How Are Mountains Formed? Colliding Plates, Volcanoes and the Rise of the Himalayas

Mountains look like the most permanent things on Earth. Generations live and die beneath the same peaks. Yet on geological timescales, mountains are born, grow and wear away, and some are still rising today. Their story begins deep below our feet, with a planet whose surface is constantly on the move.

A planet of moving plates

Earth’s rigid outer layer, the lithosphere, is broken into about a dozen large tectonic plates and many smaller ones. They float on hotter, slowly flowing rock beneath and move at roughly the speed your fingernails grow, a few centimetres per year. According to the US Geological Survey, this theory of plate tectonics, accepted in the 1960s, explains earthquakes, volcanoes and the formation of most mountain ranges.

Where plates pull apart, new crust forms, as along the Mid-Atlantic Ridge, an underwater mountain chain. Where they collide, crust is crushed, thickened and pushed upwards. Where they slide past each other, they cause earthquakes, as along the North Anatolian Fault in Turkey.

Four ways to build a mountain

Fold mountains are the most common type of great range. When two continents collide, neither sinks easily, so the crust between them is squeezed like a rug pushed against a wall, folding and stacking up into enormous ridges.

Volcanic mountains grow where magma reaches the surface, often above subduction zones where one plate dives beneath another and melts, or over hot spots like Hawaii. Turkey’s highest peak, Mount Ararat (Ağrı Dağı), at 5,137 metres, is a volcano.

Fault-block mountains form when the crust is stretched or stressed until it breaks along faults. Some blocks are pushed up while others drop down, creating steep ranges next to flat valleys.

The birth of the Himalayas

The Himalayas are the most dramatic example. Over 100 million years ago, the Indian plate broke away from an ancient southern supercontinent and began moving north, quite fast for a plate, at up to about 15 centimetres a year. An ocean called the Tethys lay between India and Asia.

About 50 million years ago, India collided with Asia. The ocean closed, and its sea-floor sediments were scraped up and crumpled. Because both landmasses were thick, light continental crust, neither could sink, so the crust thickened and rose. The result is the highest mountain range on Earth, home to all fourteen peaks over 8,000 metres.

That is why the summit of Everest is made of limestone containing fossils of sea creatures: those rocks once lay on the floor of the Tethys Ocean. India is still pushing into Asia by about 4 to 5 centimetres a year, and parts of the Himalayas are still rising by a few millimetres a year. The same collision causes frequent, sometimes devastating, earthquakes in the region.

How high can mountains get?

Everest is the highest mountain above sea level, at about 8,849 metres, a figure announced jointly by China and Nepal in 2020. But it depends how you measure:

• From base to summit, Mauna Kea in Hawaii rises about 10,000 metres from the sea floor.
• Because Earth bulges at the equator, Chimborazo in Ecuador has the summit farthest from Earth’s centre.

Mountains cannot grow forever. The higher they rise, the heavier they are, and the crust beneath sinks into the softer mantle, a balance called isostasy. Like icebergs, big mountains have deep roots.

Erosion: tearing mountains down

As soon as mountains rise, they begin to be destroyed. Rain, frost, glaciers and rivers break rock apart and carry it away. Glaciers carve sharp peaks and U-shaped valleys; rivers cut deep gorges.

Old mountain ranges show the result. The Appalachians in North America and the Scottish Highlands were once as high as the Himalayas, but hundreds of millions of years of erosion have worn them down to rounded hills. A mountain’s height at any moment is a contest between uplift and erosion.

Why mountains matter

Mountains shape climate and life. They force air to rise, cool and drop rain, so one side of a range is often wet and green while the other lies in a dry rain shadow. Their snow and glaciers feed rivers that supply water to billions of people, and their varied heights create many habitats, making mountains hotspots of biodiversity.

In short

• Most mountains are built by moving tectonic plates, a few centimetres a year.
• Fold mountains form where plates collide; volcanic mountains where magma erupts.
• Fault-block mountains form where the crust cracks and blocks rise or sink.
• The Himalayas formed when India hit Asia about 50 million years ago and are still rising.
• Everest is about 8,849 m high, and its summit holds fossils of sea creatures.
• Erosion constantly wears mountains down; old ranges become rounded hills.