Scientists have discovered a new layer of partially molten rock beneath the Earth's crust that could help resolve a long-standing debate about how tectonic plates move. Until now, scientists had found patches of melt, but the current study, led by the University of Texas at Austin, reveals for the first time the global extent of this phenomenon. The results have just been published in the journal Nature Geoscience. The molten layer lies about 160 kilometers below the Earth's surface and is part of the so-called asthenosphere, which lies beneath Earth's tectonic plates in the upper mantle. Slow convection movements occur in the asthenosphere, which explain the movement of continents. In addition, the basalt of the asthenosphere flows by extrusion along mid-ocean ridges, constantly renewing and expanding the ocean floor. On the other hand, where expansion encounters an obstacle represented by a continent, it sinks beneath it, thus causing the material at the bottom to melt within the asthenosphere and deeper mantle, a phenomenon known as subduction. The athenosphere becomes increasingly rigid as it goes deeper: starting at 350 kilometers, it loses its "soft" property until it fuses with the lower mantle at around 850 kilometers. However, the reason for this ductility is not entirely clear. Scientists thought that molten rocks could be a factor. But this work shows that melting, in fact, does not seem to significantly influence the flow of mantle rocks. Rocks that flow like honey "When we think of something melting, we intuitively think that melting must play an important role in the viscosity of the material," explains Junlin Hua, a doctoral student leading the research. "But what we found is that even when the melt fraction is quite high, its effect on mantle flow is very small." According to the authors, heat convection and rock in the mantle are the predominant influences on plate motion. Although Earth's interior is largely solid, over long periods of time, rocks can move and flow like honey. Showing that the melt layer has no influence on plate tectonics means one less complicating variable for computer models. A diagram of the asthenosphere, which aids plate tectonics, where researchers at the UT Austin Jackson School of Geosciences say they detected a global layer of partial melt (shown in mottled red). Junlin Hua/UT Jackson School of Geosciences “We can’t rule out that local melting doesn’t matter,” says Thorsten Becker, who designs geodynamic models of the Earth. “But I think it pushes us to view these melt observations as a marker of what’s happening in the Earth, and not necessarily as actively contributing to anything.” These aren’t isolated spots; It’s an entire layer The idea to search for a new layer deep inside the Earth came to Hua while studying seismic images of the mantle beneath Turkey during his doctoral research. Intrigued by signs of partially molten rock beneath the crust, Hua compiled similar images from other seismic stations until he had a global map of the asthenosphere. What he and others had taken to be a point anomaly was, in fact, a single layer present across the entire planet, and it showed up in seismic readings where the asthenosphere was hottest. MORE INFORMATION news No Journey to Jupiter’s moons that may harbor life The next surprise came when he compared his melt map to seismic measurements of tectonic movement and found no correlation, even though the melt layer spanned nearly half of Earth. "This work is important because understanding the properties of the asthenosphere and the reasons why it's weak is critical to understanding plate tectonics," explains Karen Fischer, a seismologist and professor at Brown University.
