Continent

Continents develop and grow in several ways as a byproduct of sea floor spreading and subduction. When subduction occurs where oceanic and continental crust converge, the plate margin carrying oceanic crust invariably subducts below the other. As the oceanic plate margin subducts into the upper mantle, volatiles (primarily water) escape the subducting crust. These volatiles lower the melting temperature of the overlying mantle, producing mafic magma due to partial melting. The magma moves upwards through the base of the felsic crust overhead, causing more partial melting. The resulting magma tends to have an intermediate to felsic composition. As crystallization begins within magma, the early crystals tend to be more mafic due to their higher crystallization temperature. Therefore, this fractional crystallization further refines the magma toward a felsic composition. When the magma crystallizes, the upper crust gains additional felsic rock.

Alternatively, accumulated sediments can be scraped off the subducted plate onto the overriding one, forming an accretionary wedge. A volcanic island arc may eventually arrive at the plate boundary as well. Subduction of such a large mass is not possible, however, so part of the arc will also be welded, or accreted, to the continental margin. The accretionary wedge may be metamorphosed and welded to the crust too. Much of California formed in this way, one small piece—known as a microterrane—at a time. Finally, fragments of oceanic crust, called ophiolites, sometimes resist subduction and get shoved, or obducted, onto the continental margin as well. So continents grow by magma emplacement, accretion and obduction. In time, through intensive metamorphic and igneous activity, alteration of these microterranes will produce a more-or-less stable, homogenous, rock unit and a new terrane will become a part of the ancient craton.