Himalayas, Nepali Himalaya, great mountain system of Asia forming a barrier between the Plateau of Tibet to the north and the alluvial plains of the Indian subcontinent to the south. The Himalayas include the highest mountains in the world, with more than 110 peaks rising to elevations of 24,000 feet (7,300 metres) or more above sea level. One of those peaks is Mount Everest (Tibetan: Chomolungma; Chinese: Qomolangma Feng; Nepali: Sagarmatha), the world’s highest, with an elevation of 29,035 feet (8,850 metres; see Researcher’s Note: Height of Mount Everest. The mountains’ high peaks rise into the zone of perpetual snow.
For thousands of years the Himalayas have held a profound significance for the peoples of South Asia, as their literature, mythologies, and religions reflect. Since ancient times the vast glaciated heights have attracted the attention of the pilgrim mountaineers of India, who coined the Sanskrit name Himalaya—from hima (“snow”) and alaya (“abode”)—for that great mountain system. In contemporary times the Himalayas have offered the greatest attraction and the greatest challenge to mountaineers throughout the world.

The ranges, which form the northern border of the Indian subcontinent and an almost impassable barrier between it and the lands to the north, are part of a vast mountain belt that stretches halfway around the world from North Africa to the Pacific Ocean coast of Southeast Asia. The Himalayas themselves stretch uninterruptedly for about 1,550 miles (2,500 km) from west to east between Nanga Parbat (26,660 feet [8,126 metres]), in the Pakistani-administered portion of the Kashmir region, and Namjagbarwa (Namcha Barwa) Peak (25,445 feet [7,756 metres]), in the Tibet Autonomous Region of China. Between those western and eastern extremities lie the two Himalayan countries of Nepal and Bhutan. The Himalayas are bordered to the northwest by the mountain ranges of the Hindu Kush and the Karakoram and to the north by the high and vast Plateau of Tibet. The width of the Himalayas from south to north varies between 125 and 250 miles (200 and 400 km). Their total area amounts to about 230,000 square miles (595,000 square km).
Though India, Nepal, and Bhutan have sovereignty over most of the Himalayas, Pakistan and China also occupy parts of them. In the disputed Kashmir region, Pakistan has administrative control of some 32,400 square miles (83,900 square km) of the range lying north and west of the “line of control” established between India and Pakistan in 1972. China administers some 14,000 square miles (36,000 square km) in the Ladakh district of Kashmir and has claimed territory at the eastern end of the Himalayas within the Indian state of Arunachal Pradesh. Those disputes accentuate the boundary problems faced by India and its neighbours in the Himalayan region.

The most characteristic features of the Himalayas are their soaring heights, steep-sided jagged peaks, valley and alpine glaciers often of stupendous size, topography deeply cut by erosion, seemingly unfathomable river gorges, complex geologic structure, and series of elevational belts (or zones) that display different ecological associations of flora, fauna, and climate. Viewed from the south, the Himalayas appear as a gigantic crescent with the main axis rising above the snow line, where snowfields, alpine glaciers, and avalanches all feed lower-valley glaciers that in turn constitute the sources of most of the Himalayan rivers. The greater part of the Himalayas, however, lies below the snow line. The mountain-building process that created the range is still active. As the bedrock is lifted, considerable stream erosion and gigantic landslides occur.
The Himalayan ranges can be grouped into four parallel longitudinal mountain belts of varying width, each having distinct physiographic features and its own geologic history. They are designated, from south to north, as the Outer, or Sub-, Himalayas (also called the Siwalik Range); the Lesser, or Lower, Himalayas; the Great Himalaya Range (Great Himalayas); and the Tethys, or Tibetan, Himalayas. Farther north lie the Trans-Himalayas in Tibet proper. From west to east the Himalayas are divided broadly into three mountainous regions: western, central, and eastern.

During the Jurassic Period (about 201 to 145 million years ago), a deep crustal downwarp—the Tethys Ocean—bordered the entire southern fringe of Eurasia, then excluding the Arabian Peninsula and the Indian subcontinent. About 180 million years ago, the old supercontinent of Gondwana (or Gondwanaland) began to break up. One of Gondwana’s fragments, the lithospheric plate that included the Indian subcontinent, pursued a northward collision course toward the Eurasian Plate during the ensuing 130 to 140 million years.
The Indian-Australian Plate gradually confined the Tethys trench within a giant pincer between itself and the Eurasian Plate. As the Tethys trench narrowed, increasing compressive forces bent the layers of rock beneath it and created interlacing faults in its marine sediments. Masses of granites and basalts intruded from the depth of the mantle into that weakened sedimentary crust. Between about 40 and 50 million years ago, the Indian subcontinent finally collided with Eurasia. The plate containing India was sheared downward, or subducted, beneath the Tethys trench at an ever-increasing pitch.

During the next 30 million years, shallow parts of the Tethys Ocean gradually drained as its sea bottom was pushed up by the plunging Indian-Australian Plate; that action formed the Plateau of Tibet. On the plateau’s southern edge, marginal mountains—the Trans-Himalayan ranges of today—became the region’s first major watershed and rose high enough to become a climatic barrier. As heavier rains fell on the steepening southern slopes, the major southern rivers eroded northward toward the headwaters with increasing force along old transverse faults and captured the streams flowing onto the plateau, thus laying the foundation for the drainage patterns for a large portion of Asia. To the south the northern reaches of the Arabian Sea and the Bay of Bengal rapidly filled with debris carried down by the ancestral Indus, Ganges (Ganga), and Brahmaputra rivers. The extensive erosion and deposition continue even now as those rivers carry immense quantities of material every day.
The Himalayas are drained by 19 major rivers, of which the Indus and the Brahmaputra are the largest, each having catchment basins in the mountains of about 100,000 square miles (260,000 square km) in extent. Five of the 19 rivers, with a total catchment area of about 51,000 square miles (132,000 square km), belong to the Indus system—the Jhelum, the Chenab, the Ravi, the Beas, and the Sutlej—and collectively define the vast region divided between Punjab state in India and Punjab province in Pakistan. Of the remaining rivers, nine belong to the Ganges system—the Ganges, Yamuna, Ramganga, Kali (Kali Gandak), Karnali, Rapti, Gandak, Baghmati, and Kosi rivers—draining roughly 84,000 square miles (218,000 square km) in the mountains, and three belong to the Brahmaputra system—the Tista, the Raidak, and the Manas—draining another 71,000 square miles (184,000 square km) in the Himalayas.

The major Himalayan rivers rise north of the mountain ranges and flow through deep gorges that generally reflect some geologic structural control, such as a fault line. The rivers of the Indus system as a rule follow northwesterly courses, whereas those of the Ganges-Brahmaputra systems generally take easterly courses while flowing through the mountain region.
To the north of India, the Karakoram Range, with the Hindu Kush range on the west and the Ladakh Range on the east, forms the great water divide, shutting off the Indus system from the rivers of Central Asia. The counterpart of that divide on the east is formed by the Kailas Range and its eastward continuation, the Nyainqêntanglha (Nyenchen Tangla) Mountains, which prevent the Brahmaputra from draining the area to the north. South of that divide, the Brahmaputra flows to the east for about 900 miles (1,450 km) before cutting across the Great Himalaya Range in a deep transverse gorge, although many of its Tibetan tributaries flow in an opposite direction, as the Brahmaputra may once have done.