Himalayas (हिमालय) are a mountain range extending from north India to north-eastern India, separating the Indian subcontinent from the Tibetan Plateau. Himālaya, in Sanskrit, means "the abode of snow" (from hima "snow", and ālaya "abode". Himalayas are the earth's highest and greatest mountain range.

The Himalayas stretch across India, Bhutan, Tibet, Nepal, Afghanistan and Pakistan. It is the source of two major river systems, the Indus Basin, and the Ganga-Brahmaputra Basin. An estimated 750 million people live in the watershed area of the Himalayan rivers. The Himalayas, geologically young and structurally fold mountains stretch over the northern borders of India. These mountain ranges run in a west-east direction from the Indus to the Brahmaputra. The Himalayas form an arc, which covers a distance of about 2,400 km. Their width varies from 400 km in Kashmir to 150 km in Arunachal Pradesh. The altitudinal variations are greater in the eastern half than those in the western half. The Himalayas consists of three parallel ranges in its longitudinal extent. A number of valleys lie between these ranges. The northern most range is known as the Great or Inner Himalayas or the 'Himadri'.

Ecology

The flora and fauna of the Himalayas varies with rainfall, altitude, and soils. The climate ranges from tropical at the base of the mountains to permanent ice and snow at the highest elevations. The amount of yearly rainfall increases from west to east along the front of the range. This diversity of altitude, rainfall and soil conditions generates a variety of distinct plant and animal communities, or ecoregions.

Lowland forests: On the Indo-Gangetic plain at the base of the mountains, an alluvial plain drained by the Indus and Ganges-Brahmaputra river systems, vegetation varies from west to east with rainfall. The xeric Northwestern thorn scrub forests occupy the plains of Pakistan and the Indian Punjab. Further east lie the Upper Gangetic plains moist deciduous forests of Uttar Pradesh and Lower Gangetic plains moist deciduous forests of Bihar and West Bengal. These are monsoon forests, with drought-deciduous trees that lose their leaves during the dry season. The moister Brahmaputra Valley semi-evergreen forests occupy the plains of Assam.

Terai belt: Above the alluvial plain lies the Terai strip, a seasonally marshy zone of sand and clay soils. The Terai has higher rainfall than the plains, and the downward-rushing rivers of the Himalaya slow down and spread out in the flatter Terai zone, depositing fertile silt during the monsoon season and receding in the dry season. The Terai has a high water table, and the central part of the Terai belt is occupied by the Terai-Duar savanna and grasslands, a mosaic of grasslands, savannas, deciduous and evergreen forests that includes some of the world's tallest grasslands.

Bhabhar belt: Above the Terai belt is an upland zone known as the Bhabhar, with porous and rocky soils, made up of debris washed down from the higher ranges. The Bhabhar and the lower Siwalik ranges have a subtropical climate. The Himalayan subtropical pine forests occupy the western end of the subtropical belt, with forests dominated by Chir Pine (Pinus roxburghii). The central part of the range is home to the Himalayan subtropical broadleaf forests, dominated by Sal (Shorea robusta).

Montane forests: At the middle elevations of the range, the subtropical forests yield to a belt of temperate broadleaf and mixed forests, with the Western Himalayan broadleaf forests at the western end of the range, and the Eastern Himalayan broadleaf forests in Assam and Arunachal Pradesh. Above the broadleaf forests are the Western and Eastern Himalayan sub alpine conifer forests.

Alpine shrub and grasslands: Above the tree line are the Northwestern, Western, and Eastern Himalayan alpine shrub and meadows, which yield to tundra in the higher Himalayan range.

Geological origin

The Himalayas are among the youngest mountain ranges on the planet. Their formation is a result of a continental collision along the convergent boundary between the Indo-Australian Plate and the Eurasian Plate. The collision began in the Upper Cretaceous period about 70 million years ago. By about 50 million years ago this fast moving Indo-Australian plate had completely closed the Tethys Ocean, whose existence has been determined by sedimentary rocks settled on the ocean floor. The Indo-Australian plate continues to be driven horizontally below the Tibetan plateau, which forces the plateau to move upwards. The Indo-Australian plate is moving at 67 mm/year, and this leads to the Himalayas rising by about 5 mm/year. The movement of the Indian plate into the Asian plate also makes this region seismically active, leading to earthquakes from time to time.

Lakes

The Himalaya region is dotted with hundreds of lakes. Most lakes are found at altitudes of less than 5,000 m, with the size of the lakes diminishing with altitude. The largest lake is the Pangong Tso, which is spread across the border between India and Tibet. It is situated at an altitude of 4,600 m, and is 8 km wide and nearly 134 km long. Another notable high altitude lake is the Gurudogmar in north Sikkim at an altitude of 5,148 m. Other major lakes include the Tsongmo lake, near the Indo-Tibetan border in Sikkim and Tilicho lake. The mountain lakes are known to geographers as tarns if they are caused by glacial activity. Tarns are found mostly in the upper reaches of the Himalaya, above 5,500 m.

River systems

The higher regions of the Himalayas are snowbound throughout the year, and they form the sources for several perennial rivers, most of which combine into two large river systems; the western rivers combining into the Indus Basin and the eastern rivers into the Ganga-Brahmaputra Basin. The Indus begins in Tibet and flows southwest through India and Pakistan to the Arabian Sea. It is fed by the Jhelum, the Chenab, the Ravi, the Beas, and the Sutluj rivers. The Ganga originates from the Gangotri glacier and flows southeast through the plains of north India, fed by the Alaknanda and the Yamuna among other tributaries. The Brahmaputra originates as the Tsangpo in western Tibet, and flows east through Tibet and west through the plains of Assam. The Ganga and the Brahmaputra meet in Bangladesh, and drain into the Bay of Bengal. The eastern-most Himalayan rivers feed the Ayeyarwady River, which originates in eastern Tibet and flows south through Myanmar to drain into the Andaman Sea.

Glaciers

The Himalayan range encompasses a very large number of glaciers, notable among which is the Siachen Glacier, the largest in the world outside the polar region. Some of the other more famous glaciers include the Gangotri and Yamunotri (Uttarakhand), Nubra, Biafo and Baltoro (Karakoram region), Zemu (Sikkim) and Khumbu glaciers (Mount Everest region).