The plate-tectonic revolution was initially “kinematic”—a description of plate motions across Earth’s surface. Plate tectonics is now recognized as the surface manifestation of a greater process—circulation of the solid earth. Magma ascends to the surface at mid-ocean-ridge spreading centers to cool and form oceanic crust, which millions of years later returns to the mantle at subduction zones. Formation of oceanic crust is the greatest contribution of flow from our planet’s interior, as two-thirds of the earth is resurfaced about every 100 million years. Partial melting of the mantle at spreading centers is the mechanism by which this flow takes place, and thus is fundamental to understanding solid-earth circulation.