A layer of partially molten rock about 22 to 75 miles underground can't be the only mechanism that allows continents to gradually shift their position over millions of years, according to a NASA-sponsored researcher. The result gives insight into what allows plate tectonics – the movement of the Earth's crustal plates – to occur.
Although the basic process that drives plate tectonics is understood, many details remain a mystery. "Something has to decouple the crustal plates from the asthenosphere so they can slide over it," says Schmerr. "Numerous theories have been proposed, and one of those was that a melt-rich layer lubricates the boundary between the lithosphere and the asthenosphere, allowing the crustal plates to slide. However, since this layer is only present in certain regions under the Pacific plate, it can't be the only mechanism that allows plate tectonics to happen there. Something else must be letting the plate slide in areas where the melt doesn't exist."
Other possible mechanisms that would make the boundary between the lithosphere and the asthenosphere flow more easily include the addition of volatile material like water to the rock and differences in composition, temperature, or the grain size of minerals in this region. However, current data lacks the resolution to distinguish among them.
Schmerr made the discovery by analyzing the arrival times of earthquake waves at seismometers around the globe. Earthquakes generate various kinds of waves; one type has a back-and-forth motion and is called a shear wave, or S-wave. S-waves traveling through the Earth will bounce or reflect off material interfaces inside the Earth, arriving at different times depending on where they interact with these interfaces.