About 140 million years ago, the Indian plate broke off the Gondwana supercontinent and began drifting north at an unusual 15–20 centimetres per year. As the Tethys Ocean closed, its oceanic crust subducted beneath Eurasia; about 50 million years ago the two continental masses came into full contact. India did not stop on impact: the plate continues to push north at roughly 4 cm per year, and the mountains are still rising.

The geological consequences have no parallel. Continental crust is far thicker and lighter than oceanic crust; with neither side able to subduct, the crust has had to stack, fold, and thicken. Today the crust beneath the Himalayas is more than 70 km thick — a value found nowhere else on Earth. That thickening makes all 14 of the world's 8,000-metre peaks, the Tibetan Plateau, the Karakoram, and the Hindu Kush parts of one continuous orogen.

The climatic effect is equally transformative. The rise of the Tibetan Plateau created the Asian monsoon system, watering the continent's southern flank seasonally and drying its northern interior. Rapid erosion of the Himalayas accelerated the weathering of silicate rocks, drawing carbon dioxide out of the atmosphere — a mechanism widely cited as a contributor to long-term global cooling over the Cenozoic. India's collision is the largest reshaping of Earth's surface in the past 50 million years.