MORGAN1 has convincingly demonstrated a case for the upward rise of deep mantle material along narrow plumes which give rise to positive gravity anomalies and hot spots on the Earth's surface. These hot spots are reflected in large volcanoes and, as Wilson2 suggested, where the lithosphere moves over hot spots, volcanic chains are produced. Perhaps the most striking support of Morgan's hypothesis are the parallel ‘dog legs’ in hot spot trajectories given by the Hawaii-Emperor, Austral-Marshall/Gilbert, and Tuamotu-Line chains, most readily explicable as generated by motion of the Pacific plate over three fixed hot spots.