Scientists have discovered a new layer of partly molten rock under the Earth's crust that might help settle a long-standing debate about how tectonic plates move. The molten layer is located about 100 miles from the surface and is part of the asthenosphere, which is important for plate tectonics because it forms a relatively soft boundary that lets tectonic plates move through the mantle. The researchers found, however that the melt does not appear to notably influence the flow of mantle rocks. Instead, they say, the discovery confirms that the convection of heat and rock in the mantle are the prevailing influence on the motion of the plates.