Mountain

• 1. Introduction

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  A mountain is an elevated portion of a planet's surface, usually steep-sided, often baring the bedrock beneath. Mount Everest, in the Himalayas of Asia, lifts its summit 8850 m above mean sea level. Yet the tallest on Earth depends entirely on where you start measuring. Mauna Kea in Hawaii rises 9,330 m from its base at the bottom of the ocean. And beyond Earth, Olympus Mons on Mars climbs to 21171 m, the highest known mountain anywhere in the Solar System. So what counts as a mountain at all? Where do they come from, and why do they wear away? What lives on their cold flanks, and why do so few people live there? These questions have no single clean answer, and that uncertainty begins with the word itself.

• 2. The Problem Of Definition

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  There is no universally accepted definition of a mountain. Elevation, volume, relief, steepness, spacing, and continuity have all served as criteria. The Oxford English Dictionary calls it a natural elevation that rises more or less abruptly and reaches an altitude that is impressive or notable relative to its surroundings. Local usage muddies the picture further. John Whittow's Dictionary of Physical Geography notes that some authorities treat eminences above 600 m as mountains and anything lower as a hill. Yet some ranges averaging between 1500 m and 2000 m are still called hills, like the Sivalik Hills in the Himalayas and the Black Hills in the United States. National rules diverge sharply. In the United Kingdom and the Republic of Ireland a summit at least 2000 ft high usually qualifies, matching the official UK access definition. Some definitions add a prominence requirement, such as rising 300 m above the surrounding terrain. The United States once used a 1000 ft threshold through its Board on Geographic Names, then abandoned it in the 1970s. Today the United States Geological Survey concludes the terms carry no technical definition there. The UN Environmental Programme sidesteps the argument with a seven-class system, ranging from elevations above 4500 m down to isolated basins under 25 km2 surrounded by higher classes. By those rules, mountains cover 33% of Eurasia and 24% of the Earth's land mass overall.

• 3. How Mountains Are Built

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  Three main types account for the world's mountains: volcanic, fold, and block. All three trace back to plate tectonics, the movement, crumpling, and diving of portions of the Earth's crust. Volcanoes form where one plate is pushed beneath another, or at a mid-ocean ridge or hotspot. At a depth of around 100 km, water entering the rock above the descending slab triggers melting and produces magma. When that magma reaches the surface it can build a shield volcano or a stratovolcano, as with Mount Fuji in Japan and Mount Pinatubo in the Philippines. Magma that solidifies below ground without surfacing can still raise a dome mountain, like Navajo Mountain in the United States. Fold mountains arise when two plates collide, shortening the crust along thrust faults and overthickening it. Because the lighter continental crust floats on denser mantle rock, the weight pushed upward must be balanced by a far greater volume forced downward, so the crust runs much thicker beneath mountains. The upfolds are anticlines and the downfolds synclines, with recumbent and overturned folds in asymmetric cases. The Balkan Mountains and the Jura Mountains are examples. Block mountains, by contrast, come from faults where rock has moved past rock. Uplifted blocks become horsts, and the dropped blocks between them form graben that can grow into rift valley systems. This landscape appears in East Africa, the Vosges and Rhine valley, and the Basin and Range Province of Western North America, where the crust is stretched and thinned. These forces act on time scales of up to tens of millions of years.

• 4. The Slow Unmaking

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  Once mountain building ceases, the agents of erosion go to work: water, wind, ice, and gravity wear the uplifted land back down. Because of this, the surface of a mountain is younger than the rock that forms its body. Glaciers carve the most distinctive signatures, leaving pyramidal peaks, knife-edge arêtes, and bowl-shaped cirques that can cradle lakes. Slumping and other forms of mass wasting strip material away alongside the rivers and ice. Some mountains are not raised peaks at all but the leftovers of a worn-down plateau. The Catskills are plateau mountains, shaped by the erosion of an uplifted plateau rather than by folding or faulting. Even at rest, a mountain is never truly finished.

• 5. Cold Air And Latent Heat

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  Climate on a mountain turns colder with height, the product of a tug-of-war between radiation and convection. Sunlight in the visible spectrum heats the ground, which in turn heats the air at the surface. If radiation alone moved that heat, the greenhouse effect would hold the ground near 333 K, with temperature decaying exponentially upward. Hot air, however, expands and grows less dense, so it rises and carries heat with it. A rising parcel of air exchanges almost no heat with its surroundings, an adiabatic process whose temperature falls as pressure drops. That dry adiabatic lapse rate runs about 9.8 °C per kilometre. Water complicates everything, because water vapor carries latent heat of vaporization. As air rises and cools it saturates, the vapor condenses into cloud, and the released heat shifts the figure to the moist adiabatic lapse rate of about 5.5 °C per kilometre. The practical consequence is striking. Climbing 100 m up a mountain roughly equals traveling 80 kilometres toward the nearest pole. Leslie Holdridge captured this link to ecology in 1947 through biotemperature, the mean temperature with all readings below 0 °C counted as 0 °C. Below freezing, plants go dormant. The peaks of mountains under permanent snow can hold a biotemperature below 1.5 C.

• 6. A Sensitive Frontier

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  Mountain environments are undergoing alterations unprecedented in the last 10,000 years, and they are unusually sensitive to anthropogenic climate change. Observational studies show highlands warming faster than nearby lowlands, though the effect disappears when measured globally. Precipitation in highland areas is not rising as fast as in lowlands, and climate models send mixed signals about whether any given highland will get wetter or drier. The physical consequences are already visible. In recent decades ice caps and glaciers have shed ice at an accelerating pace, and the melting of glaciers, permafrost, and snow has left underlying surfaces increasingly unstable. Landslip hazards have grown in both number and magnitude. River discharge patterns are shifting too, which matters because nearly half of mountain areas supply essential or supportive water for mainly urban populations, especially during the dry season and in semiarid regions such as central Asia. Many mid-latitude mountains serve as cold climate refugia, where ecosystems occupy small environmental niches and changing soils add an indirect threat to the direct one.

• 7. Zones Of Life

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  A particular set of plants and animals tends to adapt to a relatively narrow band of climate, so mountain ecosystems stack along elevation bands of roughly constant conditions. This is altitudinal zonation, and dry regions sharpen it further, since mountains there gather both higher precipitation and lower temperatures. The pattern follows a familiar order from top to bottom. At the highest elevations trees cannot grow, and life takes an alpine form resembling tundra. Just below the tree line lie subalpine forests of needleleaf trees built for cold, dry conditions. Lower still grow montane forests, needleleaf in temperate latitudes but broadleaf rainforest in the tropics. Some species become trapped, because the conditions just above and below their band are inhospitable and block their movement. These stranded ecological systems are known as sky islands, isolated worlds perched on the slopes.

• 8. The Limits Of Habitation

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  The highest known permanently tolerable altitude sits at 5950 m. Above that, falling atmospheric pressure means less oxygen for breathing and weaker protection from solar radiation. Above 8000 m there is not enough oxygen to support human life, a region called the death zone, which contains the summits of both Mount Everest and K2. The numbers of who lives high are small. While 7% of Earth's land lies above 2500 m, only 140 million people live above that line, and just 20 to 30 million above 3000 m, about half of them in the Andes, central Asia, and Africa. Only a handful of communities exist above 4000 m. La Rinconada, Peru, a gold-mining town, is the highest human habitation at 5100 m, while El Alto, Bolivia, at 4150 m, breaks the pattern with a diverse service and manufacturing economy and a population near 1 million. Mountain economies lean on agriculture with high risk of crop failure, on mining where minerals occur, and increasingly on tourism around national parks and ski resorts. Roughly 80% of mountain people live below the poverty line. These same heights hold deep meaning. Mount Olympus was held to be the home of the gods, the 3,776.24 m volcano of Mount Fuji draws tens of thousands of Japanese climbers each year, and Mount Kailash in the Tibet Autonomous Region of China is sacred to Hinduism, Bon, Buddhism, and Jainism. Irish Catholics make pilgrimages up the 952 m Mount Brandon, and Nanda Devi, tied to the goddesses Nanda and Sunanda, has been off-limits to climbers since 1983.