Himalayas

• 1. Introduction

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  The Himalayas hold more than 100 peaks that climb above 7200 metres, including Mount Everest, the highest point on Earth. This mountain range in Asia separates the plains of the Indian subcontinent from the Tibetan Plateau. It curves across five countries: Nepal, India, China, Bhutan, and Pakistan. Some 53 million people live within the Himalayas themselves. Far more depend on what the range produces. Its rivers feed a drainage basin home to roughly 600 million people. So how did these peaks rise so high, and why do they keep rising? Why are some of their summits forbidden to climbers? And how does a wall of rock decide where rain falls and where deserts form? The answers begin deep underground, with a collision that has not yet ended.

• 2. A Collision Still Unfolding

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  Seventy million years ago, India began splitting away from the supercontinent Gondwana. The Indian continental plate, carrying the Neo-Tethys oceanic plate above it, drove northward. Geologists Wolfgang Frisch, Martin Meschede, and Ronand Blakey describe the pace bluntly: India marched north at about 20 centimetres a year, a plate velocity that exceeds any modern example. When India met Eurasia, the denser oceanic plate slid under and sank into the deeper asthenosphere. The buoyant continental plate could not. Its lower crust and mantle slipped beneath, while one layer of upper crust piled into stacked sheets called nappes. Geophysicist Peter Molnar noted that most of the Himalayas are slices of rock that once were the top part of India's crust. This is orogeny, the building of mountains. The collision did not finish the job. The Indian plate still moves at 67 millimetres a year, and over the next 10 million years it will travel 1500 kilometres into Asia. About 20 millimetres a year of that convergence is absorbed by thrusting along the southern front. The result lifts the Himalayas roughly 5 millimetres higher every year, keeping them geologically and seismically alive.

• 3. Anchors And Arc

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  The range runs west-northwest to east-southeast in an arc 2400 kilometres long. Its western anchor, Nanga Parbat, lies just south of the northernmost bend of the Indus river and rises over 8000 metres above the Indus valley. Its eastern anchor, Namche Barwa, sits inside the great bend of the Yarlung Tsangpo river in Tibet. These two points are syntaxes, places where mountain ranges built by forces at different angles converge. At both, rock once buried deep has been pushed upward fast, an estimated 7 millimetres per year, or 7 kilometres per million years. The protruding regions reach 8125 metres and 7756 metres and hold the greatest topographical relief found in the interior of any continent. Between the anchors, four parallel ranges climb from south to north: the Sivalik Hills, the Lower Himalayan Range, the Great Himalayas, and the Tibetan Himalayas. The range narrows as it travels, from 350 kilometres wide in the west to 151 kilometres in the east. At its centre rise the 8000-metre peaks of Dhaulagiri and Annapurna in Nepal, split by the Kali Gandaki Gorge. The pass at the head of that gorge, the Kora La, is the lowest point on the ridgeline between Everest and K2.

• 4. Rivers That Predate The Peaks

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  Despite their scale, the Himalayas do not form a major continental divide. A number of rivers cut straight through the range, especially in the east, so the main ridge is not clearly defined and passes matter less than they do elsewhere. The Indus and the Yarlung Tsangpo are older than the mountains around them. Both rise along the Indus-Yarlung suture zone, the scar where India welded to Eurasia, and the rising peaks repeatedly diverted their courses. Now they bend around Nanga Parbat and Namche Barwa before reaching the Indian Ocean. The western drainage gathers into the Indus Basin, fed by the Jhelum, Chenab, Ravi, Beas, and Sutlej, the five rivers of the Punjab. The eastern drainage becomes the Ganges-Brahmaputra Basin. The Brahmaputra begins as the Yarlung Tsangpo in western Tibet, flows east through Tibet, then west across the plains of Assam. The Ganges and the Brahmaputra meet in Bangladesh and empty into the Bay of Bengal through the Sunderbans, the world's largest river delta.

• 5. The Third Pole

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  After Antarctica and the Arctic, the great ranges of central Asia hold the world's third-largest deposit of ice and snow, which is why some call this region the Third Pole. The Himalayan range encompasses about 15,000 glaciers storing roughly 12000 cubic kilometres of fresh water. Among them are the Gangotri, Yamunotri, and Khumbu glaciers, the Langtang glacier, and the Zemu in Sikkim. Because the mountains sit near the Tropic of Cancer, their permanent snow line is among the highest in the world, around 5500 metres. Equatorial mountains in New Guinea, the Rwenzoris, and Colombia carry a snow line some 900 metres lower. That ice is shrinking. Studies measured an overall decrease of about 13 percent in glacial coverage over the last 40 to 50 years, with a marked acceleration in mass loss since 1975. The region is also dotted with hundreds of lakes. Pangong Tso, spread across the India-China border, covers about 700 square kilometres. Some lakes carry a darker risk. The Tsho Rolpa glacier lake in the Rowaling Valley, at an altitude of 4580 metres, is rated the most dangerous for a glacial lake outburst flood, having grown considerably over the last 50 years.

• 6. A Wall That Makes Weather

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  Temperatures in the Himalayas cool by 2.0 degrees Celsius for every 300 metres of altitude gained. That gradient, stretched across more than eight degrees of latitude, packs temperate and subtropical zones into a single range. The wall does something larger. It blocks the colder air of Central Asia from spilling south, which pushes the tropical zone farther north in South Asia than anywhere else on Earth. The Southwest Monsoon is the climate's defining force. In winter, high pressure over Central Asia drives dry air south across the mountains. In summer the Central Asian plateau heats faster than the ocean, air rises into a thermal high, and moist air from the Indian Ocean is pulled inland. When that air meets the mountains it lifts, cools, and releases heavy rain, the orographic effect. The monsoon weakens as it moves west. Darjeeling in the east can receive as much as 2030 millimetres in the monsoon season, while Shimla in the west gets only 975 millimetres in the same period. The contrast can be sharper still over short distances. Upper Mustang, sheltered by the Annapurna and Dhaulagiri massifs, sees around 300 millimetres a year, while Pokhara on the southern side of those massifs draws about 3900 millimetres. Beyond the range, the rain shadow of the Himalayas is believed to help form Central Asian deserts such as the Taklamakan and Gobi.

• 7. Life At The Edges

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  At the highest reaches, the elusive snow leopard is the main predator, hunting members of the goat family across alpine pasture and rock, including the endemic bharal, or Himalayan blue sheep. The Himalayan musk deer lives at high altitude too, hunted for its musk until it became rare and endangered. Endemic or near-endemic herbivores include the Himalayan tahr, the takin, the Himalayan serow, and the Himalayan goral. A critically endangered subspecies of brown bear appears sporadically along the range, alongside the Asian black bear. In the mixed deciduous and conifer forests of the eastern Himalayas, red pandas feed in dense understories of bamboo. Lower down, the foothill forests hold primates such as the endangered Gee's golden langur and the Kashmir gray langur, each confined to narrow ranges in the east and west. Climate change is already reshaping this wealth. Warming is pushing species to higher elevations, pine forest is invading oak forest in the Garhwal Himalayan region, and trees such as rhododendron, apple, and box myrtle are flowering and fruiting early. The highest known tree species in the Himalayas, Juniperus tibetica, grows at 4900 metres in Southeastern Tibet.

• 8. Sacred Summits

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  Several Himalayan summits are off-limits to climbers, sacred ground in Hinduism and Buddhism: Kangchenjunga from the Indian side, Gangkhar Puensum, Machapuchare, Nanda Devi, and Kailash in the Tibetan Transhimalaya. For Hindus, the range is personified as Himavat, king of all mountains and father of the goddess Parvati, and also father of the goddess Ganga. Two of their most sacred pilgrimage places are the temple complexes at Pashupatinath and Muktinath, the latter known as Shaligrama for its sacred black rocks. In Jainism, Mount Ashtapada is where the first tirthankara, Rishabhanatha, attained moksha. Buddhism has deep roots here as well. Paro Taktsang in Bhutan is the holy place where Padmasambhava is said to have founded Buddhism, and Tibet once held over 6,000 monasteries, including the residence of the Dalai Lama. The people carry their diversity in architecture, language, dialect, ritual, and dress. Handwoven textiles show colors and patterns unique to each ethnic background, and Rai and Limbu women wear big gold earrings and nose rings to display their wealth. The forests sustain life in another way too. A bark extract from an Abies pindrow tree treats coughs and bronchitis, and locals across some Asian and African countries depend on such medicinal plants rather than prescriptions, even as illegal clearing of those forests for wood puts the supply at risk.