Maurice Ewing. Portrait of William Maurice Ewing (1906-74), American geologist who discovered the thickness of the Earth's crust beneath the oceans. Using marine seismic techniques, Ewing showed that the crust beneath the oceans was only 5-8 km thick as compared with the 40 km beneath continental land masses. Ewing also discovered the extent of the global extent of mid-ocean ridges now known to be caused as plates in the crust move apart. In 1957 he showed that the mid-Atlantic ridge had a deep central rift in it. He also showed that the depth of sediment on the ocean floor grows with increasing distance from a ridge, providing evid- ence in support of Hess' plate tectonic theory.