Mountain ranges form mainly because a cooling Earth contracts and wrinkles.

A cooling object contracts. In the nineteenth century, geologists applied that simple idea to the whole planet.

If Earth had once been much hotter and was gradually shrinking as it cooled, its rigid outer shell might have become too large for the smaller interior. The crust could then wrinkle and crumple, rather like the skin of a drying fruit. Mountain ranges seemed like the obvious wrinkles.

The model had some appeal because mountains contain strongly folded and compressed rocks.

The larger pattern was harder to explain. Mountain belts, deep ocean trenches, volcanoes, and earthquakes occur in particular global zones rather than as random wrinkles spread across a uniformly shrinking planet.

Mid-twentieth-century exploration of the ocean floor supplied a different mechanism. New crust forms along mid-ocean ridges and moves outward. Elsewhere, one tectonic plate can sink beneath another at a subduction zone, while continents carried by converging plates can collide.

Those collisions and subduction zones compress and thicken crust, drive faulting and volcanism, and raise large mountain belts. Erosion and continued uplift then reshape them over time.

Earth has cooled over its history, but global contraction is not the main process organizing modern mountain ranges. Most major belts are products of a mobile surface divided into tectonic plates.