1. Dynamic Earth
Class 15
28 February 2005

2. The Flow of the Continents (Chapter 5) Building Mountains: Rock Deformation and Earthquakes

3. Rock Deformation
Large scale deformation of the Earth’s crust = Plate Tectonics
Smaller scale deformation = structural geology

4. Deformation
Changes in volume or shape of a rock body
= strain

5. Deformation of rocks
Folds and faults are geologic structures
Structural geology is the study of the deformation of rocks and the effects of this movement

6. Small-Scale Folds

7. Stress
The force that acts on a rock unit to change its shape and/or its volume
Causes strain or deformation
Stress
Compression
Tension
Shear

8. Compression
Action of coincident oppositely directed forces acting towards each other

9. Tension
Action of coincident oppositely directed forces acting away from each other

10. Shear
Action of coincident oppositely directed forces acting parallel to each other across a surface in a couple

11. Differential stress

12. Strength Ability of an object to resist deformation
Compressive or tensile

13. Strain
Any change in original shape or size of an object in response to stress acting on the object

14. Kinds of deformation
Elastic vs Plastic
Brittle vs Ductile

15. Elastic Deformation
Temporary change in shape or size that is recovered when the deforming force is removed

17. Ductile (Plastic) Deformation
Permanent change in shape or size that is not recovered when the stress is removed
Occurs by the slippage of atoms or small groups of atoms past each other in the deforming material, without loss of cohesion

18. Increasing stress
Increasing strain

19. Brittle Deformation (Rupture)
Loss of cohesion of a body under the influence of deforming stress
Usually occurs along sub-planar surfaces that separate zones of coherent material

20. Increasing stress
Increasing strain

21. Factors that affect deformation
Temperature
Pressure
Strain rate
Rock type
The variation of these factors determines if a rock will fault or fold.

22. Rocks are: Elastic and brittle near the earth's surface
More plastic and ductile deeper in the crust
Because of the increasing temperature and pressure

23. Folds
Most common ductile response to stress on rocks in the earth's crust

24. Experimental Deformation of Marble
Brittle Deformation
(low confining pressure)
Ductile Deformation
(high confining pressure)

25. Tectonic Forces and Resulting Deformation

26. Geometry of Anticlines & Synclines

27. Types of Folds (bent planar structures)
anticline: older rocks on the inside
syncline: older rocks on the outside
(scale - from mm to tens of km)

28. Anticlines and Synclines

29. Syncline

30. Fold Terms
axial plane: the plane of mirror symmetry dividing the fold into two limbs
axis: line formed by the intersection of the axial plane and a bedding plane
horizontal fold: where the fold axis is horizontal
plunging fold: where the fold axis is not horizontal

31. Fold Terminology

32. Symmetrical, Asymmetrical and Overturned Folds

33. Asymmetric Folds

34. Overturned Folds

35. Map View of Plunging Folds

36. Oil Field at Crest of Plunging Anticline

40. Fracture Most common brittle response to stress
With No displacement = Joint
With displacement = Fault

41. Joints commonly form when the surface of a volcanic rock cools and contracts

43. Devil’s Tower, Wyoming

44. Faults
Occur when large stresses build up in the crust, often due to lava movement into shallow magma chambers
Classified according to the kind of movement that has occurred along them

46. Normal Fault
Hanging wall
drops down

47. Tom Bean

49. Rift Valley Formed by Extension

51. Wildrose Graben, Southern California

52. Reverse Fault
Hanging wall
moves up

53. Reverse Fault

54. Large-Scale Overthrust Sheet

55. Keystone Thrust Fault, S. Nevada
Cambrian Limestone
Jurassic
Sandstone

56. Cross Section of the Keystone Thrust West of Las Vegas

57. French Thrust, Wyoming
Mississippian
Limestone
Cretaceous Shale

58. Strike-slip
Fault

62. Faults may move several meters in a second or so
This movement within the earth's crust usually generates an earthquake

63. Global Locations of Earthquakes

64. First-motion studies of earthquakes indicate direction of movement on faults
Fig

65. Tuesday
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