The magma that melts in the upper mantle and erupts at the mid-ocean ridge has a different composition (higher in Ca, Mg, Fe and relatively lower in Si and Al) than the composition of the magma (more Si, Al, Na, K rich in relative terms) that built the continents. Melting different degrees (2%, 5%, 30%) of a rock with a mantle-like composition generates magma of different Al, Si contents. Continents grow at their edges by partial melting of oceanic crust that is being subducted, generating magmas of a somewhat more Si, Al-rich composition than the subducting slab itself. The chemistry and physics of melting a mixture of minerals rather than one homogeneous substance has the wonderful outcome of creating two distinct types of crust:
