orogenesis

Orogenesis, or orogeny, is the geological process by which mountains are formed. It encompasses the structural deformation of the crust, crustal thickening, folding and faulting, magmatic intrusion, metamorphism, and surface uplift that accompany the assembly of continents and the interaction of tectonic plates. Orogenic processes operate primarily at convergent plate boundaries, where plates collide or subduct, and at accretionary margins and intracontinental belts, over timescales of tens to hundreds of millions of years.

Key mechanisms include crustal compression leading to thickening and thrust faulting, nappe stacking, and development of dipping folds; subduction generates magmatic arcs and volcanic activity; continent-continent collision produces high-grade metamorphism, extreme crustal thickening, and uplift. Exhumation and erosion reveal deep crustal rocks in mountain belts and create foreland basins as the crust responds to load.

Orogenic belts form the major landscapes of the Earth and display characteristic structures such as thrust nappes, folded sedimentary and metamorphic rocks, metamorphism, and granitoid intrusions. The cycle often includes accretion of lithospheric blocks (terrane accretion), followed by regional metamorphism, and eventual surface uplift and erosion.

Famous examples include the Himalayas, formed by the ongoing collision of the Indian and Eurasian plates; the Andes, built by subduction of the oceanic Nazca plate beneath South America; the Alps, from the collision of Africa with Europe; and the ancient Appalachians, products of several Paleozoic orogenies.

Orogeny contrasts with extensional tectonics, such as rifting, and with post-orogenic collapse, where gravitational spreading and erosion modify previously elevated belts.