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CHAPTER 11 Crustal Deformation and Mountain Building
1. Whether rock deformation is ductile or brittle depends on four variables:
• temperature. If the rock is warm, it is more likely to flow than break.
• confining pressure. If high, the rock is more likely to flow than break.
• rock type. At shallow depth, crystalline igneous and metamorphic rocks deform (fault) in a brittle
manner whereas sedimentary strata deform (fold) in a ductile manner.
• length of time over which stress is applied. A rapid increase of strain is likely to end in breakage
whereas a slow increase of strain can be continued by ductile flow.
2. Which of the following statements about rock deformation is false?
a. deep crustal rocks are more likely to deform ductily than shallow crustal rocks;
b. hotter rocks are more likely to deform ductily than cooler rocks;
c. most sedimentary rocks are more deformable than igneous rocks;
d. under low confining pressure rocks are more likely to deform ductily than under high confining pressure.
3. Structures formed by brittle deformation are faults and joints.
4. What types of tectonic forces cause faulting?
a) compressive forces; b) tensional forces; c) shearing forces; d) all of these.
5. Sketch and briefly describe the relative motion of rock bodies located on opposite sides of normal,
reverse, and thrust faults as well as both types of strike-slip faults.
6. The offset on a fault may be determined by comparing the blocks of rock on either side of the fault surface.
If the movement is in the direction of the fault's dip (or inclination), then the rock above the fault plane is the
hanging wall block, and the rock below the fault is the footwall block. If the hanging wall moves:
• down relative to the footwall, the fault is a normal fault.
• up relative to the footwall, the fault is a reverse fault.
7. What type of fault is characterized by the rocks above the fault plane moving downward relative to the
rocks below the fault plane?
a) normal faults; b) reverse faults; c) strike-slip; d) all of these.
8. Faults that intersect Earth's surface may produce a "step" in the land known as a fault scarp. Areas of
tectonic extension, such as the Basin and Range Province, produce fault-block mountains—horsts separated
by neighboring grabens or half-grabens.
9. Infer the differential stresses and the type of plate boundary that were responsible for the Rift Valley
of East Africa.
10. Large normal faults with low dip angles are detachment faults. Large reverse faults with low dip angles
are thrust faults.
11. Areas of tectonic compression, such as mountain belts, are dominated by reverse faults that shorten the
crust horizontally while thickening it vertically.
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12. Strike-slip faults have most of their movement in a horizontal direction along the trend of the fault trace.
Transform faults are strike-slip faults that serve as tectonic boundaries between lithospheric plates.
13. Which of the following is an example of a fault where the motion is primarily horizontal?
a) a strike-slip fault; b) a right-lateral fault; c) a transform fault;
d) all of these.
14. Joints form in the shallow crust when rocks are stressed under brittle conditions. They facilitate
groundwater movement and mineralization of economic resources, and they may result in human hazards.
15. Structures formed by ductile deformation are folds.
16. Folds are wavelike undulations in layered rocks that develop through ductile deformation in rocks
undergoing compressional stress.
17. Which of the following conditions would favor folding rather than faulting?
a) low temperatures and low confining pressures;
b) low temperatures and high confining pressures;
c) high temperatures and low confining pressures;
d) high temperatures and high confining pressures.
18. Folds may be described in terms of their geometric configuration: If the limbs of a fold dip away from the
hinge, the fold has an overall arch-like structure and is called an anticline. If the limbs of a fold dip upward,
the fold has an overall trough-like structure and is called a syncline. Anticlines and synclines may be
symmetrical, asymmetrical, overturned, or recumbent.
19. The two sides of a fold are called its ________ .
a) anticlines; b) synclines; c) limbs; d) axial planes.
20. An overturned fold is characterized by ___________ .
a) two limbs at right angles to one another;
b) two limbs dipping in the same direction - with one tilted beyond vertical;
c) two limbs dipping in opposite directions;
d) two limbs not parallel to each other.
21. The shape of a fold does not necessarily correlate to the shape of the landscape above it. Rather, surface
topography usually reflects patterns of differential weathering.
22. Explain why the shape of the landscape can be opposite to the shape of the folds below.
23. A fold is said to plunge when its axis penetrates the ground at an angle. This results in a V-shaped outcrop
pattern of the folded layers.
24. If the sedimentary rocks on a geologic map form a zigzag pattern, the underlying structure probably
consists of _________.
a) horizontal anticlines and synclines; b) plunging anticlines and synclines; c) domes and basin;
d) strike-slip faults.
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25. Domes and basins are large folds that produce roughly circular-shaped outcrop patterns. The overall shape
of a dome or basin is a saucer or a bowl, either right-side-up (basin) or inverted (dome),
26. What do domes and anticlines have in common? What sets them apart from one another?
27. Monoclines are large step-like folds in otherwise horizontal strata that result from subsurface faulting.
Imagine a carpet draped over a staircase to envision how the strata can go from horizontal to tilted and back to
horizontal again.
28. Orogenesis is the process of mountain formation, especially by a folding and faulting of Earth's crust.
29. Orogenesis most often occurs along convergent plate boundaries, where compressional forces cause
folding and faulting of the rock, thickening the crust vertically and shortening it horizontally. Some mountain
belts are very old, while others are actively forming today.
30. Infer the differential stresses and the type of plate boundary that were responsible for the Canadian
Rockies
31. Sediment scraped off the subducting plate builds up in an accretionary wedge. The thickness of the wedge
varies in accordance with the amount of sediment available. Between the accretionary wedge and the volcanic
arc is a relatively calm site of sedimentary deposition, the forearc basin.
32. If a subducted plate is overridden by oceanic lithosphere, island arc-type mountain building results, with a
thick accumulation of erupted volcanic rocks mixed with sediment scraped off the subducted plate.
Andean-type mountain building occurs where subduction takes place beneath continental lithosphere.
33. Explain why the Andes Mountains are located on the western margin of South America rather than
the eastern margin.
34. Release of water from a subducted slab triggers melting in the wedge of mantle above the slab. These
primary magmas have a basaltic (mafic) composition. They rise to the base of the continental crust and pond.
Fractional crystallization then separates ferromagnesian minerals from a melt that grows increasingly granitic
(felsic) in composition.
35. The evolved magma rises further and may either cool and crystallize below the surface to form a batholith
or erupt at the surface and form a continental volcanic arc.
36. Sketch a cross section of an Andean-type mountain belt and describe how its major features are
generated.
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37. Terranes are relatively small crustal fragments (microcontinents, volcanic island arcs, or oceanic
plateaus). Terranes may be accreted to continents when subduction brings them to a trench, but they cannot
subduct due to their relatively low density. The terranes are "peeled off" the subducted slab and thrust onto
the leading edge of the continent.
38. The geography of central California preserves an accretionary wedge (Coast Ranges), a forearc basin
(Great Valley), and the roots of a continental volcanic arc (Sierra Nevada).
39. Himalayas are geologically young fold mountains formed by the collision of India and Eurasia starting
around 50 million years ago. They are still rising today.
40. Isostasy is the principle that the elevation of the crust can change in accordance with gravitational
balance. If additional weight is placed on the crust (a flood basalt, a thick layer of sediments, or an ice sheet),
the crust will sink into the mantle until it comes to equilibrium with the extra load. If the new mass is
removed, the crust will rebound upward.
41. When mountains form and rock mass is pushed up to the height of a young compressional mountain belt
such as the Himalayas, the rocks at the base of the mountains become warmer and weaker. As the overlying
rock mass pushes down on them through gravity, these deeper rocks flow out of the way. This causes the
mountain belt to collapse outward into a wider, lower-elevation region.
42. Describe how isostasy will affect elevation of the highest peaks in a mountain range if rivers and
glaciers eroded deep valleys through the mountains and removed large amounts of rock.
accretionary wedge
anticline
basin
brittle deformation
compressional stress
compressional mountain
deformation
detachment fault
differential stress
dip-slip fault
dome
ductile deformation
elastic deformation
fault-block mountain
fault scarp
fault
fold
footwall block
forearc basin
graben
gravitational collapse
half-graben
hanging wall block
horst
isostasy
isostatic adjustment
joint
microcontinent
monocline
normal fault
orogenesis
reverse fault
rock structure (geologic structure)
shear
strain
stress, confining pressure
strike-slip fault
suture
syncline
tensional stress
terrane
thrust fault
transform fault