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waves struck the coast of the Indian Ocean, with an estimated
death toll of about 295,000 and over a million left homeless.
The world’s largest recorded earthquakes have all oc-
curred where one tectonic plate subducts beneath another.
These include the magnitude-9.5 1960 Chile earthquake, the
magnitude-9.2 1964 Prince William Sound, Alaska, earth-
quake, the magnitude-9.1 1957 Andreanof Islands, Alaska,
earthquake, and the magnitude-9.0 1952 Kamchatka, Russia,
earthquake. Such earthquakes often create large tsunamis that
result in death and destruction over a wide area.

19.4 ORIGIN OF MOUNTAINS
Most of the interesting features of Earth’s surface have been
created by folding and faulting, and the most prominent of
these features are mountains. Mountains are elevated parts of
Earth’s crust that rise abruptly above the surrounding surface.
Most mountains do not occur in isolation but in groups that are
associated in chains or belts. These long, thin belts are generally
found along the edges of continents rather than in the continen-
tal interior. There are a number of complex processes involved
in the origin of mountains and mountain chains, and no two
mountains are exactly alike. For convenience, however, moun-
tains can be classified according to three basic origins: (1) fold-
ing, (2) faulting, and (3) volcanic activity.

FOLDED AND FAULTED MOUNTAINS
The major mountain ranges of Earth—the Appalachian, Rocky,
and Himalayan mountains, for example—have a great vertical re-
lief that involves complex folding on a very large scale. The crust FIGURE 19.17 The folded structure of the Appalachian Moun-
was thickened in these places as compressional forces produced tains, revealed by weathering and erosion, is obvious in this Skylab
tight, almost vertical folds. Thus, folding is a major feature of the photograph of the Virginia-Tennessee-Kentucky boundary area. The
major mountain ranges, but faulting and igneous intrusions are clouds are over the Blue Ridge Mountains.
invariably also present. Differential weathering of different rock
types produced the parallel features of the Appalachian Mountains
that are so prominent in satellite photographs (Figure 19.17). The Arizona, Utah, and southeastern California have large numbers
folded sedimentary rocks of the Rockies are evident in the almost of fault block mountains that generally trend north and south.
upright beds along the flanks of the front range.
A broad arching fold, which is called a dome, produced
the Black Hills of South Dakota. The sedimentary rocks from VOLCANIC MOUNTAINS
the top of the dome have been weathered away, leaving a some- Lava and other materials from volcanic vents can pile up to moun-
what circular area of more resistant granite hills surrounded by tainous heights on the surface. These accumulations can create
upward-tilting sedimentary beds (Figure 19.18). The Adiron- local volcano-formed mountains near mountains produced by
dack Mountains of New York are another example of this type of folding or faulting. Such mixed-origin mountains are common in
mountain formed from folding, called domed mountains. northern Arizona, New Mexico, and western Texas. The Cascade
Compressional forces on a regional scale can produce Mountains of Washington and Oregon are a series of towering
large-scale faults, shifting large crustal blocks up or down volcanic peaks, most of which are not active today. As a source of
relative to one another. Huge blocks of rocks can be thrust mountains, volcanic activity has an overall limited impact on the
to mountainous heights, creating a series of fault block mountains. continents. The major mountains built by volcanic activity are the
Fault block mountains rise sharply from the surrounding land mid-oceanic ridges formed at diverging plate boundaries.
along the steeply inclined fault plane. The mountains are not Deep within Earth, previously solid rock melts at high tem-
in the shape of blocks, however, as weathering has carved them peratures to become magma, a pocket of molten rock. Magma is
into their familiar mountainlike shapes (Figure 19.19). The Teton not just melted rock alone, however, as the melt contains variable
Mountains of Wyoming and the Sierra Nevadas of California are mixtures of minerals (resulting in different types of lava flows).
classic examples of fault block mountains that rise abruptly from It also includes gases such as water vapor, sulfur dioxide, hydro-
the surrounding land. The various mountain ranges of Nevada, gen sulfide, carbon dioxide, and hydrochloric acid. You can often

19-13 CHAPTER 19 Building Earth’s Surface 489