The main features of are: The Earth's surface is covered by a series of crustal plates. The ocean floors are continually, moving, spreading from the center, sinking at the edges, and being regenerated. Convection currents beneath the plates move the crustal plates in different directions. The source of heat driving the convection currents is radioactivity deep in the Earths mantle. Advances in sonic depth recording during World War II and the subsequent development of the nuclear resonance type magnometer (proton-precession magnometer) ....