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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.
Over the past 65 million years, powerful global plate-tectonic forces have moved the Earth’s crust to form the band of Eurasian mountain ranges—including the Himalayas—that stretch from the Alps to the mountains of Southeast Asia.
During the Jurassic Period (about 200 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 million years. This 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 this weakened sedimentary crust. About 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; this 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 today’s drainage patterns. 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 these rivers carry immense quantities of material every day.
Finally, some 20 million years ago, during the early Miocene Epoch, the tempo of the crunching union between the two plates increased sharply, and Himalayan mountain building began in earnest. As the Indian subcontinental plate continued to plunge beneath the former Tethys trench, the topmost layers of old Gondwana metamorphic rocks peeled back over themselves for a long horizontal distance to the south, forming nappes. Wave after wave of nappes thrust southward over the Indian landmass for as far as 60 miles (about 100 km). Each new nappe consisted of Gondwana rocks older than the last. In time these nappes became folded, contracting the former trench by some 250 to 500 horizontal miles (400 to 800 km). All the while, downcutting rivers matched the rate of uplift, carrying vast amounts of eroded material from the rising Himalayas to the plains where it was dumped by the Indus, Ganges, and Brahmaputra rivers. The weight of this sediment created depressions, which in turn could hold more sediment. In some places the alluvium beneath the Gangetic Plain now exceeds 25,000 feet (7,600 metres) in depth.
Probably only within the past 600,000 years, during the Pleistocene Epoch (roughly 2,600,000 to 11,700 years ago), did the Himalayas become the highest mountains on Earth. If strong horizontal thrusting characterized the Miocene and the succeeding Pliocene Epoch (about 23 to 2.6 million years ago), intense uplift epitomized the Pleistocene. Along the core zone of the northernmost nappes—and just beyond—crystalline rocks containing new gneiss and granite intrusions emerged to produce the staggering crests seen today. On a few peaks, such as Mount Everest, the crystalline rocks carried old fossil-bearing Tethys sediments from the north piggyback to the summits.
Once the Great Himalayas had risen high enough, they became a climatic barrier: the marginal mountains to the north were deprived of rain and became as parched as the Plateau of Tibet. In contrast, on the wet southern flanks the rivers surged with such erosive energy that they forced the crest line to migrate slowly northward. Simultaneously, the great transverse rivers breaching the Himalayas continued their downcutting in pace with the uplift. Changes in the landscape, however, compelled all but these major rivers to reroute their lower courses because, as the northern crests rose, so also did the southern edge of the extensive nappes. The formations of the Siwalik Series were overthrust and folded, and in between the Lesser Himalayas downwarped to shape the midlands. Now barred from flowing due south, most minor rivers ran east or west through structural weaknesses in the midlands until they could break through the new southern barrier or join a major torrent.
In some valleys, such as the Vale of Kashmir and the Kathmandu Valley of Nepal, lakes formed temporarily and then filled with Pleistocene deposits. After drying up some 200,000 years ago, the Kathmandu Valley rose at least 650 feet (200 metres), an indication of localized uplift within the Lesser Himalayas.
The Outer Himalayas comprise flat-floored structural valleys and the Siwalik Range, which borders the Himalayan mountain system to the south. Except for small gaps in the east, the Siwaliks run for the entire length of the Himalayas, with a maximum width of 62 miles (100 km) in the northern Indian state of Himachal Pradesh. In general, the 900-foot (275-metre) contour line marks their southern boundary; they rise an additional 2,500 feet (760 metres) to the north. The main Siwalik Range has steeper southern slopes facing the Indian plains and descends gently northward to flat-floored basins, called duns. The best-known of these is the Dehra Dun, in southern Uttarakhand state, just north of the border with northwestern Uttar Pradesh state.
To the north the Siwalik Range abuts a massive mountainous tract, the Lesser Himalayas. In this range, 50 miles (80 km) in width, mountains rising to 15,000 feet (4,500 metres) and valleys with elevations of 3,000 feet (900 metres) run in varying directions. Neighbouring summits share similar elevations, creating the appearance of a highly dissected plateau. The three principal ranges of the Lesser Himalayas—the Nag Tibba, the Dhaola Dhar, and the Pir Panjal—have branched off from the Great Himalaya Range lying farther north. The Nag Tibba, the most easterly of the three ranges, is some 26,800 feet (8,200 metres) high near its eastern end, in Nepal, and forms the watershed between the Ganges and Yamuna rivers in Uttarakhand.
To the west is the picturesque Vale of Kashmir, in Jammu and Kashmir state (the Indian-administered portion of Kashmir). A structural basin (i.e., an elliptical basin in which the rock strata are inclined toward a central point), the vale forms an important section of the Lesser Himalayas. It extends from southeast to northwest for 100 miles (160 km), with a width of 50 miles (80 km), and has an average elevation of 5,100 feet (1,600 metres). The basin is traversed by the meandering Jhelum River, which runs through Wular Lake, a large freshwater lake in Jammu and Kashmir.
The backbone of the entire mountain system is the Great Himalaya Range, rising into the zone of perpetual snow. The range reaches its maximum height in Nepal; among its peaks are 10 of the 13 highest in the world, each of which exceeds 26,250 feet (8,000 metres) in elevation. From west to east those peaks are Nanga Parbat, Dhaulagiri 1, Annapurna 1, Manaslu 1, Xixabangma (Gosainthan), Cho Oyu, Mount Everest, Lhotse, Makalu 1, and Kanchenjunga 1.
The range trends northwest-southeast from Jammu and Kashmir to Sikkim, an old Himalayan kingdom that is now a state of India. East of Sikkim it runs east-west for another 260 miles (420 km) through Bhutan and the eastern part of Arunachal Pradesh as far as the peak of Kangto (23,260 feet [7090 metres]) and finally bends northeast, terminating at Namcha Barwa.
There is no sharp boundary between the Great Himalayas and the ranges, plateaus, and basins lying to the north of the Great Himalayas, generally grouped together under the names of the Tethys, or Tibetan, Himalayas and the Trans-Himalayas, which extend far northward into Tibet. In Kashmir and in the Indian state of Himachal Pradesh, the Tethys are at their widest, forming the Spiti Basin and the Zaskar Range.
here is no sharp boundary between the Great Himalayas and the ranges, plateaus, and basins lying to the north of the Great Himalayas, generally grouped together under the names of the Tethys, or Tibetan, Himalayas and the Trans-Himalayas, which extend far northward into Tibet. In Kashmir and in the Indian state of Himachal Pradesh, the Tethys are at their widest, forming the Spiti Basin and the Zaskar Range.
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. Of the other rivers, five belong to the Indus system—the Jhelum, the Chenab, the Ravi, the Beas, and the Sutlej—with a total catchment area of about 51,000 square miles (132,000 square km); nine belong to the Ganges system—the Ganges, Yamuna, Ramganga, Kali (Kali Gandak), Karnali, Rapti, Gandak, Baghmati, and Kosi rivers—draining another 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 this 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 this 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.
The Great Himalayas, which normally would form the main water divide throughout their entire length, function as such only in limited areas. This situation exists because the major Himalayan rivers, such as the Indus, the Brahmaputra, the Sutlej, and at least two headwaters of the Ganges—the Alaknanda and the Bhagirathi—are probably older than the mountains they traverse. It is believed that the Himalayas were uplifted so slowly that the old rivers had no difficulty in continuing to flow through their channels and, with the rise of the Himalayas, acquired an even greater momentum, which enabled them to cut their valleys more rapidly. The elevation of the Himalayas and the deepening of the valleys thus proceeded simultaneously. As a result, the mountain ranges emerged with a completely developed river system cut into deep transverse gorges that range in depth from 5,000 to 16,000 feet (1,500 to 5,000 metres) and in width from 6 to 30 miles (10 to 50 km). The earlier origin of the drainage system explains the peculiarity that the major rivers drain not only the southern slopes of the Great Himalayas but, to a large extent, its northern slopes as well, the water divide being north of the crest line.
The role of the Great Himalaya Range as a watershed, nevertheless, can be seen between the Sutlej and Indus valleys for 360 miles (580 km); the drainage of the northern slopes is carried by the north-flowing Zaskar and Dras rivers, which drain into the Indus. Glaciers also play an important role in draining the higher elevations and in feeding the Himalayan rivers. Several glaciers occur in Uttarakhand, of which the largest, the Gangotri, is 20 miles (32 km) long and is one of the sources of the Ganges. The Khumbu Glacier drains the Everest region in Nepal and is one of the most popular routes for the ascent of the mountain. The rate of movement of the Himalayan glaciers varies considerably; in the neighbouring Karakoram Range, for example, the Baltoro Glacier moves about 6 feet (2 metres) per day, while others, such as the Khumbu, move only about 1 foot (30 cm) daily. Most of the Himalayan glaciers are in retreat, at least in part because of climate change.
The north-facing slopes generally have a fairly thick soil cover, supporting dense forests at lower elevations and grasses higher up. The forest soils are dark brown in colour and silt loam in texture; they are ideally suited for growing fruit trees. The mountain meadow soils are well developed but vary in thickness and in their chemical properties. Some of the wet deep upland soils of this type in the eastern Himalayas—for example, in the Darjiling (Darjeeling) Hills and in the Assam valley—have a high humus content that is good for growing tea. Podzolic soils (infertile acidic forest soils) occur in a belt some 400 miles (640 km) long in the valleys of the Indus and its tributary the Shyok River, to the north of the Great Himalaya Range, and in patches in Himachal Pradesh. Farther east, saline soils occur in the dry high plains of the Ladakh region. Of the soils that are not restricted to any particular area, alluvial soils (deposited by running water) are the most productive, though they occur in limited areas, such as the Vale of Kashmir, the Dehra Dun, and the high terraces flanking the Himalayan valleys. Lithosols, consisting of imperfectly weathered rock fragments that are deficient in humus content, cover many large areas at high altitudes and are the least-productive soils.
The Himalayas, as a great climatic divide affecting large systems of air and water circulation, help determine meteorological conditions in the Indian subcontinent to the south and in the Central Asian highlands to the north. By virtue of its location and stupendous height, the Great Himalaya Range obstructs the passage of cold continental air from the north into India in winter and also forces the southwesterly monsoon (rain-bearing) winds to give up most of their moisture before crossing the range northward. The result is heavy precipitation (both rain and snow) on the Indian side but arid conditions in Tibet. The average annual rainfall on the south slopes varies between 60 inches (1,530 mm) at Shimla, Himachal Pradesh, and Mussoorie, Uttarakhand, in the western Himalayas and 120 inches (3,050 mm) at Darjiling, West Bengal state, in the eastern Himalayas. North of the Great Himalayas, at places such as Skardu, Gilgit, and Leh in the Kashmir portion of the Indus valley, only 3 to 6 inches (75 to 150 mm) of precipitation occur.
Local relief and location determine climatic variation not only in different parts of the Himalayas but even on different slopes of the same range. Because of its favourable location on top of the Mussoorie Range facing the Dehra Dun, the town of Mussoorie, for example, at an elevation of about 6,100 feet (1,900 metres), receives 92 inches (2,335 mm) of precipitation annually, compared with 62 inches (1,575 mm) in the town of Shimla, which lies some 90 miles (145 km) to the northwest behind a series of ridges reaching 6,600 feet (2,000 metres). The eastern Himalayas, which are at a lower latitude than the western Himalayas, are relatively warmer. The average minimum temperature for the month of May, recorded in Darjiling at an elevation of 6,380 feet (1,945 metres), is 52 °F (11 °C). In the same month, at an elevation of 16,500 feet (5,000 metres) in the neighbourhood of Mount Everest, the minimum temperature is about 17 °F (−8 °C); at 19,500 feet (6,000 metres) it falls to −8 °F (−22 °C), the lowest minimum having been −21 °F (−29 °C); during the day, in areas sheltered from strong winds that often blow at more than 100 miles (160 km) per hour, the sun is often pleasantly warm, even at high elevations.
There are two periods of precipitation: the moderate amounts brought by winter storms and the heavier precipitation of summer, with its southwesterly monsoon winds. During winter, low-pressure weather systems advance into the Himalayas from the west and cause heavy snowfall. Within the regions where western disturbances are felt, condensation occurs in upper air levels; as a result, precipitation is much greater over the high mountains. During this season snow accumulates around the Himalayan high peaks, and precipitation is greater in the west than the east. In January, for example, Mussoorie in the west receives almost 3 inches (75 mm), whereas Darjiling to the east receives less than 1 inch (25 mm). By the end of May the meteorological conditions have reversed. Southwesterly monsoon currents channel moist air toward the eastern Himalayas, where the moisture rising over the steep terrain cools and condenses to fall as rain or snow; in June, therefore, Darjiling receives about 24 inches (600 mm) and Mussoorie less than 8 inches (200 mm). The rain and snow cease in September, after which the finest weather in the Himalayas prevails until the beginning of winter in December.
Himalayan vegetation can be broadly classified into four types—tropical, subtropical, temperate, and alpine—each of which prevails in a zone determined mainly by elevation and precipitation. Local differences in relief and climate, as well as exposure to sunlight and wind, cause considerable variation in the species present within each zone. Tropical evergreen rainforest is confined to the humid foothills of the eastern and central Himalayas. The evergreen dipterocarps—a group of timber- and resin-producing trees—are common; their different species grow on different soils and on hill slopes of varying steepness. Mesua ferrea (Ceylon ironwood) is found on porous soils at elevations between 600 and 2,400 feet (180 and 720 metres); bamboos grow on steep slopes; oaks and chestnuts grow on the lithosol, covering sandstones from Arunachal Pradesh westward to central Nepal at elevations from 3,600 to 5,700 feet (1,100 to 1,700 metres). Alder trees are found along the watercourses on the steeper slopes. At higher elevations these species give way to mountain forests in which the typical evergreen is Pandanus furcatus, a type of screw pine. Besides these trees, some 4,000 species of flowering plants, of which 20 are palms, are estimated to occur in the eastern Himalayas.
With decreasing precipitation and increasing elevation westward, the rainforests give way to tropical deciduous forests, where the valuable timber tree sal is the dominant species; wet sal forests thrive on high plateaus at elevations of 3,000 feet (900 metres), while dry sal forests prevail higher up, at 4,500 feet (1,400 metres). Farther west, steppe forest (i.e., expanse of grassland dotted with trees), steppe, subtropical thorn steppe, and subtropical semidesert vegetation occur successively. Temperate mixed forests extend from about 4,500 to roughly 11,000 feet (1,400 to 3,400 metres) and contain conifers and broad-leaved temperate trees. Evergreen forests of oaks and conifers have their westernmost outpost on the hills above Murree, some 30 miles (50 km) northwest of Rawalpindi, in Pakistan; these forests are typical of the Lesser Himalayas, being conspicuous on the outer slopes of the Pir Panjal, in Jammu and Kashmir state. Pinus roxburghii (chir pine) is the dominant species at elevations from 2,700 to 5,400 feet (800 to 1,600 metres). In the inner valleys this species may occur even up to 6,300 feet (1,900 metres). Deodar cedar, a highly valued endemic species, grows mainly in the western part of the range. Stands of this species occur between 6,300 and 9,000 feet (1,900 and 2,700 metres) and tend to grow at still higher elevations in the upper valleys of the Sutlej and Ganges rivers. Of the other conifers, blue pine and spruce first appear between about 7,300 and 10,000 feet (2,200 and 3,000 metres).
The alpine zone begins above the tree line, between elevations of 10,500 and 11,700 feet (3,200 and 3,600 metres), and extends up to about 13,700 feet (4,200 metres) in the western Himalayas and 14,600 feet (4,500 metres) in the eastern Himalayas. In this zone can be found all the wet and moist alpine vegetation. Juniper is widespread, especially on sunny sites, steep and rocky slopes, and drier areas. Rhododendron occurs everywhere but is more abundant in the wetter parts of the eastern Himalayas, where it grows in all sizes from trees to low shrubs. Mosses and lichens grow in shaded areas at lower levels in the alpine zone where the humidity is high; flowering plants are found at high elevations.
The fauna of the eastern Himalayas is similar to that of the southern Chinese and Southeast Asian region. Many of these species are primarily found in tropical forests and are only secondarily adapted to the subtropical, mountain, and temperate conditions prevailing at higher elevations and in the drier western areas. The animal life of the western Himalayas, however, has more affinities with that of the Mediterranean, Ethiopian, and Turkmenian regions. The past presence in the region of some African animals, such as giraffes and the hippopotamuses, can be inferred from fossil remains in deposits found in the Siwalik Range. The animal life at elevations above the tree line consists almost exclusively of cold-tolerant endemic species that evolved from the wildlife of the steppes after the uplift of the Himalayas. Elephants and rhinoceroses are restricted to parts of the forested Tarai region—moist or marshy areas, now largely drained—at the base of the low hills in southern Nepal. Asiatic black bears, clouded leopards, langurs (a long-tailed Asian monkey), and Himalayan goat antelopes (e.g., the tahr) are some of the denizens of the Himalayan forests. The Indian rhinoceros was once abundant throughout the foothill zone of the Himalayas but is now endangered, as is the musk deer; both species are dwindling, and few live, other than those in a handful of reserves set up to protect them. The Kashmir stag, or hangul, is near extinction.
In remote sections of the Himalayas, at higher elevations, snow leopards, brown bears, lesser pandas, and Tibetan yaks have limited populations. The yak has been domesticated and is used as a beast of burden in Ladakh. Above the tree line the most numerous animals, however, are diverse types of insects, spiders, and mites, which are the only animal forms that can live as high up as 20,700 feet (6,300 metres).
Fish of the genus Glyptothorax live in most of the Himalayan streams, and the Himalayan water shrew inhabits stream banks. Lizards of the genus Japalura are widely distributed. Typhlops, a genus of blind snake, is common in the eastern Himalayas. The butterflies of the Himalayas are extremely varied and beautiful, especially those in the genus Troides.
The bird life is equally rich but is more abundant in the east than in the west. In Nepal alone almost 800 species have been observed. Among some of the common Himalayan birds are different species of magpies (including the black-rumped, the blue, and the racket-tailed), titmice, choughs (related to the jackdaw), whistling thrushes, and redstarts. A few strong fliers, such as the lammergeier (bearded vulture), the black-eared kite, and the Himalayan griffon (an Old World vulture), also can be seen. Snow partridges and Cornish choughs are found at elevations of 18,600 feet (5,700 metres).
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