The movements of continental plates over the mantle whose currents pull them around the Earth's surface. As far back as 1912, Alfred Wegener noted that the continents fit together like puzzle pieces into some lost whole (see Pangaea, a name he coined). He was ignored, but in 1929, Arthur Holmes decided that thermal convection currents in the mantle could move continents around. These ideas also account for where the seafloor comes from (molten material emitted through cracks between plates), how some of it disappears (when the edge of one plate vanishes under another--subduction), and why volcanic and mountain-building activity so often surfaces in belts (plate edges).