orogénesis

Orogénesis, orogenesis in English, is the geological process by which mountains form through deformation of the Earth's lithosphere. It encompasses crustal shortening, thickening, uplift, folding and faulting, regional metamorphism, and magmatic activity, driven mainly by plate tectonics at convergent boundaries and, to a lesser extent, at other boundary types. Orogenesis operates over millions of years and yields mountain belts and elevated landscapes.

Primary mechanisms include continental collision, subduction of oceanic lithosphere, and the accretion of crustal blocks. In continental collision, converging plates fuse and the crust shortens and thickens, producing thrust faults, nappes, and high-grade metamorphism. Subduction creates volcanic arcs and uplift in the overriding plate, while accretion adds crustal blocks that are deformed and sutured into existing massifs. Crustal shortening is partly balanced by vertical uplift through isostasy, and magmatic intrusions contribute plutonic rocks and silicic volcanism.

Characteristically, orogenic belts show folded sedimentary sequences, thrust-related structures, metamorphic aureoles, and extensive igneous activity. The cycles involve long phases of uplift and deformation followed by erosion, which shapes peaks and valleys and exposes deeper rocks. Later tectonics can rework or reactivate former structures, continuing landscape evolution within the same belts.

Examples include the Himalayan system formed by India-Asia collision; the Andean system above subduction of the Nazca Plate; and the Alpine belt surrounding the Mediterranean region. Older belts such as the Caledonian and Variscan/Hercynian orogenies illustrate different tectonic histories that nevertheless rely on the same fundamental processes of crustal deformation and mountain building.