Mountains

I

Introduction

Mountains, name usually applied to any region of land that is raised rather steeply above the surrounding terrain. Mountains are distinguishable from plateaux by their usually limited summit area; and they are distinguishable from what are comonly called hills by their generally higher elevation. The elevation, or altitude, of a mountain is given as the height of the summit above sea level. Therefore, a mountain with an elevation of, say, 4,000 m (13,100 ft) may rise to only 3,000 m (9,840 ft) above the surrounding land.

Mountains are normally found in groups or ranges consisting of peaks, ridges, and intermontane valleys. Apart from certain mountains that occur singly, the smallest unit is the range, comprising either a single complex ridge or a series of ridges generally alike in origin, age, and form. Several closely related ranges in a parallel alignment or chain-like cluster are known as a mountain system; an elongated series of systems forms a mountain chain; and an extensive complex of ranges, systems, and chains is known as a belt, or cordillera.

II

Formation

Geologists believe that most mountains are formed by movements in the Earth's crust. The plate tectonics model has helped to explain many of these processes. Briefly, this concept visualizes the crust of the Earth as made up of a number of vast plates that move about, at the rate of a few centimetres a year, leading to the collision and separation of continents and the subsequent development of mountain belts.

Movements that result in collisions between plates tend to raise the crust by faulting, folding, or arching up layers of rock strata. Movements that result in separation, or rifting, of plates cause some blocks of crust to sink or founder, leaving other blocks to tower above them. Volcanic eruptions also raise mountains, and much of the world's volcanic activity is concentrated along the active margins of crustal plates. Finally, some ranges of low mountains are created by so-called nontectonic processes, chief among these being the sculpturing effects of differential erosion.

A

Uplift

Collisions between plates of the Earth's crust trigger various geologic processes that result in crustal uplift. A common process, produced by horizontal compression, is the deformation of crustal strata into folds or wrinkles or the thrusting of vast, thick sheets of rock over one another. The Himalaya, for example, were raised by the compression that accompanied the collision of the Indian plate with the Eurasian one. Europe's Alps and Jura Mountains also formed because of horizontal compression, generated in their case by collision of the African plate with the Eurasian one. Similarly, the Appalachian Mountains rose in response to forces caused by repeated collisions between the North American plate and the African and European ones, and most of the Rocky Mountains rose because of the compressive effects generated by collisions between the North American and Pacific plates.

Formation of a basin range structure, best exemplified by the Great Basin area of Nevada and Arizona, is the result of movement of rock along faults, or major deep cracks, in the crust of the Earth. Occasionally, such movement, or so-called faulting, causes rock bordering on faults to be uplifted vertically in great blocks. The raised edges of the blocks then appear as mountains, the depressed edges as valleys. Such mountains are also widely known as fault-block, or block, mountains.

A third type of mountain formed by uplifting is the so-called dome. Such mountains are created by deep-seated intrusion of igneous, or molten, rock that arches the rocks at the surface.

B

Erosion

Rock on the surface of the Earth is constantly exposed to erosive actions. Because rocks of different composition resist erosion differently, however, areas of relatively hard rock may stand high 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 the upturned 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 an uncovered intrusive igneous mass. Parts of the Ozark Plateau (also known as the Ozark Mountains) in Arkansas and Missouri in the United States are good examples of mountains created by the forces of erosion.