Oceanic Crust

Oceanic crust is the basaltic outer layer that floors the deep ocean basins, formed at mid-ocean ridges by decompression melting of the mantle. It is about 7 km thick and shows a layered structure: pillow basalts on top, a sheeted dike complex below, then gabbro above the Moho. Its density near 2.9 g/cm3 exceeds continental crust, so it subducts back into the mantle, and no oceanic crust older than about 180 to 200 million years survives. It records geomagnetic reversals as symmetric magnetic-anomaly stripes, the Vine-Matthews-Morley evidence for seafloor spreading.