The process of mountain building or formation; especially, by folding and faulting of the earth's crust and by plastic folding, metamorphism, and the intrusion of magmas in the lower parts of the lithosphere.

Unlike epeirogeny, orogeny usually affects smaller regions and is associated with evidence of folding and faulting.

The long chains of mountains often seen on the edges of continents form through orogeny.

As traditionally used, the term orogeny refers to the development of long, mountainous belts on the continents that are called orogenic belts or orogens.

• These include the Appalachian and Cordilleran orogens of North America, the Andean orogen of western South America, the Caledonian orogen of northern Europe and eastern Greenland, and the Alpine-Himalayan orogen that stretches from western Europe to eastern China.
• It is important to recognize that these systems represent only the most recent orogenic belts that retain the high relief characteristic of mountainous regions.
• In fact, the continents can be viewed as a collage of ancient orogenic belts, most of which are so deeply eroded that no trace of their original mountainous topography remains.
• By comparing characteristic rock assemblages from more recent orogens with their deeply eroded counterparts, geologists surmise that the processes responsible for mountain building today extended back through most (if not all) of geologic time and played a major role in the growth of the continents.
• The construction of mountain belts is best understood in the context of plate tectonics theory.
• Orogenic belts form at convergent boundaries, where lithosphere plates collide.
• There are two basic kinds of convergent plate boundaries, leading to the development of two end-member classes of orogenic belts.
• Oceanic subduction boundaries are those at which oceanic lithosphere is thrust (subducted) beneath either continental or oceanic lithosphere.
• The process of subduction leads to partial melting near the plate boundary at depth, which is manifested by volcanic and intrusive igneous activity in the overriding plate.
• Where the overriding plate consists of oceanic lithosphere, the result is an intraoceanic island arc; such as, the Japanese islands.
• Where the overriding plate is continental, a continental arc is formed. The Andes of western South America is an example.