Top: Schematic cross section of a mid-ocean ridge. A mid-ocean ridge marks the boundary between two plates that moving apart. As the plates move apart, the mantle rises upward to fill the gap between the diverging plates. Partial meltingof the upper mantle then yields magma, some of which is erupted on the seafloor to produce the basaltic crust. As the two plates pull apart, normal faults form to accommodate the stretching. Modified from Hamblin & Christiansen (1998). Bottom: Evolution of oceanic crust formed at a mid-ocean ridge. The processes described above are repeated continuously. New oceanic crust is created at the mid-ocean ridge, and half becomes part of the tectonic plate containing continent A, and half becomes part of the plate containing continent B. Over time, the width of the oceanic crust gets larger, as does the distance between continents A and B. Modified from Marshak (2001).