Although the evidence for continental drift became overwhelming, no mechanism was proposed until the early 1960's when Harry Hess and Robert Dietz described a process of ocean floor spreading supported by the symmetric banding of the magnetic signature of the sea floor parallel to ocean ridges.
Currents form within any liquid when it is heated, just as they do in a pot of boiling soup. Similar currents form with the Earth's thick, dense mantle. The source of heat is the decay of radiogenic istopes. Arthur Holmes was the first to propse this mechanism in 1945.
As the mantle is heated, it rises, creating massive, slow convection currents within the Earth. The heated rock spreads i laterally at the base of the solid lithosphere, dragging the frgmented crust with it. Volcanoes and earthquakes occur as a direct consquence of plate movements.
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