Learn · Mountains · 7 minute read
How mountains rise
There is sea floor at the top of Everest. How continents collide, why mountains have roots, and where Scotland’s Alps went.
Mountains look permanent. To a geologist they are some of the most short-lived things on the Earth's surface: pushed up by moving plates, and attacked from the moment they rise by frost, rain, rivers and ice. Every mountain range is a race between the two.
When continents collide
The Himalaya stretch 2,900 km along the border between India and Tibet. Their story starts far to the south. When Pangaea broke apart, India began moving north, across an ocean called Tethys. About 80 million years ago it was roughly 6,400 km south of Asia and moving north at about 9 metres a century.
Between 40 and 50 million years ago India reached Asia. Continental crust is too light to sink into the mantle, so instead of one plate diving under the other, the crust crumpled and stacked up. India's advance slowed by about half, and the Himalaya and the Tibetan Plateau began to rise fast. India is still pushing north today, and the Himalaya are still going up by more than 1 cm a year.
A sea floor on the roof of the world
The very top of Everest, 8,848.86 m above sea level, is made of grey limestone. It was laid down on the floor of a shallow sea in the Ordovician period, about 450 million years ago. In it are broken bits of sea creatures: trilobites, crinoids and the tiny shells of ostracods. The collision lifted them nearly 9 km into the sky.
Why mountains have roots
The Earth's crust floats on the denser mantle beneath it, rather as ice floats on water. A thick pile of crust stands higher, but it also sinks deeper. Under most continents the crust is about 30 km thick. Under big mountain ranges it can be up to 100 km thick, with a root reaching far down into the mantle. Geologists call this balance isostasy.
It has a strange result. As the top of a mountain range is worn away, the whole range rises a little to make up for it, as an iceberg rises when its tip melts. That is how rocks that were once buried tens of kilometres deep end up at the surface.
Britain's lost mountains
Britain's highest peak, Ben Nevis, is 1,345 m high. It and the rest of the Scottish Highlands are the worn-down remains of a far greater range. When the Iapetus Ocean closed and the old continents collided, more than 400 million years ago, they raised the Caledonian mountains, which have been compared in scale to the Alps or even the Himalaya. Pieces of the same range now lie in Norway and in the Appalachians of North America, scattered when the Atlantic opened. Over 400 million years, weather and ice have planed them down to their roots.