Evidence shows that volcanic island chains and aseismic ridges are formed by plate motion over fixed-mantle "hot-spots" (Iceland, Hawaii, Galapagos, etc.) and new arguments link these hot-spots with the driving mechanism of continental drift. It is assumed that the hot-spots are surface expressions of deep mantle plumes roughly 150 km in diameter, rising 2 m/year, and extending to the lowest part of the mantle. The rising material spreads out in the asthenosphere, producing stresses on the plate bottoms. Order-of-magnitude estimates show these stresses are sufficiently large to influence plate motion significantly. the total upward flow in the plumes is estimated at 500 cu km/year, which would require the entire mantle to overturn once each 2 billion years.