Key Terms
accretionary wedge (p. 426)
active continental
margins (p. 426)
Andean-type plate
margins (p. 426)
anticlines (p. 418)
basins (p. 420)
brittle deformation (p. 416)
compressional
mountains (p. 425)
deformation (p. 416)
dip-slip faults (p. 422)
dome (p. 420)
ductile deformation (p. 416)
elastic deformation (p. 416)
faults (p. 421)
fault-block mountains (p. 431)
fault scarps (p. 422)
folds (p. 418)
grabens (p. 422)
gravitational collapse (p. 433)
half-grabens (p. 422)
hogbacks (p. 420)
horsts (p. 422)
isostasy (p. 432)
isostatic adjustment (p. 432)
joints (p. 424)
microcontinents (p. 428)
monoclines (p. 420)
normal faults (p. 422)
orogenesis (p. 425)
passive continental
margin (p. 426)
reverse fault (p. 423)
stress (p. 416)
strike-slip fault (p. 423)
synclines (p. 418)
terrane (p. 427)
thrust faults (p. 423)
transform fault (p. 423)
volcanic island arc (p. 426)
CHAPTER 17 Crustal Deformation and Mountain Building
434
C H A P T E R
S E V E N T E E N
Crustal Deformation and Mountain Building
in Review
Deformation refers to changes in the shape, size (volume) or
orientation of a rock body and is most pronounced along plate margins. Rocks deform differently depending on the environment (temperature
and confining pressure); the composition and texture of the rock; and the
length of time stress is maintained. Rocks first respond by deforming
elastically and will return to their original shape when the stress is
removed. Once their elastic limit (strength) is surpassed, rocks either
deform by ductile flow or they fracture. Ductile deformation is a solid-state
flow that changes the shape of an object without fracturing. Ductile flow
may be accomplished by gradual slippage and recrystallization along planes
of weakness within the crystal lattice of mineral grains. Ductile deformation
tends to occur in a high-temperature/high-pressure environment. In a nearsurface environment, most rocks deform by brittle failure.
Among the most basic geologic structures associated with rock
deformation are folds (flat-lying sedimentary and volcanic rocks bent
into a series of wavelike undulations). The two most common types of
folds are anticlines, formed by the upfolding, or arching, of rock layers,
and synclines, which are downfolds. Most folds are the result of horizontal compressional stresses. Folds can be symmetrical, asymmetrical, or,
if one limb has been tilted beyond the vertical, overturned. Domes
(upwarped structures) and basins (downwarped structures) are circular
or somewhat elongated folds formed by vertical displacements of strata.
Faults are fractures in the crust along which appreciable
displacement has occurred. Faults in which the movement is primarily
vertical are called dip-slip faults. Dip-slip faults include both normal and
reverse faults. Low-angle reverse faults are called thrust faults. Normal
faults indicate tensional stresses that pull the crust apart. Reverse and
thrust faulting indicate that compressional forces are at work. Large thrust
faults are associated with subduction zones and other convergent
boundaries where plates collide. Strike-slip faults exhibit mainly
horizontal displacement parallel to the strike of the fault surface.
Large strike-slip faults, called transform faults, accommodate
displacement between plate boundaries.
Joints are fractures along which no appreciable displacement has
occurred. Joints generally occur in groups with roughly parallel
orientations and are the result of brittle failure of rock units located in
the outermost crust.
The name for the processes that collectively produce a compressional
mountain belt is orogenesis. Most compressional mountains consist of folded
and faulted sedimentary and volcanic rocks, portions of which have been
strongly metamorphosed and intruded by younger igneous bodies.
Subduction of oceanic lithosphere beneath a continental block gives
rise to an Andean-type plate margin that is characterized by a continental
volcanic arc and associated igneous plutons. In addition, sediment
derived from the land, as well as material scraped from the subducting
plate, becomes plastered against the landward side of the trench,
forming an accretionary wedge.
Mountain belts can develop as a result of the collision and merger of
one or more crustal fragments, including island arcs and oceanic plateaus
to a continental block. Many of the mountain belts of the North American
Cordillera were generated in this manner.
Continued subduction of oceanic lithosphere beneath an Andeantype continental margin will eventually close an ocean basin. The result
will be a continental collision and the development of compressional
mountains that are characterized by shortened and thickened crust. The
development of a major mountain belt is often complex, involving two
or more distinct episodes of mountain building. Continental collisions
have generated most of Earth’ s major mountain belts, including the
Himalayas, Alps, Urals, and Appalachians.
Although most mountains form along convergent plate boundaries, other
tectonic processes, such as continental rifting, can produce topographic
mountains. The fault-block mountains that form in these settings are bounded
by high-angle normal faults. The Basin and Range Province in the western
United States consists of hundreds of tilted, faulted blocks that give rise to
nearly parallel mountain ranges that stand above sediment-laden basins.
Earth’ s crust floats on top of the deformable rocks of the mantle,
much like wooden blocks float in water. The concept of a floating crust
in gravitational balance is called isostasy. Most mountains consist of
crustal rocks that have been shortened and thickened producing deep
crustal roots that isostatically support them. As erosion lowers the
peaks, isostatic adjustment gradually raises the mountains in response.
The processes of uplift and erosion will continue until the mountain
block reaches “normal” crustal thickness. Gravity can also cause these
elevated structures to collapse under their own “weight.”
accretionary wedge (p. 426)
active continental
margins (p. 426)
Andean-type plate
margins (p. 426)
anticlines (p. 418)
basins (p. 420)
brittle deformation (p. 416)
compressional
mountains (p. 425)
deformation (p. 416)
dip-slip faults (p. 422)
dome (p. 420)
ductile deformation (p. 416)
elastic deformation (p. 416)
faults (p. 421)
fault-block mountains (p. 431)
fault scarps (p. 422)
folds (p. 418)
grabens (p. 422)
gravitational collapse (p. 433)
half-grabens (p. 422)
hogbacks (p. 420)
horsts (p. 422)
isostasy (p. 432)
isostatic adjustment (p. 432)
joints (p. 424)
microcontinents (p. 428)
monoclines (p. 420)
normal faults (p. 422)
orogenesis (p. 425)
passive continental
margin (p. 426)
reverse fault (p. 423)
stress (p. 416)
strike-slip fault (p. 423)
synclines (p. 418)
terrane (p. 427)
thrust faults (p. 423)
transform fault (p. 423)
volcanic island arc (p. 426)
CHAPTER 17 Crustal Deformation and Mountain Building
434
C H A P T E R
S E V E N T E E N
Crustal Deformation and Mountain Building
in Review
Deformation refers to changes in the shape, size (volume) or
orientation of a rock body and is most pronounced along plate margins. Rocks deform differently depending on the environment (temperature
and confining pressure); the composition and texture of the rock; and the
length of time stress is maintained. Rocks first respond by deforming
elastically and will return to their original shape when the stress is
removed. Once their elastic limit (strength) is surpassed, rocks either
deform by ductile flow or they fracture. Ductile deformation is a solid-state
flow that changes the shape of an object without fracturing. Ductile flow
may be accomplished by gradual slippage and recrystallization along planes
of weakness within the crystal lattice of mineral grains. Ductile deformation
tends to occur in a high-temperature/high-pressure environment. In a nearsurface environment, most rocks deform by brittle failure.
Among the most basic geologic structures associated with rock
deformation are folds (flat-lying sedimentary and volcanic rocks bent
into a series of wavelike undulations). The two most common types of
folds are anticlines, formed by the upfolding, or arching, of rock layers,
and synclines, which are downfolds. Most folds are the result of horizontal compressional stresses. Folds can be symmetrical, asymmetrical, or,
if one limb has been tilted beyond the vertical, overturned. Domes
(upwarped structures) and basins (downwarped structures) are circular
or somewhat elongated folds formed by vertical displacements of strata.
Faults are fractures in the crust along which appreciable
displacement has occurred. Faults in which the movement is primarily
vertical are called dip-slip faults. Dip-slip faults include both normal and
reverse faults. Low-angle reverse faults are called thrust faults. Normal
faults indicate tensional stresses that pull the crust apart. Reverse and
thrust faulting indicate that compressional forces are at work. Large thrust
faults are associated with subduction zones and other convergent
boundaries where plates collide. Strike-slip faults exhibit mainly
horizontal displacement parallel to the strike of the fault surface.
Large strike-slip faults, called transform faults, accommodate
displacement between plate boundaries.
Joints are fractures along which no appreciable displacement has
occurred. Joints generally occur in groups with roughly parallel
orientations and are the result of brittle failure of rock units located in
the outermost crust.
The name for the processes that collectively produce a compressional
mountain belt is orogenesis. Most compressional mountains consist of folded
and faulted sedimentary and volcanic rocks, portions of which have been
strongly metamorphosed and intruded by younger igneous bodies.
Subduction of oceanic lithosphere beneath a continental block gives
rise to an Andean-type plate margin that is characterized by a continental
volcanic arc and associated igneous plutons. In addition, sediment
derived from the land, as well as material scraped from the subducting
plate, becomes plastered against the landward side of the trench,
forming an accretionary wedge.
Mountain belts can develop as a result of the collision and merger of
one or more crustal fragments, including island arcs and oceanic plateaus
to a continental block. Many of the mountain belts of the North American
Cordillera were generated in this manner.
Continued subduction of oceanic lithosphere beneath an Andeantype continental margin will eventually close an ocean basin. The result
will be a continental collision and the development of compressional
mountains that are characterized by shortened and thickened crust. The
development of a major mountain belt is often complex, involving two
or more distinct episodes of mountain building. Continental collisions
have generated most of Earth’ s major mountain belts, including the
Himalayas, Alps, Urals, and Appalachians.
Although most mountains form along convergent plate boundaries, other
tectonic processes, such as continental rifting, can produce topographic
mountains. The fault-block mountains that form in these settings are bounded
by high-angle normal faults. The Basin and Range Province in the western
United States consists of hundreds of tilted, faulted blocks that give rise to
nearly parallel mountain ranges that stand above sediment-laden basins.
Earth’ s crust floats on top of the deformable rocks of the mantle,
much like wooden blocks float in water. The concept of a floating crust
in gravitational balance is called isostasy. Most mountains consist of
crustal rocks that have been shortened and thickened producing deep
crustal roots that isostatically support them. As erosion lowers the
peaks, isostatic adjustment gradually raises the mountains in response.
The processes of uplift and erosion will continue until the mountain
block reaches “normal” crustal thickness. Gravity can also cause these
elevated structures to collapse under their own “weight.”