physical geology and the environment

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PHYSICAL GEOLOGY AND THE ENVIRONMENT (2ND CANADIAN EDITION) Chapter 11: Geologic Structures Chapter Summary: Tectonic forces result in deformation of the Earth's crust. Stress (force per unit area) is a measure of the tectonic force and confining pressure acting on bedrock. Stress can be compressive, tensional, or shearing. Strained (changed in size or shape) rock records past stresses, usually as joints, faults, or folds.

A geologic map shows the structural characteristics of a region. Strike and dip symbols on geologic maps indicate the attitudes of inclined surfaces such as bedding planes. The strike and dip of a bedding surface indicate the relationship between the inclined plane and a horizontal plane.

If rock layers bend (ductile behaviour) rather than break, they become folded. Rock layers are folded into anticlines and synclines and recumbent folds. If the hinge line of a fold is not horizontal, the fold is plunging. Older beds exposed in the core of a fold indicate an anticline, whereas younger beds in the centre of the structure indicate a syncline. In places where folded rock has been eroded to a plain, an anticline can usually be distinguished from a syncline by whether the beds dip toward the centre (syncline) or away from the centre (anticline). Also, the oldest rocks are found in the centre of an eroded anticline whereas the youngest rocks are found in the centre or core of a syncline.

Fractures in rock are either joints or faults. A joint indicates that movement has not occurred on either side of the fracture; displaced rock along a fracture indicates a fault. Dip-slip faults are either normal or reverse, depending on the motion of the hanging-wall block relative to the footwall block. The relative motion of the hanging wall is upward in a reverse fault and downward in a normal fault. A reverse fault with a low angle of dip for the fault plane is a thrust fault. Reverse faults accommodate horizontal shortening of the crust, whereas normal faults accommodate horizontal stretching or extension.

In a strike-slip fault, which can be either left-lateral or right- lateral, horizontal movement

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PHYSICAL GEOLOGY AND THE ENVIRONMENT (2ND CANADIAN EDITION) parallel to the strike has occurred.

Questions: 1. Which of the following is a type of stress? (Page 277-278) A. Shear B. Compression C. Tension D. All of these are types of stress 2. The compass direction of a line formed by the intersection of an inclined plane and a horizontal plane is called: A.

the cross-cutting angle

B.

the intersection angle

C.

strike

D.

dip

3. Folding occurs when rocks behave as: (Page 282) A.

brittle solids

B.

fluids

C.

ductile solids

D.

none of these

4. Anticlines: A.

form in rocks that are resistant to folding

B.

form in rocks as a result of brittle deformation

C.

are upward-arching folds

D.

are downward-arching folds

5. Synclines: A.

are never observed in nature

B.

are downward-arching folds

C.

are upward-arching folds

D.

form in rocks that are resistant to folding

6. In a syncline, the oldest rocks will be found: (Page 282-286)

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PHYSICAL GEOLOGY AND THE ENVIRONMENT (2ND CANADIAN EDITION) A.

on the limbs of the fold

B.

near the axis of the fold

C.

at the bottom of the fold

D.

none of these

7. In a syncline, all rock layers: (Page 282-286) A.

dip toward the fold axis

B.

dip away from the fold axis

C.

have vertical dips

D.

have horizontal dips

8. In a syncline, the youngest rock layers: (Page 282-286) A.

are found on the limbs of the fold

B.

are found nearest the fold axis

C.

dip away from the fold axis

9. In an plunging anticline, the fold axis plunges: (Page 282-286) A.

very steeply

B.

toward the flanks of the fold

C.

toward the youngest rocks

D.

none of these

10. A structural basin is a special case of: (Page 286,287) A.

a dome

B.

a syncline

C.

an anticline

D.

a freak of nature

11. This type of fault is observed where the hanging wall is displaced upward relative to the footwall. (Page 289,290) A.

normal fault

B.

reverse fault

C.

a left-lateral strike-slip fault

D.

a right-lateral strike-slip fault

12. The San Andreas Fault is an example of: (Box 11.4, Page 294)

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PHYSICAL GEOLOGY AND THE ENVIRONMENT (2ND CANADIAN EDITION) A.

a normal fault

B.

a reverse fault

C.

a thrust fault

D.

none of these

13. Faults where displacement is both vertical and horizontal are called: (Page 289,290) A.

radical faults

B.

complex faults

C.

slippery faults

D.

oblique faults

14. A fault that displays mostly vertical displacement is: (Page 289-291) A.

a dip-slip fault

B.

a strike-slip fault

C.

a transform fault

D.

none of these

15. Strike-slip faults can also be: (Page 291-293) A.

dip-slip faults

B.

transform faults

C.

anticlines

D.

syncline

16. Faults result from: (Page 278, 287) A.

ductile deformation of rocks

B.

folding of rocks

C.

brittle deformation of rocks

D.

all of these

17. Fractures result from: (Page 278, 287) A.

brittle deformation of rocks

B.

ductile deformation of rocks

C.

slow strain rates

D.

all of these

18. Folds whose limbs are horizontal are known as: (Page 287, 288)

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PHYSICAL GEOLOGY AND THE ENVIRONMENT (2ND CANADIAN EDITION) A.

horizontal layers

B.

overturned folds

C.

massively thrusted folds

D.

recumbent folds

19. Which of the following is not a fold type? (Page 287, 288) A.

syncline

B.

anticline

C.

dome

D.

thrust

20. Which geologic setting has the most obvious dip-slip faults? (Ch.11) A.

The Appalachian Mountains

B.

The Basin and Range Province of North America

C.

California

D.

The Canadian Shield

21. The San Andreas Fault is an example of a left-lateral strike-slip fault. (Box 11.4, Page 294) True False 22. An anticline is a downwarped fold. (Page 285) True False 23. A syncline is an upwarped fold. (Page 285) True False 24. In a syncline, the youngest rocks are on the limbs of the structure. (Page 285) True False 25. In a syncline, all rock layers dip toward the fold axis. (Page 285) True False 26. In a plunging syncline, the structure opens in the direction of plunge. (Page 285-286)

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PHYSICAL GEOLOGY AND THE ENVIRONMENT (2ND CANADIAN EDITION) True False 27. In an anticline, the hanging wall moves upward relative to the footwall. (Page 285) True False 28. In an anticline, the oldest rock layers are found near the fold axis. (Page 285) True False 29. In a plunging anticline, the oldest rock layers are found near the fold axis. (Page 285-286) True False 30. A dome is a special case of an anticlinal structure. (Page 286-287) True False 31. Folds can be many kilometers long and many kilometers wide. (Page 282-284) True False 32. Dip-slip faults are characterized by vertical displacements. (Page 290) True False 33. In a normal fault, the hanging wall is displaced downward relative to the footwall. (Page 290) True False 34. Strike-slip faults are often associated with transform plate boundaries. (Page 292-293) True False 35. Rocks deep beneath Earth's surface behave as plastic solids. (Page 292-293) True False 36. Folds result from brittle deformation of rocks. (Page 287)

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PHYSICAL GEOLOGY AND THE ENVIRONMENT (2ND CANADIAN EDITION) True False 37. There are no known left-lateral strike-slip faults in Earth. (Page 292-293) True False 38. Strike-slip faults are characterized by displacement parallel with the strike of the fault. (Page 292-293) True False 39. Dip-slip faults are characterized by mostly vertical displacements. (Page 290) True False 40. Stress and strain are the same thing. (Page 277) True False

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