The mid-ocean ridge is a 75 000-kilometer network of undersea mountain chains that, like the seam of a baseball, winds around the globe from the Arctic Ocean to the Atlantic, around Africa, Asia, and Australia, under the Pacific, and skirts the west coast of North America. This fundamental feature of Earth’s superstructure delimits the intersection of tectonic plates and is the source of 85% of Earth’s volcanism. As plates separate and cracks form, the mantle wells up to fill the gap. Partial melting of the solid mantle produces magma that percolates upward through pores and grain boundaries in the rock and then crystallizes into basalt—new layers of Earth’s crust. The process prompted oceanographer Bruce Heezen to characterize the ridge system as “the wound that never heals.”
But if the mechanics of tectonic fracture were all that controlled the accretion of crust, one might expect uniform depth everywhere as the plates...
