Magmatism

Magmatism is the emplacement of magma within and at the surface of the outer layers of a terrestrial planet, which solidifies as igneous rocks. It does so through magmatic activity or igneous activity, the production, intrusion and extrusion of magma or lava. Volcanism is the surface expression of magmatism. Magmatism is the emplacement of magma within and at the surface of the outer layers of a terrestrial planet, which solidifies as igneous rocks. It does so through magmatic activity or igneous activity, the production, intrusion and extrusion of magma or lava. Volcanism is the surface expression of magmatism. Magmatism is one of the main processes responsible for mountain formation. The nature of magmatism depends on the tectonic setting. For example, andesitic magmatism associated with the formation of island arcs at convergent plate boundaries or basaltic magmatism at mid-ocean ridges during sea-floor spreading at divergent plate boundaries. On Earth, magma forms by partial melting of silicate rocks either in the mantle, continental or oceanic crust. Evidence for magmatic activity is usually found in the form of igneous rocks – rocks that have formed from magma. Magmatism is associated with all stages of the development of convergent plate boundaries, from the initiation of subduction through to continental collision and its immediate aftermath. The subduction of oceanic crust, whether beneath oceanic of continental crust, is associated in almost all cases with partial melting of the overlying asthenosphere due to the addition of volatiles (especially water) expelled from the downgoing slab. Only when the slab fails to reach sufficient depth as in the earliest stages of subduction or where there are periods of flat-slab subduction that completely pinch out the asthenosphere, is magmatism absent. The magmatism is mostly calc-alkaline in type along a well-defined curvilinear magmatic arc. Only the volcanic parts of modern arcs are exposed at the surface and the understanding of the underlying magma chambers relies on geophysical methods. Ancient arc sequences that formed on continental crust or that have become accreted to continental crust are often deeply eroded and the plutonic equivalents of the arc volcanoes become exposed. Continental collisions are accompanied by major crustal thickening, leading to heating and anatexis within the crust, generally in the form of peraluminous granitic intrusions. Post-collisional magmatism is a result of decompression melting associated with isostatic rebound and possible extensional collapse of the thickened crust formed during the collision. Slab detachment has also been proposed as a cause of late to post-collisional magmatism.