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oceanic crust The oceanic rocks which form 65% of the Earth's surface, and are the upper part of the oceanic lithosphere, overlying the Mohorovičić seismic discontinuity. The oceanic crust comprises four seismic layers, commencing at an average depth of 4.5 km below sea level. The uppermost layer (sediments) varies in thickness from being absent over the oceanic ridges to 2–3 km near the continental shelves. The other three layers are of remarkably constant thickness and seismic velocity. Layer 2 is formed by basaltic lavas with P-wave velocities of 5 km/s and a dyke complex just under 2 km thick, overlying a 5 km thick gabbroic layer (layer 3) with a P-wave velocity of 6.7 km/s. Layer 4 is a thin (less than 0.5 km) layer with a P-wave velocity of 7.4 km/s, immediately overlying the mantle where the P-wave velocity is 8.1 km/s. The total thickness is about 11 km and shows little variation throughout the ocean basins, despite the change in ocean bathymetry from the ridges to the trenches. The crust is cut by fracture zones. It is largely aseismic away from spreading ridges and subduction zones. The presence of linear magnetic anomalies, mostly paralleling the present mid-oceanic ridges, allows the dating of the ocean floors, as does the dating of sediments immediately overlying the igneous part of the crust and obtained by drilling during the DSDP and IPOD projects. The oldest oceanic crust, within the present ocean basins, is less than 200 million years old; it is found in the W. Pacific and the N. Atlantic, and was formed by igneous activity at spreading (accretionary) plate margins as part of the sea-floor spreading process. Layer 3 represents the cooled magma chamber that originally fed the overlying basaltic dykes and lavas.