In the last half dozen years the nature of the crust under the oceans has been determined by seismic refraction work (Ewing, Worzel, Raitt, Hill, Hersey, Gaskell, and others). The Mohorovicic discontinuity is near 10 km in depth, and above this there are approximately 4 km of rock having the properties of basalt which are overlain by sediments of perhaps ¾ km thickness.
Hess, H. H.. 1955. "The oceanic crust." Journal of Marine Research 14, (4). https://elischolar.library.yale.edu/journal_of_marine_research/854