
Two sides of a formation are pushed in opposite directions (sliding past one another)

9. Geologic Structures Plate tectonics—primary stressor
3 main types of structures
Folds—ductile deformation (primarily)
Faults—brittle deformation
Joints—brittle deformation

10. Folds A fold is a bend in the rock Three characteristics
Plastic deformation
Three characteristics
Usually results from compression
Folding always shortens the horizontal distances in rock
A fold usually occurs as part of a group of folds

11. Anticline Vs. Syncline

12. Plunging Folds

13. Overturned and Asymmetric

14. Circular Features Domes Basin
Circular anticlinal structure (both pics)
Beds dip away from center
Basin
Circular synclinal structure
Beds dip towards center

15. All My Faults are Stress Related!!!
Brittle deformation
Faults
Cracks in rock along which motion has occurred
Movement may be gradual or sudden (earthquake)
Usually associated with other faults in a fault zone
Question of the hour (#2): Why do rocks move repeatedly along faults and fault zones?

16. Nature is Lazy
Rock moves repeatedly along faults and fault zones because
Tectonic forces often affect one location for a long time
It’s easier
Three types of faults
Depend on tectonic forces

17. Normal Faulting

18. Reverse Fault (part 1)

19. Reverse Fault (part 2)

20. Strike-slip Fault

21. Ah, How My Joints Ache…
A joint is a fracture in rock along which no motion has occurred
Planes of weakness—like mineral cleavage
Good for mining

22. Styles of Continental Deformation
Relating Small Structures to Big Picture

23. Question of the Hour (#3)
Where do mountains come from?
In a subducting zone
Magmas/lavas—stall/erupt
High temp—rocks expand when heated
Continent/continent collision—underthrusting
Crust is shoved under other crust
Compression squeezes crust—thickness increases

24. Tensional Tectonics Plates can split apart
Rift valleys – long narrow troughs formed by a block that has dropped down between two big normal faults

25. Rifting and Normal Faulting

27. Compressive Tectonics
Fold and thrust belts:
When two continental plates collide, crust compressed
Crust thickened by underthrusting—thicken crust 2x

28. Transverse Margins