Seismic exploration at sea has established that the oceanic crust is completely different from that of the continents. If we accept continental drift, it is therefore necessary to invoke a mechanism for the evolution of new oceanic crust. An attempt is made to locate regions where new oceanic crust may be forming and it is suggested that these regions are related to regions of uprising convection in the mantle. The crustal structures beneath the Red Sea and the Gulf of California are very similar and closer to oceanic than continental. As these are seismically active regions of extension, it seems reasonable to suppose that they represent areas where new oceanic crust is evolving in regions of continental break-up. These rift zones are in continuity with the seismically active oceanic rifts where similar seismic velocities (about 7 km/s) have been found and it is inferred that the oceanic rifts also represent regions where new oceanic crust is evolving. These are generally near the centres of the oceans. The tensional rift zones which are regions of shallow seismicity help to locate regions of rising convection currents in the mantle. It is further suggested that regions of deep and intermediate focus earthquakes locate regions of descending convection currents and maps of earthquake distributions are used to reveal a possible large-scale pattern of mantle convection. It is supposed that new oceanic crust evolves over the rising convection currents. A study is therefore made of the crustal sections for the Red Sea, Gulf of California and mid-oceanic rift regions and these are compared with typical continental and oceanic crusts. A possible mechanism for the evolution of new oceanic crust is given based on the isostatic equilibrium of oceans and continents.