MECHANISM OF PLATE MOVEMENT AND DIRECTION CHANGE

Imaging through the Earth’s mantle by earthquake waves shows that subducting slabs descend all the way to the Earth’s core. Hence all 2800 kilometers of the subplate mantle overturn beneath large tectonic plates. Spreading axes between the plates are offset into stairstep patterns by long transform faults, suggesting that the spreading that occurs there is largely a passive response to the divergence of the plates. The 90-km-thick mantle part of the plate, beneath the thin oceanic crust, is chilled by its nearness to the Earth’s surface. This makes it heavier than the main part of the mantle, so slabs of it penetrate downward into subduction zones as fast as 10 cm/yr. The downward-moving slabs pull the plates behind them, and both together cause an overturning of a mantle cell beneath the plate. An adjacent mantle cell in contact with the descending slab, but opposite from the moving plate, rotates in the opposite direction. Hotspot plume tracks, such as the Hawaiian and Emperor Ridges, mark the direction and approximate rate of plate movement. Paleomagnetic measurements show that hotspots move backward at about one third the rate that the plates move, because the hotspot source near the base of the convection cell moves opposite to the plate. The Pacific Plate changed about 43 million years ago from moving northward parallel with the Emperor Ridge to moving northwestward parallel with the Hawaiian Ridge. This happened when new subduction began to curl downward along the former Mariana Transform Fault. Later, the Pacific Slab descended at full speed into the Mariana Subduction Zone, and the Pacific Convection Cell reoriented to its present movement.