Orogeny

Orogeny refers to forces and events leading to a large structural deformation of the Earth's lithosphere (crust and uppermost mantle) due to the interaction between tectonic plates. Orogens or orogenic belts develop when a continental plate is crumpled and is pushed upwards to form mountain ranges, and involve a great range of geological processes collectively called orogenesis.

Orogeny is the primary mechanism by which mountains are built on continents. The word "orogeny" comes from Ancient Greek (ὄρος oros, "mountain" + γένεσις genesis for "creation, origin"). Though it was used before him, the term was employed by the American geologist G.K. Gilbert in 1890 to describe the process of mountain building as distinguished from epeirogeny.

Formation of an orogen is accomplished in part by the tectonic processes of subduction (where a continent rides forcefully over an oceanic plate (noncollisional orogens)) or convergence of two or more continents (collisional orogens).

Orogeny usually produces long arcuate (from Latin arcuare, "to bend like a bow") structures, known as orogenic belts. Generally, orogenic belts consist of long parallel strips of rock exhibiting similar characteristics along the length of the belt. Orogenic belts are associated with subduction zones, which consume crust, produce volcanoes, and build island arcs. Geologists attribute the arcuate structure to the rigidity of the descending plate, and island arc cusps relate to tears in the descending lithosphere. These island arcs may be added to a continent during an orogenic event.

The processes of orogeny can take tens of millions of years and build mountains from plains or from the seabed. The topographic height of orogenic mountains is related to the principle of isostasy, that is, a balance of the downward gravitational force upon an upthrust mountain range (composed of light, continental crust material) and the buoyant upward forces exerted by the dense underlying mantle.

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