Mountains

Simply defined, mountains are large land-forms that rise more or less abruptly from the surrounding low places. In general, mountains

Mountains near Asheville, North Carolina (From the collections of the Library of Congress)

are parts of the earth's crust that have been raised above their surroundings by upwarp-ing, folding, buckling, faulting, or volcanic eruptions. These deformations and resulting uplifts are directly or indirectly the consequence of plate tectonics.

When a continental plate collides with an oceanic plate, the oceanic plate is subducted below the much thicker continental plate, and sediments from the seafloor are scraped off and pushed, folded, and pasted onto the adjacent continent—often forming a series of parallel mountain belts. Associated with the folded and faulted rocks are volcanoes whose magma is derived from the melting of the subducted ocean plate, which intrudes and erupts within and on the sediments. A good example of this type of mountain is the Andes Mountains on the western side of South America.

When continental plates collide they crush against each other, resulting in massive folded belts, and remain towering features as long as the tectonic zone remains active. The Himalayan Mountains, driven up by the collision between the Indian and Eurasian plates, are a classic example. Uplift rates of about 1 cm per year are great enough to create very high and steep topography. Mountains such as these are high enough to alter atmospheric circulation and present a barrier to the migration of plants and animals. They, like lower mountains, produce a feature termed the rain shadow, by means of which moist air currents forced up the windward sides of mountains cool and drop their moisture as rain or snow. Once over the mountain the dried-out air moves across a parched terrain because it supplies little if any precipitation. The Rocky Mountains of Colorado are a good example and illustrate how mountains can control the types of plants and dependent animals in a region.

Once mountain-building activity has shifted elsewhere, erosion becomes the dominant process, and the mountains wear away. However, the worn down mountains—such as the Appalachians of the eastern United States— mark the place of former plate interaction. They are an example of one of the most striking features on the earth's surface, the long mountain belts that are found on all continents, composed of numerous parallel ridges that resulted from plate collision.

Where the accumulative stresses are ten-sional rather than compressional, a series of fault block mountains can result. In the Basin and Range of Nevada and adjacent states, tension has pulled the earth apart as a result of arching and subsequent downward movement of segments of the earth's crust caused by collapse. The consequence is a series of more or less parallel high mountains interspersed with the down-dropped basins. Fault block mountains such as the Sierra Nevada occur where the eastern side has dropped downward along a fault and the mountain mass has rotated upward.

Volcanic mountains, cone-shaped edifices resulting from the extrusion of magma from a vent, may occur singly or, usually, in a series. Once volcanism ceases the volcano erodes rapidly and may eventually leave a feature called a neck—the material that solidified within the vent. Eventually the neck will dis appear also. In the western United States, Mt. Saint Helens and Mt. Rainer are examples of active volcanoes, ones that have erupted in historical times.

Calderas are volcanic mountains that have collapsed as a result of the rapid extrusion of magma, leaving a large circular basin that is usually filled with water. Calderas form during an eruptive phase when large amounts of magma are extruded, leaving behind a partially empty magma chamber into which the overlying volcanic edifice collapsed.

The major mountain system in the world is the midocean rift, built along divergent plate boundaries; it is about 60,000 km long and 2,000 km wide. Iceland, like all oceanic mountains, is volcanic and is one of the few places where the midocean ridge is above sea level. Rates of spreading movement along the rift are 2 to 6 cm per year.

Many oceanic mountains result from "hot spots" that bring large volumes of magma to the surface, and frequently form high volcanic islands aligned in a row. Iceland stands above sea level because it is located where a hot spot coincides with the midocean ridge. Hot spots also produce massive volcanoes on the land; none have been active recently, however, so they have eroded forms and do not form imposing features. Examples of this type of volcano are found in Yellowstone National Park in Wyoming, and Lake Toba in Sumatra.

Island arc mountains are created where an oceanic plate dives downward, underthrusting the adjacent oceanic plate and creating a series of volcanoes. The Aleutian Islands and the Japanese Islands in the Pacific Ocean are examples. The rate of decent is 5 to 12 cm per year.

—Sidney Horenstein

See also: Glaciation; Volcanoes

Bibliography

Keller, Edward A., and Nicholas Pinter. 1996. Active

Tectonics: Earthquakes, Uplift, and Landscape. Upper

Saddle River, NJ: Prentice Hall; Montgomery, Carla. 1996. Fundamentals of Geology, 3rd ed. New York: McGraw Hill Professional Publishing; Ollier, Cliff, and Colin Pain. 2000. The Origin of Mountains. New York: Routledge; Pinter, Nicholas, and Mark T. Brandon. 1997. "How Erosion Builds Mountains." Scientific American 276:74-79; Plummer, Charles C., David McGeary, and Diane Carlson. 2002. Physical Geology, 9th ed. New York: McGraw-Hill.

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