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, and the Appalachians and the Scottish Highlands are the worn stumps of a range built when the continents collided to form Pangaea three hundred 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 nearly three 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.

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.