1. Deformation, Mountain Building, and Earth's Crust
Chapter 10
Deformation, Mountain Building, and Earth's Crust

2. Introduction
Deformation is a general term for any change in the shape or volume of a rock, such as when a rock is folded or fractured.
Deformation occurs in building large mountain ranges at convergent boundaries through:
Emplacement of plutons
Volcanism
Metamorphism
Continental accretion

3. Rock Deformation - How Does it Occur?
Stress is a force of deformation.
Strain is the change in shape that results when stress is applied.
Fig. 10.1a, p. 234

4. Rock Deformation- How Does it Occur?
Stress and Strain
When subjected to stress (force), ice on a pond may bend (elastic deformation), or if the stress is great enough, it will fracture, that is, the ice strained or deformed in response to stress.
Fig. 10.2, p. 235

5. Rock Deformation- How Does it Occur?
Types of Strain
Compression
Tension
Shear

6. Rock Deformation- How Does it Occur?
Types of Strain
Compression
In compression, the rocks are squeezed along the same line.
Compression shortens the rock layers by folding or faulting.
Fig. 10.3a, p. 236

7. Rock Deformation- How Does it Occur?
Types of Strain
Tension
In tension, the forces along the same line act in opposite directions.
Tension lengthens the rocks or pulls them apart; fractures and faults form.
Fig. 10.3b, p. 236

8. Rock Deformation- How Does it Occur?
Types of Strain
Shear
In shear, the forces act parallel to one another, but in opposite directions.
Deformation occurs along closely spaced planes like the slip between cards in a deck.
Fig. 10.3c, p. 236

9. (a) Compression (b) Tension (c) Shear stress Stepped Art
Fig. 10-3, p. 236

10. Rock Deformation- How Does it Occur?
Types of Strain
Rocks will deform elastically until they reach the elastic limit unless the force is applied quickly.
Elastic deformation or strain occurs if rocks return to their original shape when the stress is released.
Fig. 10.4, p. 236

11. Rock Deformation- How Does it Occur?
Types of Strain
Rocks will deform elastically until they reach the elastic limit unless the force is applied quickly.
Plastic deformation or strain occurs when rocks fold or fracture when stress is applied and do not recover their original shape.
Fig. 10.4, p. 236

12. Rock Deformation- How Does it Occur?
Types of Strain
What determines whether a rock will bend elastically, plastically or fracture?
Type of stress applied
Pressure and temperature
Rock type
Length of time
Fig. 10.4, p. 236

13. Rock Deformation- How Does it Occur?
Types of Strain
Rocks will deform elastically until they reach the elastic limit unless the force is applied quickly.
Ductile rocks show a great amount of plastic strain (they bend) before they fracture.
Brittle rocks fracture after only a small amount of plastic strain.
Fig. 10.4, p. 236

14. Strike and Dip-The Orientation of Deformed Rock Layers
Principle of original horizontality states that most rocks are originally laid down horizontally.
When we see rocks inclined, they have been deformed by folding and/or fracturing.
Fig. 10.6b, p. 238

15. Strike and Dip-The Orientation of Deformed Rock Layers
Strike and dip describe a rock layer's orientation with respect to the horizontal.
Strike is the intersection of a horizontal plane with an inclined plane.
Dip is the maximum angle of an inclined plane.
Fig. 10.5, p. 237

16. Deformation and Geologic Structures
Geologic structures are rocks that have been deformed.
Deformation includes fracturing and/or folding.
Fig. 10.1, p. 234

17. Deformation and Geologic Structures
Folded Rock Layers
Folds are layers of rock that were once planar that are bent or crumpled.
Folds form during compression and undergo plastic strain.
Fig. 10.1, p. 234

18. Deformation and Geologic Structures
Folded Rock Layers
Folding occurs deep in the crust where rock behavior is ductile.
There are 3 kinds of folds:
Monoclines
Anticlines
Synclines

19. Deformation and Geologic Structures
Folded Rock Layers
Folds have an axial plane that divides the fold in half.
Each half is called a limb.
The axis is an imaginary line formed by the intersection of the axial plane and the folded beds.
Figure 10.8, p. 238

20. Deformation and Geologic Structures
Folded Rock Layers
Monoclines
A monocline is a flexure in otherwise horizontal or uniformly dipping rock layers.
One limb is horizontal.
The fold axis is inclined.
Fig. 10.6, p. 238

21. Deformation and Geologic Structures
Folded Rock Layers
Anticlines and Synclines
Anticlines are up-arched folds. The oldest rocks are in the core.
Synclines are down-arched folds. The youngest rocks are in the core.
Figure 10.8, p. 254

22. Deformation and Geologic Structures
Folded Rock Layers
Anticlines and Synclines
Upright folds
Axial plane is vertical
Both limbs dip at the same angle.
Figure 10.7, p. 251

23. Upright, non-plunging folds
Fig. 10.9, p. 239

24. Deformation and Geologic Structures
Folded Rock Layers
Inclined and Overturned folds
In these folds the axial plane is inclined
Usually form under compression at convergent boundaries
Overturned folds have both limbs dipping in the same direction
Fig a,b, p. 239

25. Deformation and Geologic Structures
Folded Rock Layers
Recumbent folds
In these folds, the axial plane is horizontal or nearly horizontal.
Usually form under compression at convergent boundaries
Fig c, p. 239

26. Deformation and Geologic Structures
Folded Rock Layers
Plunging folds
Fold axis is not horizontal
Axial plane may be vertical or inclined
Fig a,b, p. 240

27. Plunging fold
Fig c, p. 240

28. Deformation and Geologic Structures
Folded Rock Layers
Domes and basins
Domes and basins are circular to oval structures which have rock layers occurring in age-position contexts which are the same as anticlines and synclines, respectively.
Fig , p. 241

29. Fig , p. 241

30. Black Hills of South Dakota: A Dome
Geo-Recap Fig. 1 p. 255

31. Deformation and Geologic Structures
Joints
Joints are fractures along which no movement has taken place parallel to fracture surface, although movement may occur perpendicular to the surface. They are not faults.
Fig a, p. 241

32. Deformation and Geologic Structures
Joints
Joints occur in almost all surface rocks.
They form in response to compression, tension, and shearing.
Joints may occur in two or three prominent sets.
Fig a, p. 241

33. Deformation and Geologic Structures
Faults are fractures along which the opposite sides have moved relative to one another and parallel to the fracture surface.
Fig a, p. 242

34. Deformation and Geologic Structures
Faults are fractures along which the opposite sides have moved relative to one another and parallel to the fracture surface.
Fig b, p. 242

35. Deformation and Geologic Structures
Faults are fractures along which the opposite sides have moved relative to one another and parallel to the fracture surface.
Fig c, p. 242

36. Deformation and Geologic Structures
Faults
Dip-slip Faults
All movement is in the direction of dip along dip-slip faults.
Dip-slip faults are categorized as normal or reverse.
Fig a, p. 242

37. Deformation and Geologic Structures
Faults
Dip-slip Faults
Normal faults form in response to tensional forces. The hanging wall moves down relative to the footwall.
Fig a, p. 243

38. Deformation and Geologic Structures
Faults
Normal faults
Fig a, p. 244

39. Deformation and Geologic Structures
Faults
Normal faults form from tension.
Fig , p. 248

40. Deformation and Geologic Structures
Faults
Dip-slip faults
Reverse faults form in response to compressional forces.
Fig b, p. 243

41. Deformation and Geologic Structures
Faults
Dip-slip faults
Thrust faults are a type of reverse fault that dip at less than 45 degrees, often as low as 5 degrees!
Fig c, p. 243

42. Deformation and Geologic Structures
Faults
Reverse Faults
Fig b, p. 244

43. Deformation and Geologic Structures
Faults
Strike-slip faults
Faults in which all movement is in the direction of the strike of the fault plane are known as strike-slip faults.
Strike-slip faults are classified as right-lateral or left-lateral, depending on the apparent direction of the offset between blocks.
Fig d, p. 243

44. Deformation and Geologic Structures
Faults
Strike-slip faults – San Andreas fault (right- lateral)
Fig a, p. 245

45. Deformation and Geologic Structures
Faults
Oblique-slip faults
Oblique-slip faults have both strike-slip and dip- slip components of movement.
Fig e, p. 243
Fig b, p. 245

46. Deformation and the Origin of Mountains
A mountain is an area of land that stands at least 300 meters above the surrounding country and has a restricted summit area.
A mountain range is a group of linear peaks and ridges that formed together.
A mountain system is a complex group of linear peaks and ridges that is composed of several mountain ranges. Mountain systems are the result of plate movements and interactions along plate boundaries.

47. Deformation and the Origin of Mountains
The Tetons Range is part of the Rocky Mountain system.
The Smoky Mountains are a range in the Appalachian mountain system.

48. Deformation and the Origin of Mountains
Mountain Building
Mountain building can involve faulting and folding, but can arise without these types of deformation.
Ways mountain form:
Volcanism
Erosion: mesas and buttes
Compression
Block-faulting - tension
Fig a, p. 248

49. Deformation and the Origin of Mountains
Plate Tectonics and Mountain Building
Orogeny – an episode of mountain building
Most orogenies are produced along convergent plate boundaries where one plate is subducted beneath another or where two continents collide.

50. Deformation and the Origin of Mountains
Plate Tectonics and Mountain Building
Orogeny – an episode of mountain building
Orogenies are accompanied by the emplacement of batholiths, metamorphism and thickening of the Earth’s crust.

51. Deformation and the Origin of Mountains
Plate Tectonics and Mountain Building
Orogeny – an episode of mountain building
Sedimentary rocks that formed in marine environments are often found emplaced high in the mountains as a result of orogenies.

52. Deformation and the Origin of Mountains
Plate Tectonics and Mountain Building
Orogenies are also closely associated with:
Mass Wasting, including land-slides
Glaciers
Running Water
Erosion is responsible for carving the most majestic peaks!

53. Deformation and the Origin of Mountains
Plate Tectonics and Mountain Building
Orogenies at Oceanic-Oceanic Plate Boundaries
Fig a, p. 249

54. Deformation and the Origin of Mountains
Plate Tectonics and Mountain Building
Orogenesis along oceanic-oceanic plate boundaries includes deformation, igneous activity, island arc formation, and metamorphism.
Fig b, p. 249

55. Deformation and the Origin of Mountains
Plate Tectonics and Mountain Building
Orogenies at Oceanic-Continental Plate Boundaries
Subduction of oceanic lithosphere along an oceanic-continental plate boundary also results in orogeny.
Fig c, p. 249

56. Deformation and the Origin of Mountains
Plate Tectonics and Mountain Building
Orogenies at Oceanic-Continental Plate Boundaries
Fig d, p. 249

57. Deformation and the Origin of Mountains
Plate Tectonics and Mountain Building
Andes Mountains of South America
Fig , p. 250

58. Passive continental margin
lithosphere
Passive continental margin
Sea level
Oceanic
Asthenosphere
Sediments
Oceanic
lithosphere
Continental
Asthenosphere
Sea level
Active continental margin
Oceanic
lithosphere
Continental
Deformation
Sea level
Asthenosphere
Stepped Art
Fig , p. 250

59. Deformation and the Origin of Mountains
Plate Tectonics and Mountain Building
Continent-Continent Boundaries
Mountain systems, such as the Himalayas, occur within continents, distant from present plate boundaries as the result of continent-continent collisions and suturing.
Fig , p. 251

60. Deformation and the Origin of Mountains
Terranes and the Origin of Mountains
Continental accretion – a process of adding material to a continent
Includes preexisting crust, as well as new plutons and volcanic rocks

61. Deformation and the Origin of Mountains
Terranes and the Origin of Mountains
Terranes are exotic pieces, fragments of seamounts or small pieces of continents that get transported on the plates.
Common along convergent oceanic- continental plate boundaries

62. Earth’s Crust
Continental crust is less dense and much thicker than oceanic crust.
This helps explain why mountains on land stand higher on continents than in ocean basins.

63. Earth’s Crust Floating continents?
Gravity allows the continents to "float" on the asthenosphere.
A gravimeter detects gravity anomalies.
Fig , p. 253

64. Earth’s Crust
Principle of Isostasy – Earth’s crust floats on the denser mantle.
Airy and Pratt’s Models
Airy model states that mountains have a low density root that allows them to project both far above and far below the surface.
Pratt model states that mountains are high because they are less dense than the adjacent rocks (like the mantle and the ocean crust).
Both models explain the behavior of the crust.

65. Earth’s Crust
Principle of Isostasy
Fig , p. 253

66. Earth’s Crust
Isostatic Rebound – When large glaciers melt or mountains erode away, the crust rises back up to its equilibrium level.
Rebound occurs slowly
Fig , p. 254

67. Earth’s Crust Isostatic Rebound
The ice sheets that covered Alaska and most of Europe 10,000 years ago have melted away but the continents are still rebounding!
Fig , p. 254

68. Earth’s Crust Isostatic Rebound
Scandinavia is rebounding at about 1 meter per century!
Fig , p. 254

69. Mountains Continental crust Mantle Low-density mountain root
Deposition
Uplift
Subsidence
Erosion
Transport
Stepped Art
Fig , p. 254

70. End of Chapter 10