Oceans apart
Three-fifths of Earth’s crust lies underwater, spread out along the seafloor. More than four cubic miles of ocean crust forms each year, constantly regenerating like new skin across the globe. This ocean crust arises along mid-ocean ridges — underwater mountain ranges that ripple along the ocean floor like jagged scars. These ridges line the boundaries of tectonic plates, which slowly shift around the planet. As plates pull apart, magma from the underlying mantle erupts at the surface, eventually solidifying as new crust. In time, this newly formed crust moves with the migrating plate away from the ocean ridge, leaving room for newer crust to take its place. The speed of crust formation varies from ridge to ridge: Some fast-spreading ridges produce up to six inches of new crust per year, while slower-spreading ridges creep along at just two inches per year.While the general process of ocean-crust formation — also known...