Himalayas, The world's highest and youngest mountain belt. It is not a single range or chain but consists of a series of chains running roughly parallel to one another for long distances and converging at places, with numerous valleys and plateaus or duns between them. Rising as a series of low hills from the plains they go on getting higher and higher till their highest ranges and their renowned peaks such as the Everest (8,848m), Mt K2 (8,610m), the Kanchenjunga (8,585m), etc almost whisper to the skies. The general trend of a few hill ranges is transverse to that of the principal Himalayan belt. These are the Assam ranges, the Manipur ranges, the Arakan Yoma, the chittagong hill tracts, the Pegu Yoma, etc.
The Himalayas and other central Asian mountains take their rise or radiate from the Pamirs or the 'roof of the world'. From there the Himalayas extend to the southeast bordering the northern, western and eastern frontiers of the Indian subcontinent. In their eastern parts the mountain ranges appear to be compressed and are closer together than in the western parts and accordingly they attain their greatest height rather abruptly and for that reason their snow capped peaks such as the Kanchenjunga is visible from the northern parts of Bangladesh.
The principal chains of the mountains extend for some 3,000 km between the borders of Afghanistan and Myanmar. Geographically, the Himalayas extend from the Indus river in Gilgit in Pakistan through India, Tibet, Nepal, East India, and Bhutan to the southern bend of the brahmaputra river in SE Tibet. The Himalayas have been classified transversely by some of the prominent streams that cut across them, such as the Indus, the Sutlej, the Kali, tista and the Brahmaputra. The portions of the mountains between the Indus and the Sutlej are known as the Punjab and the Kashmir Himalaya (563.15 km long). The next block between the Sutlej and the Kali is called the Kumaon Himalaya (321.80 km long) and the third between the Kali and the Tista the Nepal Himalaya (804.50 km long). The fourth segment is situated between the Tista and the Brahmaputra and is called the Assam Himalaya (724.05 km long).
Geographically, the Himalayan mountain belt can be classified into the Sub-Himalaya (consists of the Siwalik sediments); the Lower Himalaya (elevations from 1,500 to 3,000m); the Higher Himalaya (elevations from 3,000 to 8,000 m); the Indus-Zangbo Suture Zone; and the Trans Himalaya. The Higher Himalayas contain a multitude of snow covered peaks of over 8,000 m high, such as Everest, Annapurna and Manaslu.
The Himalayas were principally formed as a result of the collision between the Indian and the Asian plates. After splitting from Gondwanaland, India drifted northwards to collide with the Asian landmass about 40 million years ago. The intervening tethys ocean was closed by northwards subduction beneath southern Tibet, and the collision created the Himalayan orogenic belt. Continuing northward movement of India at a rate of about 5 cm per year over the last 40 million years has caused it to indent Asia, and the resultant massive shortening is expressed by thrusting of the northern margin of India, by faulting and earthquakes in the Himalayas and China, by rifting and faulting in Tibet, and by the uplift of the Himalayas which is still continuing at rates of upto several millimetres per year.
Several models have been proposed for Himalayas geologic evolution. Fundamentally, these models agree on the northward drift of India in the Mesozoic, the consumption of the Tethyan ocean along the Indus-Tsangpo suture zone in the Cretaceous-early Tertiary, and the India-Asia collision and its attendant compression and deformation in the Cenozoic, giving rise to the Himalayas. The time of continental collision between India and Asia has been estimated to be 65-45 Ma. The uplift-denudation of the Himalayas since the continental collision has not been a uniform event throughout time, but rather has occurred in an episodic manner. A synthesis of available information points to the following scenario: Early Miocene (21-17 Ma) phase of uplift-denudation, Late Miocene (11-7 Ma) phase of uplift-denudation, and Quaternary phase of uplift-denudation.
The bengal basin of Bangladesh is filled mainly by orogenic sediment derived from the eastern Himalayas to the north and the Indo-Burman ranges to the east. A major palaeo-drainage system (siwalik river or Indo-Brahm river) flowed from the eastern Himalayas draining through present day Assam and bringing orogenic sediments into the Sylhet trough (surma basin) from the east. The Indo-Burman ranges contributed sediment into the Chittagong area, but the major sediment source for Miocene strata of the Bengal Basin was likely the early uplifts of the eastern Himalayas.
Detailed investigation on the Himalayas began in the late 19th century, but most knowledge has accumulated since about 1950. Sir Edmund Hillary and Tenzing Norgay first climbed Everest in 1953.
The upheaval of the Himalayas represents a significant event in the Cenozoic history of our planet, not only for its geological implications, but also for the impact it has had on the ecology of Asia. The only home of the glaciers in the vast subcontinent is the Himalayas where its highest ranges remain perpetually covered by snowfields of great magnitude and from which descend numerous tongues of ice, some large and some small, called glaciers. Examples of such spectacular glaciers near Bangladesh border are Zemu (Sikkim) and Kanchenjunga some 26 and 16 km in length.
The Himalayan peaks have long been the goal of mountaineers. The towering ranges are almost inaccessible to travel, even by air. Railways reach only the southern foothills from where the main route follows footpaths across primitive bridges, ropeways, and high mountain passes. Metalled roads run between Kashmir and China and from India through Nepal and Sikkim to China, and there are major airports at Kathmandu and Srinagar. The snow covered and extensively glaciated southern slopes give rise to the Indian subcontinent's major rivers, including the Indus, Sutlej, ganges and the Brahmaputra. Little of the Himalayan region is habitable. A characteristic feature of the Lesser Himalayan ranges is that their southern slopes facing the plains of India are much steeper than their northern slopes. Again the northern slopes of the ranges are almost everywhere clothed with dense jungle growth, getting thinner and stunted higher up till it is too cold for any vegetation to grow and the ranges are capped with masses of snow and ice. The southern slopes are too steep and exposed to the sun for the growth of any vegetation or the accumulation of snow. The southern piedmont plains of Tarai are jungle and swamps with many wild animals.
The vegetation is almost luxuriant where the mountains are fully exposed to monsoon approaching from the bay of bengal as in Sikkim. Vegetation changes from tropical rain forest to wet, moist and dry deciduous forests of sal and finally desert in the valleys of Indus and Kunar. Grazing is possible on some of the gentler slopes and farming is carried on in the valleys. The Himalayan fauna in eastern India (close to Bangladesh border) is mainly characterised by monkey, elephant, rhinoceros, tiger, leopard, mongoose, red panda etc. The bird life of the Himalayas is wonderfully rich. Among the snakes the python and the cobra are worth mentioning. Limited amounts of iron ore, gold and sapphires are found in the west. The Himalayan rivers offer much scope for hydroelectric power and irrigation development. Hill resorts such as Simla, Nainital, Mussoorie and Darjeeling are popular summer retreats from the heat of the Indian plains. In a fine clear winter morning one can see the snow capped peak of Kanchenjunga from Rangpur and Panchagarh districts of Bangladesh. [Sifatul Quader Chowdhury]