The two most important structural features of the lithosphere in terms of plate tectonics are the oceanic ridges and the zones of subduction. The energy to create both is provided by the heat-driven convection cells within the asthenosphere. The tensional forces responsible for the rifting of the lithosphere to create the oceanic ridges are created by the riding portion of the convection cell while the compressive forces that break the lithosphere and create the zones of subduction are the result of the down-going portion of the convection cell. As the rifting along the oceanic ridge continues, basaltic magma rises from the top of the asthenosphere and cools to form the gabbroic rocks of the bottom of the lithosphere while the magmas that rise to the surface form the upper portion of the lithosphere. Simultaneously with the creation of new oceanic lithosphere at the oceanic ridge, old oceanic lithosphere is consumed as the oceanic lithosphere is thrust down and under the edge of the adjoining continental crust. Within the zone of subduction, andesitic magmas are created and rise to the surface to form either continental arc volcanic mountains such as the Andes or oceanic arc volcanic islands such as the Aleutians. While some basaltic magmas are formed within the zone of subduction, the great masses of magma are the granitic magmas that are eventually cooled, solidified and exposed by erosion as mountains such as the Sierra Nevada.
oceanic ridge, zone of subduction