Study of the Oceanic Crust by Seismic Refraction and Gravimetry

The phenomenon of sea floor spreading cannot be understood without a knowledge of the structure of the crust and the underlying mantle. The large number of seismic profiles carried out in the last few years has considerably changed the picture gained from the study of near earthquakes more than 20 y. ago. At that time the continental crust was imagined as a series of geological layers defined by the average velocities of seismic waves in them. Beneath a highly variable sedimentary layer a so-called granitic layer was recognized, separated from the next layer, termed basaltic or gabbroic, by the doubtful Conrad discontinuity. The crest was separated from the upper mantle, thought to be composed of peridotites rieh in olivine (Mg, Fe)2SiO4 by the Mohorovicic discontinuity, or Moho. Just below the Moho, the velocity of P waves was 8.1 km/s, subsequently increasing with depth.