What happens when plates collide
Most of the world's great mountain ranges were pushed up where two of the Earth's plates collided. About fifty million years ago, the land that is now India, drifting north, crashed into Asia. Neither side could sink, so the crust crumpled and piled upwards into the HIMALAYAS, and India is still pushing north today, so they are still rising by a few millimetres a year. Rocks near the top of EVEREST contain fossils of sea creatures, because they formed on an ancient seabed before being lifted nearly nine kilometres into the air.
The ALPS were made the same way, by Africa pushing into Europe, and the ANDES and ROCKIES rose where ocean plates slide under the edge of a continent. Other peaks, like FUJI, ETNA and KILIMANJARO, are a VOLCANO rather than a fold in the crust. Our Earth's layers page covers the plates themselves.
Which mountain is the tallest?
Everest has the highest summit above sea level, at about 8,849 metres, a figure agreed by China and Nepal after a joint survey in 2020. But it depends how you measure. Mauna Kea in Hawaii starts deep on the ocean floor, and from base to peak it is over 10,000 metres, taller than Everest. And because the Earth bulges at the equator, the summit of Chimborazo in Ecuador is the point on the planet's surface furthest from the Earth's centre.
The name Everest comes from Sir George Everest, a surveyor of India who objected to the honour and pronounced his own name eve-rest. Locally the mountain has long been called Chomolungma in Tibetan and Sagarmatha in Nepali. Edmund Hillary and Tenzing Norgay were the first known people to reach its top, on 29 May 1953.
Thin air and moving ice
The higher you go, the thinner the air. At the top of Everest each breath holds only about a third of the oxygen it would at sea level, and most climbers use bottled oxygen. At high ALTITUDE the temperature also drops by about six or seven degrees for every thousand metres, so a SNOWCAP can sit on a mountain near the equator, like Kilimanjaro, although its ice has been shrinking fast. Mountains matter far beyond their slopes: melting snow and ice from mountain ranges supply fresh water to around half of the world's people, feeding rivers such as the Ganges, the Yangtze and the Rhine.
Glaciers, avalanches and famous faces
A GLACIER is a river of ice that creeps down a VALLEY, often only a few centimetres a day, carving it into a deep U shape. An AVALANCHE is a sudden slide of snow down a steep SLOPE. The MATTERHORN, with its sharp pyramid shape, was carved by glaciers eating away at it from several sides, and the north face of the EIGER in Switzerland is one of the most famous climbs in the world. DENALI in Alaska, the highest peak in North America, rises further from its base to its SUMMIT than Everest does from the high plateau around it. Every PEAK, RIDGE, CLIFF and RANGE in the grid is a challenge for a CLIMBER.