Geophysical contrasts between continents and oceans are described and discussed. Seismic-refraction measurements have established beyond doubt that the silicic crust under ocean basins is no more than one-fifth as thick as that under continents, and corresponds in composition to the more mafic parts of the continental crust. This crustal structure, revealed by seismic measurements, confirms the isostatic equilibrium between continents and ocean basins determined by gravity measurements. Remarkable uniformity of crustal structure within continents or within ocean basins is revealed when obviously anomalous areas such as margins, mountains, and trenches are omitted. Magnetic measurements are meager, but those available indicate that over certain large oceanic areas the magnetic field is unusually smooth.

The borderland between continents and ocean basins represents one of the most difficult areas for geophysical investigation but is crucial in most studies. Existing data reveal a tendency for abnormally low seismic velocities in the oceanic crust near continental margins.