Publisher Summary This chapter describes the structural features and the formation of the oceanic lithosphere. The oceanic crust has a seismically well-defined layered structure. The first layer consists mostly of sediments overlying the igneous crust of the second and third layers. The second layer is volcanic, dominated by pillow lavas and other types of basaltic lavas (above) and basaltic dykes (below), and the third layer is gabbro and represents magma that has crystallized at depth. The fourth layer is the uppermost (lithospheric) mantle. Seismic tomography and seismic reflection studies have shown that spreading axes are underlain by zones of crystal mush, at a depth equivalent to the third layer. Several sophisticated techniques—swath bathymetry, sidescan sonar, underwater photography, and submersible operations—have shown that volcanism in the median rift valley of ocean ridges is not continuous but episodic. Spreading axes are segmented on a variety of scales. The oceanic crust is thinner than normal in back-arc basins and thicker than normal in some other parts of the oceans—notably in large igneous provinces. Fractures and faults can bring rocks of deeper crustal layers to the surface. The seamounts and volcanic islands are formed by isolated submarine volcanoes, building up from the sea-bed.