scientific discipline that is concerned with the application of geophysical methods to problems of marine geology. Each of the principal branches of geophysical knowledge is involved: heat-flow data are obtained from ocean floors and from the midoceanic ridges; seismic reflection and refraction techniques are used to determine sediment thickness and the thickness of the oceanic crust; geomagnetics has been applied to samples of the oceanic crustal rocks in paleomagnetic investigations; and gravity measurements are made over the oceans (as on land) to complete knowledge of the global gravity distribution. Marine geophysics is intimately associated with the concepts and problems of seafloor spreading, continental drift, and plate tectonics.