Top: Schematic cross section of a mid-ocean ridge. The center of the ridge contains the youngest basaltic crust. The age of the crust increases away from the center of the ridge. The thickness of sediment overlying the basaltic crust increases away from the ridge, as does the age of the sediments directly above the basaltic crust. Modified from Hamblin & Christiansen (1998). Bottom: Thickness of sediment in the oceans. This work-in-progress shows that the thinnest sediments are present along the mid-ocean ridge in the south Atlantic, and that the thickest sediments are present adjacent to South America and Africa. Modified from an image produced by NOAA's National Geophysical Data Center: http://www.ngdc.noaa.gov/mgg/image/sedthick.jpg.