(Skt., “abode of snow”), mountain system, S Asia, comprising a series of parallel and converging ranges and forming the highest mountain region in the world. More than 30 peaks of the Himalayas rise to heights of 7620 m (25,000 ft) or more, and one of these, Mt. Everest (8850 m/29,035 ft), is the world's highest mountain. The vast Himalayan complex extends in an arc of about 2410 km (about 1500 mi) from the Indus R. in N Pakistan eastward across Kashmir, N India, part of S Tibet, and most of Nepal, Sikkim, and Bhutan; the system covers an area of about 594,400 sq km (about 229,500 sq mi).

Geologic Formation and Structure.

During the Paleozoic and Mesozoic eras (65 to 570 million years ago; see MESOZOIC ERA,, PALEOZOIC ERA,) the area that is now the Himalayas occupied the floor of the ancient Tethys Sea on the N part of the Indian Plate of the earth's crust. The mountains were formed as the Indian Plate, moving N, pressed against the stationary Asian landmass (see PLATE TECTONICS,). The main uplift occurred during the middle or late (q.v.; 12 to 65 million years ago). The ranges of the system developed from N to S in a series of stages. Even today the system has not reached a state of equilibrium, and earthquakes are frequent.

The Himalayas consist primarily of metamorphic rocks; extensive areas of igneous rocks are in the S. Paleozoic and Mesozoic marine sediments are found in several regions.

Topography.

The Himalayas can be divided into four parallel, longitudinal mountain belts of varying widths. From S to N these belts are the Outer Himalayas, or Sub-Himalayas; the Lesser Himalayas; the Great Himalayas; and the Tethys, or Tibetan Himalayas. The Outer Himalayas comprise the Siwalik range, which rises steeply from the N Indian plains and descends gently to flat-floored basins. To the N is the Lesser Himalayas, where the mountains rise to 4572 m (15,000 ft) and the valleys lie at altitudes of 914 m (about 3000 ft). The Great Himalayas, which form the backbone of the Himalayan system, contain the main ranges and highest peaks and rise above the snow line to an average elevation of about 6100 m (about 20,000 ft). The maximum height of these mountains is in Nepal, where 9 of the world's 14 highest peaks are found. Among these are KANCHENJUNGA,, DHAULAGIRI,, and ANNAPURNA, (qq.v.). To the N are the more complex Tibetan ranges and plateaus.

The passes in the Himalayas, which often lie along or across glaciers, are the highest in the world, averaging about 3050 m (about 10,000 ft) high. All passes above about 4880 m (about 16,000 ft) are closed by snow from November to May.

Rivers, Lakes, and Glaciers.

The Himalayas are drained by several major Asian rivers, including the Brahmaputra, Indus, and the Sutlej, as well as by many of their important tributaries. The headwaters of the Ganges R. are also in the Himalayas. Numerous small glacial lakes are found at the heads of gorges, but the largest lakes lie at comparatively low altitudes. Above the snow line, in all parts of the Himalayan system, are found small glaciers. Several glaciers are as much as 48 km (30 mi) long; the majority, however, are less than half that size.

Climate.

Three seasons are generally recognizable in the Himalayas: a cold period from October to February, a hot period from March to June, and the SW monsoon season with heavy rains (particularly in the E) from June to September. The high main range of the Himalayas forms a vast screen that intercepts and condenses nearly all the moisture carried by the monsoons. This moisture is deposited on the S face of the mountains, which has a heavy annual rainfall; the N slopes are semiarid or arid.

The altitude of the Himalayas also affects the temperature range. The climate on the S side of the system varies from subtropical at the base and valleys, through temperate at elevations of about 2130 m (about 7000 ft), to alpine, or tundra, at 3660 m (12,000 ft). The system's line of permanent snow lies at about 5030 m (about 16,500 ft). Strong winds prevail throughout in the high-altitude areas.

Vegetation and Animal Life.

Vegetation is generally more lush in the E where rainfall is heaviest. At elevations up to 610 m (2000 ft), a zone of grass is found in the W, and a dense, swampy subtropical forest known as the Terai is found in the E. At higher elevations is a monsoon forest in which sal, a close-grained hardwood, is the most characteristic tree. Evergreen oak and rhododendron predominate at 1524 to 2743 m (5000 to 9000 ft); the deodar cedar is also found here. Above this level, to an elevation of about 3658 m (12,000 ft) are coniferous forests, and at higher elevations, extending to the snowline, lies an alpine zone of low shrubs and grasses.

Elephant, buffalo, and rhinoceros are restricted to certain areas of the forested Terai. The Himalayan black bear, the clouded leopard, the langur monkey, and the goat antelope are widespread in the higher Himalayan forests. Above the tree line, the snow leopard, the brown bear, the red panda, and the Tibetan yak can occasionally be seen. The yak has been domesticated and is used as a beast of burden. Bird life is particularly rich in the E regions.        R.E.S., RODMAN E. SNEAD, M.A., Ph.D.

For further information on this topic, see the Bibliography, sections 795. Climbing, 867. Mountains, 1087. Himalayas, Himalayan states.