The friction of subduction generates heat that further melts magma in the asthenosphere at the transition zone between the two types of lithosphere. This magma migrates upward through the continental crust to produce plutons , batholiths , and caldera volcanoes at or near the Earth's surface. These features have a chemistry that is a mixture of both mafic and felsic magmas because of contact melting of continental rock. Oceanic crust is composed of relatively young mafic rocks created along volcanic rift zones found at the mid-oceanic ridges . Convectional currents within the Earth's mantle cause the movement of mafic magma from the asthenosphere to the oceanic surface in a conveyor belt fashion.
Ocean crust is recycled back into the asthenosphere at the subduction zones.
Because of this recycling process, the maximum age of mafic igneous rocks found in the ocean crust is about 180 million years old. Classification of Igneous Rocks A simple classification of igneous rocks can be accomplished by examining grain size of the mineral crystals, temperature at which solidification occurs, and the quantity of mafic and felsic minerals found in the rock. Table 10e-1 below describes the classification of nine general igneous rock types based on these criteria. The temperature of cooling determines which types of minerals are found dominating the rock's composition. Rocks that begin their cooling at low temperatures are rich in minerals composed of silicon, potassium, and aluminum.