How mountains form

More than one mechanism

Mountains are raised by more than one process. Continental collision buckles and thickens crust; subduction lifts the overriding plate and builds volcanic ranges; and the crustal roots beneath large ranges record thickening as well as uplift.^[1]

Collision: the Himalaya

When two continents meet head-on neither is subducted, because continental rocks are relatively light and, like colliding icebergs, resist downward motion. The crust buckles and is pushed upward and sideways instead.^[1,2]

India met Asia about 50 million years ago, and slow continuous convergence since has raised the Himalaya and the Tibetan Plateau, most of that growth occurring in the last 10 million years. The plateau alone averages about 4,600 metres.^[1,2,3]

Subduction: the Andes

Along the Peru–Chile trench the oceanic Nazca Plate descends beneath South America, and the overriding plate is lifted to create the Andes. Strong destructive earthquakes and rapid uplift are characteristic of the setting. No continent collided here.^[1,2]

The Cascade Range is the same family of setting in North America: a volcanic range sustained by oceanic–continental convergence.^[1]

Roots as well as summits

A range is not only what stands above the landscape. Continental crust averages about 30 kilometres thick, but beneath large ranges such as the Alps its base can lie as deep as 100 kilometres. Mountain building thickens crust downward as much as it raises it.^[1,2]

Commonly misunderstood