How mountains form

The Himalayas are still rising by more than 1 centimetre a year, the USGS says, because India is still pushing into Asia.

Mountains come from moving plates

Earth’s outer shell is broken into plates that move a few centimetres a year. Most of the world’s great mountain ranges stand where plates push together. The NGII atlas points out that ranges higher than 3,000 metres run mostly east-west across Eurasia and north-south through the Americas, and that their direction is closely tied to plate motion. The Andes, the longest mountain range on land, run along the western edge of South America, where an ocean plate sinks beneath the continent.

The Himalayas: two continents collide

The USGS tells the story of the Himalayas in numbers. About 225 million years ago, India was a large island off the coast of Australia, with an ocean called Tethys between it and Asia. When the supercontinent Pangaea broke apart about 200 million years ago, India began to move north. About 80 million years ago it was roughly 6,400 kilometres south of Asia, travelling at about 9 metres a century. Between 40 and 50 million years ago it ran into Asia. Both landmasses have about the same rock density, so neither could sink under the other. The crust crumpled and thickened instead, and the Himalayas and the Tibetan Plateau rose. The range now stretches 2,900 kilometres, and the Geological Society of London notes that the crust beneath it is about 75 kilometres thick, twice the average.

The collision has not ended. The USGS says the Himalayas continue to rise more than 1 centimetre a year, which would be 10 kilometres in a million years. They do not get that high because weathering and erosion wear them down at about the same rate, and because the crust may be spreading sideways rather than only thrusting up.

Folds

When layers of rock are squeezed, they bend before they break. National Geographic Education describes the two most common shapes. An anticline is an arch, with the oldest rocks in its centre. A syncline is a trough, shaped like the letter U, with the youngest rocks in its centre. Fold mountains such as the Himalayas, the Alps and the Appalachians are full of them. Age matters too: the Appalachians, now worn low, were once taller than the Himalayas, National Geographic notes. Given enough time, rivers and frost take any range apart.

Volcanic mountains

Volcanoes build mountains differently: they pile up their own lava, ash and cinders around a vent. The model above is a simple cone with a crater at the top. Its contour map shows the closed rings of a crater inside the closed rings of the cone, and the gullies cut into its flanks send rain off in every direction, a pattern called radial drainage. Drop rain on the volcano with the Rain drop tool and watch it run away from the summit on all sides. Hallasan on Jeju is a volcano of this kind, with a crater lake, Baengnokdam, at its summit.