How do two continents colliding build a mountain range like the Himalaya?

By crumpling and thickening the crust where they meet. Geologists call this mountain building orogeny. Ocean crust is dense, so it can sink into the mantle at a subduction zone. Continental crust is lighter and more buoyant, so it mostly can't. When two continents collide, the crust has nowhere to go but up. It folds, stacks and thickens, like a rug bunching up when you push both ends together.

The Himalaya is the textbook case. About 50 million years ago, India, drifting north, began to collide with Asia. The collision is still going on. The top layer of Indian crust folded over itself in giant sheets called nappes. A deeper layer kept pushing into Tibet and thickened its plateau. The lowest crust and the mantle below sank under Asia. The result is the range with the highest peaks on Earth: more than 100 of them top 7,200 metres, Everest included.

Everest's summit tells the story in miniature. Its rock began as seafloor mud on the edge of India, long before the crash. The collision then squeezed those layers, changed them and shoved them south and upward. Today they are the highest rocks on the planet, and they are still full of marine fossils. In 2022, they were named one of the world's 100 geological heritage sites. And the mountain is still rising, by about 2 millimetres a year.

Seashells on mountaintops puzzled people for centuries. Many blamed Noah's flood. In the 13th century, the scholar Albert the Great guessed better. Since erosion wears mountains down, something must push new ones up, and fossil-bearing rock must once have been seafloor. In the 1960s, Canadian geologist Tuzo Wilson supplied the modern answer. Oceans open and later close again. When the last ocean floor between two continents is gone, the continents collide, and mountains like the Himalaya rise.