The Himalayas
The Himalayas, the highest mountain range in the world, extend along the northern frontiers of Pakistan, India, Nepal, Bhutan, and Burma. They were formed geologically as a result of the collision of the Indian subcontinent with Asia.
This process of plate tectonics is ongoing, and the gradual northward drift of the Indian subcontinent still causes earthquakes. Lesser ranges jut southward from the main body of the Himalayas at both the eastern and western ends.
The Himalayan system, about 2,400 km in length and varying in width from 240 to 330 km, is made up of three parallel ranges - the Greater Himalayas, the Lesser Himalayas, and the Outer Himalayas - sometimes collectively called the Great Himalayan Range.
The Greater Himalayas, or northern range, average approximately 6,000 m in height and contain the three highest mountains on earth: Mount Everest (8,796 m) on the China-Nepal border; K2 (8,611 m, also known as Mount Godwin-Austen, and in China as Qogir Feng) in an area claimed by India, Pakistan, and China; and Kanchenjunga (8,598 m) on the India-Nepal border.
Many major mountains are located entirely within India, such as Nanda Devi (7,817 m) in the state of Uttar Pradesh. The snow line averages 4,500 to 6,000 m on the southern side of the Greater Himalayas and 5,500 to 6,000 on the northern side. Because of climatic conditions, the snow line in the eastern Himalayas averages 4,300 m, while in the western Himalayas it averages 5,800 m.
The Lesser Himalayas, located in northwestern India in the states of Himachal Pradesh and Uttar Pradesh, in north-central India in the state of Sikkim, and in northeastern India in the state of Arunachal Pradesh, range from 1,500 to 5,000 m in height.
Located in the Lesser Himalayas are the hill stations of Shimla (Simla) and Darjeeling. During the colonial period, these and other hill stations were used by the British as summer retreats to escape the intense heat of the plains. It is in this transitional vegetation zone that the contrasts between the bare southern slopes and the forested northern slopes become most noticeable.
The Outer or Southern Himalayas, averaging 900 to 1,200 m in elevation, lie between the Lesser Himalayas and the Indo-Gangetic Plain. In Himachal Pradesh and Uttar Pradesh, this southernmost range is often referred to as the Siwalik Hills.
It is possible to identify a fourth, and northernmost range, known as the Trans-Himalaya. This range is located entirely on the Qinghai-Xizang Plateau, north of the great west-to-east trending valley of the Yarlung Zangbo River. Although the Trans-Himalaya Range is divided from the Great Himalayan Range for most of its length, it merges with the Great Himalayan Range in the western section - the Karakoram Range - where India, Pakistan, and China meet.
The southern slopes of each of the Himalayan ranges are too steep to accumulate snow or support much tree life; the northern slopes generally are forested below the snow line. Between the ranges are extensive high plateaus, deep gorges, and fertile valleys, such as the vales of Kashmir and Kulu.
The Himalayas serve a very important purpose. They provide a physical screen within which the monsoon system operates and are the source of the great river systems that water the alluvial plains below. As a result of erosion, the rivers coming from the mountains carry vast quantities of silt that enrich the plains.
The area of northeastern India adjacent to Burma and Bangladesh consists of numerous hill tracts, averaging between 1,000 and 2,000 m in elevation, that are not associated with the eastern part of the Himalayas in Arunachal Pradesh.
The Naga Hills, rising to heights of more than 3,000 meters, form the watershed between India and Burma. The Mizo Hills are the southern part of the northeastern ranges in India.
The Garo, Khasi, and Jaintia hills are centered in the state of Meghalaya and, isolated from the northeastern ranges, divide the Assam Valley from Bangladesh to the south and west.