How mountains form

Mountains are areas of land that rise steeply and substantially above their surroundings. They cover roughly a quarter of Earth's land surface and are home to about a tenth of its people.

Every mountain range is the result of two opposing processes. Forces inside the Earth push rock upward, while weathering and erosion by water, ice and wind wear it down. Young ranges are high and jagged because uplift is winning; old ranges are lower and rounded because erosion has had longer to work.

The highest ranges form where tectonic plates collide. When two continental plates meet, neither sinks easily because continental rock is relatively light, so the crust crumples, folds and stacks up, much like a rug pushed against a wall.

The Himalayas formed this way, as the Indian plate collided with Asia starting around 50 million years ago. India is still pushing north by a few centimetres a year, and the range is still rising, though erosion removes much of that growth. The Alps formed as Africa pushed into Europe, and the Appalachians are the worn-down remains of a collision that happened hundreds of millions of years ago.

Where an ocean plate sinks beneath another plate, a process called subduction, rock melts deep underground. The magma rises and erupts, building chains of volcanoes. The Andes, which run along the western edge of South America, were built largely this way, and the Cascade Range, including Mount St Helens, is another example.

Volcanoes also form over hotspots, plumes of hot material rising from deep in the mantle. As a plate moves over a hotspot, a line of volcanoes forms. The Hawaiian Islands are such a chain, and Mauna Kea, measured from its base on the ocean floor, is taller than Mount Everest.

• Fault-block mountains form when the crust is stretched and cracks into blocks, some of which rise while others drop, as in the Sierra Nevada in California.
• Rift valleys, such as East Africa's Great Rift Valley, form where blocks drop between faults, often flanked by uplifted highlands.
• Dome mountains form when magma pushes up the overlying rock without erupting, and erosion later exposes the hardened core.
• Plateaus are broad, high areas lifted as a block, such as the Tibetan Plateau, the largest and highest on Earth.

Uplift raises rock, but erosion sculpts it. Glaciers carve bowl-shaped hollows called cirques, sharp ridges called arêtes and pyramid-shaped peaks such as the Matterhorn. They also grind river valleys from a V shape into a broad U shape.

Rivers cut deep gorges, and freeze-thaw weathering splits rocks as water freezes in cracks and expands. Material worn from mountains is carried away and deposited in lowlands and seas, where it may one day become new sedimentary rock.

Mountains shape climate. As moist air is pushed up a mountainside it cools and drops rain or snow, leaving a dry rain shadow on the far side. The Atacama Desert and the dry lands east of the Rockies are partly shaped by this effect.

Mountains are often called the water towers of the world: snowpack and glaciers store water and release it slowly, feeding rivers that supply billions of people. The glaciers of the Himalayas and neighbouring ranges feed rivers such as the Indus, Ganges and Yangtze, which is why their retreat under climate change is a serious concern.