The following statements summarize and describe many of the key terms and concepts presented
in the chapter.
- Deformation refers to changes in the shape and/or volume of a rock
body. Rocks deform differently depending on the environment (temperature and confining
pressure), the composition 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 results in a change in size and shape of rocks without fracturing. Ductile
deformation occurs in a high-temperature/high-pressure environment. In a near-surface
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.
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. Along spreading centers,
divergence can cause a central block called a graben, bounded by normal
faults, to drop as the plates separate.
- Reverse and thrust faulting indicate that compressional forces are at
work. Large thrust faults are found along subduction zones and other
convergent boundaries where plates are colliding.
- Strike-slip faults exhibit mainly horizontal displacement parallel to
the fault surface. Large strike-slip faults, called transform faults,
accommodate displacement between plate boundaries. Most transform faults cut the oceanic
lithosphere and link spreading centers. The San Andreas Fault cuts the continental
lithosphere and accommodates the northward displacement of southwestern California.
- 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 mountain system is
orogenesis. Most mountains consist of roughly parallel ridges 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 under 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. An excellent example of an inactive
Andean-type mountain belt is found in the western United States and includes the Sierra
Nevada and the Coast Range in California.
- Mountain belts can develop as a result of the collision and merger of an island arc,
oceanic plateau, or some other small crustal fragment 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 Andean-type 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 as exhibited by the Himalayas. The development of a major
mountain belt is often complex involving two or more distinct episodes of mountain building.
Continental collisions have generated many mountain belts, including the Alps, Urals, and
Appalachians.
- Although most mountains form along convergent plate boundaries, other tectonic
processes, such as continental rifting can produce uplift and the formation of topographic
mountains. The mountains that form in these settings, termed fault-block
mountains, are bounded by high-angle normal faults that gradually flatten with
depth. The Basin and Range Province in the western United States consists of hundreds of
faulted blocks that give rise to nearly parallel mountain ranges that stand above
sediment-laden basins.
- Earth's less dense crust floats on top of the denser and deformable rocks of the
mantle, much like wooden blocks floating in water. The concept of a floating crust in
gravitational balance is called isostasy. Most mountainous topography is
located where the crust has been shortened and thickened. Therefore, mountains have deep
crustal roots that isostatically support them. As erosion lowers the peaks, isostatic
adjustment gradually raises the mountains in response. The processes of uplifting
and erosion will continue until the mountain block reaches "normal" crustal thickness.
Gravity also causes elevated mountainous structures to collapse under their own
"weight."
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