Mountain - geography.

« B Erosion Rock on the surface of the earth is constantly exposed to erosion.

Because rocks of different composition resist erosion differently, areas of relatively hard rock may standhigh above areas of softer, more easily eroded rock.

Mountains resulting from this erosive sculpturing of the land may be linear in appearance if the resistant rock is theupturned edge of a sedimentary rock unit, flat-topped buttes or mesas if the harder rock is a flat-lying unit, or complex and irregular ranges if the resistant rocks are anuncovered intrusive igneous mass.

Portions of the Ozark Plateau (also known as the Ozark Mountains) in Arkansas and Missouri are good examples of mountains created bythe forces of erosion. C Volcanism Mountains formed by volcanic action are well known because of their usually isolated occurrence and periodically dangerous aspect.

Most spectacular and probably mostfamiliar are the conical peaks composed of lava and volcanic debris, such as Mount Rainier and Mount Saint Helens in the western United States, Mount Erebus in Antarctica,Mount Vesuvius in Italy, and Fuji in Japan.

Many of these volcanic mountains have summit craters that still emit steam and debris; others no longer showing signs ofvolcanic activity may be only dormant, not extinct.

Shield volcanoes, typified by Mauna Loa and Mauna Kea in Hawaii, are less spectacular even when quite high, since thephysical properties of their formative material have shaped them into broad shieldlike masses that deemphasize their height. III IMPORTANCE Mountains affect life in many ways.

Apart from their mineral, forest, agricultural, and recreational resource value, they exert a significant influence on climate and determinethe course of economic or historical trends.

Especially high mountain ranges, such as the Sierra Nevada in the U.S., the Andes in South America, and the Himalayas in Asia,markedly affect climate and weather patterns over vast areas of the earth because they stand as barriers to regularly circulating air masses.

Moisture carried inland bywinds from the Pacific Ocean, for example, is lost in the form of rain and snow on the windward sides of the Sierra Nevada and Andes; the leeward, or inland, side is drier,and the land beyond is frequently arid. The importance of mountains with regard to the history and economy of various nations can be shown by their influence upon the development of the western UnitedStates.

The first travelers and settlers, and then the earliest railroads, avoided mountain crossings because of the dangers and costs involved.

Later, however, the vastdeposits of minerals that became so vitally important to the development of several western states were found exposed principally in mountainous areas, and the lure of“striking it rich” drew people and railroads west despite the hardships encountered in traversing the passes.

As a result, transportation routes and patterns, with largepopulations centered about them, were established; most of these remain today. The political significance of mountains has been noticeable throughout human history.

Mountain barriers with their relatively narrow and easily defendable passes havemade various ranges throughout the world natural political boundaries, second in strategic importance only to oceans and seas. Most of the world’s highest mountains lie in the great Himalayan system and the cordillera stretching through North and South America.

The accompanying table includesthe highest mountains found on each continent. See Erosion; Geology; Volcano.

For additional information, see separate articles on most mountains and mountain systems mentioned. Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation.

All rights reserved.. »