How Do Mountains Form? Collision, Uplift and the Slow Work of Erosion

The Earth's outer shell is broken into a dozen or so large plates that move a few centimetres a year, and the largest mountains are where two of them meet head-on. When two continents collide, neither is dense enough to sink under the other, so the crust between them crumples and thickens like a rug pushed against a wall. The Himalayas and the Tibetan plateau are the result of India, which broke away from Africa and travelled north across the ocean, ramming into Asia about fifty million years ago and continuing to push into it since. The Alps came from Africa's slower push against Europe. The Scottish Highlands are the worn stumps of mountains raised over 400 million years ago, and the Appalachians took their final shape when the continents collided to form Pangaea around 300 million years ago.

When an ocean plate meets a continent, the denser ocean floor slides beneath, and the result is a different kind of range: a line of volcanoes fed by the melting of the descending plate, standing on crust thickened by the collision. The Andes, the longest mountain chain on land, are of this kind, as are the Cascades of the American northwest, and both are still active.

Not all mountains come from squeezing. Where the crust is being pulled apart it breaks into blocks along faults, and some blocks drop while their neighbours stand high as ranges. The Basin and Range country of Nevada and Utah is a series of such blocks, and the Sierra Nevada is a single enormous one tilted up on its eastern edge. The mountains along the East African Rift, where the continent is slowly splitting, formed the same way, with volcanoes such as Kilimanjaro rising from the cracks.

Volcanoes can also build mountains far from any plate boundary, over plumes of hot rock rising from deep in the mantle. The Hawaiian islands are the tops of volcanoes built on the ocean floor as the Pacific plate slides over such a hot spot, and Mauna Kea, measured from its base on the seabed, is taller than Everest. The four mechanisms in brief:

• •Continental collision: crust crumpled and thickened between two continents, as in the Himalayas and the Alps
• •Subduction: an ocean plate sliding beneath a continent, thickening the edge and feeding a line of volcanoes, as in the Andes
• •Rifting and faulting: crust stretched and broken into tilted blocks, as in the Basin and Range and the East African Rift
• •Hot-spot volcanism: a plume of hot mantle building volcanoes through the middle of a plate, as in Hawaii

A range rises only as long as the forces pushing it up exceed the forces wearing it down, and both grow with height. Higher mountains catch more rain and snow, feed faster rivers and larger glaciers, and are colder, so they erode faster; the Himalayas lose several millimetres a year to the monsoon rivers, which carry the debris to build the delta of Bangladesh. There is also a ceiling set by the rock itself. Beyond a certain height the base of a mountain begins to flow under the weight above it, and the crust beneath, which floats on the denser mantle like a ship, sinks. Everest is close to the limit for rock on a planet with Earth's gravity; Olympus Mons on Mars, where gravity is a third as strong, is about two and a half times higher.

A young range is sharp. The Himalayas, the Alps and the Rockies have jagged peaks, deep valleys and steep faces, because uplift is running ahead of erosion. An old range is rounded: the Appalachians, the Urals and the highlands of Scotland were once as high as the Alps and have been ground down over hundreds of millions of years to hills, and their roots, once buried tens of kilometres deep, are now exposed at the surface as gnarled metamorphic rock. Rounded summits, gentle slopes and wide valleys are the signature of a mountain range that stopped rising long ago.

The record can be read in the rock itself. Marine fossils on a summit mean the rock was once seabed; folded strata mean compression; a line of volcanic rock means subduction; and the age of the minerals, measured by radioactive decay, dates the collision. The fossils near the top of Everest were noticed by the first geologists to reach the mountain, and the story they told, of an ocean closing between two continents, was confirmed when plate tectonics gave it a mechanism in the 1960s.

A mountain range does not just sit in the weather; it reshapes it. When moist wind meets a range, the air is forced upward, and rising air expands and cools. Cooler air holds less water vapour, so clouds form and rain or snow falls on the windward slopes. By the time the air crosses the crest and sinks down the far side, it has lost much of its moisture and warms as it descends, so the leeward side stays dry. This dry zone is called a rain shadow, and you can see it on almost any map of rainfall that includes mountains.

The effects can be enormous. The Himalayas block moist monsoon air, so the southern slopes are among the wettest places on Earth while the Tibetan plateau behind them is high and arid. The Andes help keep parts of Patagonia dry, and in the Pacific northwest of America, lush forests on the coastal side of the Cascades give way to near desert only a short drive east. Warm, dry winds rushing down the lee side, called the foehn in the Alps and the chinook in the Rockies, can raise temperatures sharply within hours and melt snow remarkably quickly.

In the mid-nineteenth century surveyors mapping India found a puzzle. A plumb line, a weight on a string, should be pulled slightly sideways by the gravity of a nearby mountain range, and the vast mass of the Himalayas ought to have deflected it noticeably. The measured deflection was much smaller than calculation predicted. The astronomer George Airy proposed the explanation now broadly accepted: mountains are underpinned by deep roots of relatively light crust pushing down into the denser mantle beneath, so there is less extra mass than the visible peaks suggest. Beneath Tibet the crust is about twice its usual thickness, roughly seventy kilometres.

The roots explain a counter-intuitive effect of erosion. Because the crust floats, removing rock from the top makes the whole range lighter, and it rises in response, by most of the thickness removed. Wear a kilometre off a mountain belt and the land surface ends up only a couple of hundred metres lower, while deep valley-cutting can even push individual peaks higher. The same floating balance works in reverse after ice sheets melt. Scandinavia and Scotland, pressed down by thick ice during the last glaciation, are still rising, and around the northern Gulf of Bothnia the land gains almost a centimetre a year.

Everest's height was first calculated by the Great Trigonometrical Survey of India, which measured angles to distant Himalayan summits from stations on the plains well over a hundred kilometres away, because Nepal and Tibet were closed to the surveyors. Working through these observations, the mathematician Radhanath Sikdar is credited with recognising around 1852 that the summit catalogued as Peak XV was the highest yet measured, and in 1856 its height was announced as 29,002 feet, about 8,840 metres. It was named after George Everest, a former head of the survey, although he objected, and it already had a local name, Chomolungma in Tibetan.

Modern surveys carry satellite positioning receivers to the summit itself and use ground-penetrating radar to measure the depth of the snow cap, since the official figure refers to the snow surface while the rock beneath lies a few metres lower. In 2020 China and Nepal jointly announced a height of 8,848.86 metres. Which mountain is highest also depends on the question. Measured from base to peak, Mauna Kea wins. Measured from the centre of the Earth, the winner is Chimborazo in Ecuador, only about 6,260 metres above sea level, because the Earth bulges at the equator and its summit sits about two kilometres further out than Everest's.

Mountains form where tectonic plates collide and crumple the crust, where an ocean plate sinks beneath a continent and feeds volcanoes, where the crust stretches and breaks into tilted blocks, or where a hot-spot plume builds volcanoes through a plate. They rise until erosion and the strength of rock stop them, and a range's age shows in its shape: sharp and high while it is growing, rounded and low once the wearing-down has won.