Mountain

A mountain is any landmass on Earth's surface that rises to a great height in comparison to its surrounding landscape. Mountains usually have more-or-less steep sides meeting in a summit that is much narrower in width than the mountain's base.

Although single mountains exist, most occur as a group, called a mountain range. A group of ranges that share a common origin and form is known as a mountain system. A group of systems is called a mountain chain. Finally, a complex group of continental (land-based) ranges, systems, and chains is called a mountain belt or cordillera (pronounced kordee-YARE-ah).

The greatest mountain systems are the Alps of Europe, the Andes of South America, the Himalayas of Asia, and the Rockies of North America. Notable single peaks in these systems include Mont Blanc (Alps), Aconcagua (Andes), Everest (Himalayas), and Elbert (Rockies). The Himalayas is the world's highest mountain system, containing some 30 peaks rising to more than 25,000 feet (7,620 meters). Included among these peaks is the world's highest, Mount Everest, at 29,028 feet (8,848 meters) above sea level. North America's highest peak is Mount McKinley, part of the Alaska Range, which rises 20,320 feet (6,194 meters).

Mountains, like every other thing in the natural world, go through a life cycle. They rise from a variety of causes and wear down over time at various rates. Individual mountains do not last very long in the powerfully erosive atmosphere of Earth. Mountains on the waterless world of Mars are billions of years old, but Earth's peaks begin to fracture and dissolve as soon as their rocks are exposed to the weathering action of wind and rain. This is why young mountains are high and rugged, while older mountains are lower and smoother.

Mountain building

Mountain building (a process known as orogeny [pronounced o-RA-je-nee]) occurs mainly as a result of movements in the surface of Earth. The thin shell of rock covering the globe is called the crust, which varies in depth from 5 to 25 miles (8 to 40 kilometers). Underneath the crust is the mantle, which extends to a depth of about 1,800 miles (2,900 kilometers) below the surface. The mantle has an upper rigid layer and a partially melted lower layer. The crust and the upper rigid layer of the mantle together make up the lithosphere. The lithosphere, broken up into various-sized plates or sections, "floats" on top of the heated, semiliquid layer underneath.

The heat energy carried from the core of the planet through the semi-liquid layer of the mantle causes the lithospheric plates to move back and forth. This motion is known as plate tectonics. Plates that move toward each other are called convergent plates; plates moving away from each other are divergent plates.

When continental plates converge, they shatter, fold, and compress the rocks of the collision area, thrusting the pieces up into a mountain range of great height. This is how the Appalachians, Alps, and Himalayas were formed: the rocks of their continents were folded just as a flat-lying piece of cloth folds when pushed.

When a continental plate and an oceanic plate converge, the oceanic plate subducts or sinks below the continental plate because it is more dense. As the oceanic plate sinks deeper and deeper into Earth, its leading edge of rock is melted by intense pressure and heat. The molten rock then rises to the surface where it lifts and deforms rock, resulting in the formation of volcanic mountains on the forward edge of the continental plate. The Andes and the Cascade Range in the western United States are examples of this type of plate convergence.

The longest mountain range on Earth is entirely underwater. The Mid-Atlantic Ridge is a submarine mountain range that extends about 10,000 miles (16,000 kilometers) from Iceland to near the Antarctic Circle. The ridge is formed by the divergence of two oceanic plates. As the plates move away from each other, magma (molten rock) from inside Earth rises and creates new ocean floor in a deep crevice known as a rift valley in the middle of the ridge. On either side of the rift lie tall volcanic mountains. The peaks of some of these mountains rise above the surface of the ocean to form islands, such as Iceland and the Azores.

Other mountains on the planet form as solitary volcanic mountains in rift valleys on land where two continental plates are diverging. Mount Kilimanjaro, the highest point in Africa, is an extinct volcano that stands along the Great Rift Valley in northeast Tanzania. The highest of its two peaks, Kibo, rises 19,340 feet (5,895 meters) above sea level.

The erosive power of water on plateaus can also create mountains. Mesas, flat-topped mountains common in the southwest United States, are such a case. They form when a solid sheet of hard rock sits on top of softer rock. The hard rock layer on top, called the caprock, once covered a wide area. The caprock is cut up by the erosive action of streams. Where there is no more caprock, the softer rock beneath washes away relatively quickly. Mesas are left wherever a remnant of the caprock forms a roof over the softer rock below. Mesa Verde in Colorado and the Enchanted Mesa in New Mexico are classic examples.

Mountains and weather

Mountains make a barrier for moving air, robbing it of any precipitation. The atmosphere at higher elevations is cooler and thinner. As dense masses of warm, moist air are pushed up a mountain slope by winds, the air pressure surrounding the mass drops away. As a result, the mass becomes cooler. The moisture contained in the mass then condenses into cool droplets, and clouds form over the mountain. As the clouds continue to rise into cooler, thinner air, the droplets increase in size until they become too heavy to float in the air. The clouds then dump rain or snow on the mountain slope. After topping the crest, however, the clouds often contain little moisture to rain on the lee side of the mountain, which becomes arid. This is best illustrated in the Sierra Nevada mountains of

Because mountains work as a barrier for moving air, they are often topped with snow caused by the cold precipitation in the clouds surrounding the peaks. (Reproduced by permission of

JLM Visuals

.)

California, where tall redwood forests cover the ocean-facing side of the mountains and Death Valley lies on the lee side.

[ See also Plate tectonics ; Volcano ]