CHAPTER 9
Crags, Cracks, and Crumples: Crustal
Deformation and Mountain Building
Learning Objectives
1. Students should understand normal, reverse, thrust, and strike-slip faults, including
the orientation of the fault plane and sense of motion in each case. They should know how
faults and joints differ, and they should also be able to identify what evidence in the field is
used to diagnose the presence of a fault.
2. Students should be able to recognize the four major folds (synclines, anticlines,
domes, and basins) in both map view and cross-sectional view.
3. Students should be able to distinguish between brittle and ductile behavior,
recognizing that faults exemplify the former and folds the latter. They should know that heat,
pressure, and gradual application of stress favor ductile behavior in rocks.
4. Isostatic equilibrium denotes the balance between the weight of mountain ranges and
the buoyant support they receive from the denser mantle below. Mountain ranges are
underlain by thick crustal roots, just as most of an ice cube floats below the surface of water.
As mountain ranges erode, their crustal roots are pressed upward (an example of isostatic
compensation).
5. The familiar montane topography is the result of erosion. Once the rate of uplift
becomes less than the rate of erosion, the mountain range will begin to wear flat, a process
that generally takes tens of millions of years.
6. Flat, low-lying regions that have not been exposed to orogenic deformation for more
than 1 billion years are termed cratons. In the central portion of the craton, termed the
shield, metamorphic rocks are exposed at the surface. The region on the flanks of the craton,
where the metamorphics are overlain by sediments, is termed the platform.
Summary from the Text
Mountains occur in linear ranges called orogens. An orogen forms during an orogeny,
or mountain-building event.
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Mountain building causes rocks to bend, break, shorten, stretch, and shear. Because of
such deformation, rocks can change their location, orientation, and shape.
During brittle deformation, rocks break into pieces. During ductile deformation, rocks
change shape without breaking.
Rocks undergo three kinds of stress: compression, tension, and shear. Strain refers to
the way rocks change shape when subjected to a stress.
Deformation results in the development of geologic structures.
Joints are natural cracks in rock, formed in response to tension under brittle conditions.
Veins develop when minerals precipitate out of water passing through cracks.
Faults are fractures on which there has been shear. Geologists distinguish among
normal, reverse, strike-slip, and oblique-slip faults.
Folds are curved layers of rock. Anticlines are arch-like, synclines are trough-like,
monoclines resemble the shape of a carpet draped over a stair step, basins are bowl-shaped,
and domes resemble an overturned bowl.
Tectonic foliation forms when grains flatten, rotate, or grow so that they align parallel
with one another.
Uplift in mountains, over broad regions, is controlled by the isostasy, meaning that the
elevation of Earth’s surface reflects the level at which lithosphere naturally floats.
Large collisional mountain ranges are underlain by buoyant roots and contain folds,
faults, and foliations.
With modern GPS technology, it is now possible to measure the slow shortening and
uplift of mountains.
Once uplifted, mountains are sculpted by erosion. When crust thickens during mountain
building, the deep crust eventually becomes warm and weak, leading to orogenic collapse.
Mountain belts formed by convergent-margin tectonism may incorporate accreted
terranes.
Cratons are the old, relatively stable parts of continents. They include shields and
platforms. Broad regional domes and basins form in platform areas.
Answers to Review Questions
1. What changes do rocks undergo during formation of an orogenic belt such as the Alps?
ANS: In orogenic belts, rocks undergo deformation as a response to stress. Deformation
can include faulting, jointing, folding, and the development of metamorphic foliation.
2. Contrast brittle and ductile deformation.
ANS: Brittle deformation involves fracturing of rock; ductile deformation involves rock
bending or flowing.
3. What factors determine whether a rock will behave in a brittle or ductile fashion?
ANS: Temperature: Hot rocks are more ductile than cool rocks. Pressure: rocks under
very high pressure behave more ductilely than do those at low pressure. Deformation rate:
sudden changes, such as onset of tensile stress, are more likely to produce brittle behavior
than are gradual changes. Rock type: some rocks, such as halite, have a proclivity to deform
ductilely.
Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building | 45
4. How are stress and strain different?
ANS: Stress is applied force per unit area; strain is percent deformation in response to
stress.
5. How is a fault different from a joint?
ANS: A fault is a fracture along which there has been displacement; a joint is a fracture
without displacement.
6. Compare normal, reverse, and strike-slip faults.
ANS: Along a normal fault, the hanging-wall block moves downward relative to the
footwall block; along a reverse fault, the hanging-wall block moves upward relative to the
footwall block. Along a (vertical) strike-slip fault, one block slides horizontally past the
other.
7. How do you recognize faults in the field?
ANS: Offset of layers on opposite sides of the fault, the development of drag folds along
the fault interface, shattered rock (fault breccia), powdered rock (fault gouge), and
slickensides (polished fault surfaces) are all clues used to identify faults.
8. Describe the differences among an anticline, a syncline, and a monocline.
ANS: Anticlines (unless overturned) are convex-upward arches. Synclines (unless
overturned) are concave-upward troughs. Monoclines are step-like folds.
9. Discuss the relationship between foliation and deformation.
ANS: Foliation is a layering resulting from the alignment of mineral grains or the
development of compositional bands. Deformation involves folding and fracture of rock
bodies. Both processes take place during orogenic events: foliation at microscopic scale and
deformation at regional scale.
10. Describe the principle of isostasy.
ANS: The tendency of gravity to pull down a body of lithosphere (such as a mountain
range) is counterbalanced by the buoyant support of denser material surrounding the roots of
the range.
11. Discuss the processes by which mountain belts form in convergent margins, during
collisions, and in rifts.
ANS: At convergent margins, if the overriding plate is a continent, a continental
volcanic arc is formed. Collision may occur between the overriding continental plate and an
exotic terrane (island arc or microcontinent), which will not subduct but merge into the
continent, elevating the continental volcanic arc.
At collision zones between continents, massive mountain ranges are produced. Thrust
faulting at a fold-thrust belt loads the material from one continent above the other, leading to
deep burial of rocks and regional metamorphism.
At continental rifts, the lithosphere is thinned by stretching, and the hot asthenosphere
rises to compensate, heating the thinned lithosphere above and thereby making it less dense.
Its reduced density causes the thin lithosphere to rise up to reestablish isostatic equilibrium.
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Normal faulting produces regions of relative uplift, termed fault-block mountains, to the
sides of rift basins.
12. How are the structures of a craton different from those of an orogenic belt?
ANS: At a shield within a craton, very ancient (greater than 1 billion years old), highly
metamorphosed rocks are exposed at the surface. These metamorphic rocks—formed in ancient
orogenies and now eroded flat—were once deep within the core of a mountain range, but cycles
of erosion and isostatic uplift have worn away all other evidence of ancient mountains (such as
the faulted and folded sedimentary that once overlaid the metamorphosed basement rock).
On Further Thought
13. Imagine that a geologist sees two outcrops of resistant sandstone, as depicted in the crosssection sketch (shown in student text). The region between the outcrops is covered by soil. A
distinctive bed of cross-bedded sandstone occurs in both outcrops, so the geologist correlates
the western outcrop (on the left) with the eastern outcrop. The curving lines in the bed indicate
the shape of the cross beds.
Keeping in mind how cross beds form (see Chapter 6), sketch how the cross-bedded bed
connected from one outcrop to the other before erosion. What geologic structure have you
drawn?
ANS: The completed drawing will reveal an asymmetric syncline.
14. Imagine a coal bed in the strata of a large, cratonic-platform basin. Will mines dug to
reach the coal be deeper in the central region of the basin or along the margin?
ANS: Mines dug in the center of the basin will have to be dug deeper in order to reach
the coal because sedimentary strata are thickest in the center of a basin.