Mountain building has occurred in the recent geologic past at several locations around the world. Young mountain belts include the American Cordillera (cordillera means “spine” or “backbone”), which runs along the western margin of the Americas from southernmost South America to Alaska and includes both the Rockies and the Andes; the Alpine–Himalaya chain, which extends along the margin of the Mediterranean, through Iran to northern India and into Indochina; and the mountainous terrains of the western Pacific, which include volcanic island arcs like Japan, the Philippines, and much of Indonesia. Most of these young mountain belts have formed in the past 100 million years (Figure 1). Some, including the Himalayas, began their growth as recently as 50 million years ago.

In addition to these young mountain belts, there are several chains of Paleozoic-age mountains on Earth. Although these older mountain belts are deeply eroded and topographically less prominent, they exhibit the same structural features found in younger mountains. The Appalachians in the eastern United States and the Urals in Russia are classic examples of this group of older, well-worn mountain belts.

The term for the processes that collectively produce a mountain belt is orogenesis (oros = mountain, genesis = to come into being). An episode of mountain building is called an orogeny. Most major mountain belts display striking visual evidence of great compressional forces that have shortened the crust horizontally while thickening it vertically. These collisional mountains usually result from the collision of one or more small crustal fragments with a continental margin or from the closure of an ocean basin that results in the collision of two major landmasses. As a consequence, collisional mountains contain large quantities of preexisting sediments and sedimentary rocks that at one time lay along the margin of a continent and were subsequently faulted and contorted into a series of folds. Although folding and thrust faulting are often the most conspicuous signs of orogenesis, varying degrees of metamorphism and igneous activity are always present.

The theory of plate tectonics provides a model for orogenesis with excellent explanatory power; it accounts for the origin of virtually all the present mountain belts and most of the ancient ones. According to this model, the tectonic processes that generate Earth’s major mountainous terrains occur along convergent plate boundaries. We will next revisit the nature of convergent plate boundaries and then examine how the process of subduction has driven mountain building around the globe.

• Orogenesis is the making of mountains. An episode of orogenesis is an orogeny. Most orogenesis occurs along convergent plate boundaries, where compressional forces cause folding and faulting, thickening the crust vertically and shortening it horizontally.

• collisional mountains: Mountains in which compressive horizontal forces have shortened and thickened the crust. Most major mountain belts are of this type.

• orogenesis: The processes that collectively result in the formation of mountains.

• orogeny: A specific episode of orogenesis (mountain building).