Scientists have discovered a new layer of partially molten rock beneath the Earth’s crust that could help settle a longstanding debate about how tectonic plates move. Until now, scientists have found melting ‘patches’ of sorts, 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 is about 160 kilometers of the earth’s surface, and is part of the so-called asthenosphere, which lies beneath Earth’s tectonic plates, in the upper mantle. In the asthenosphere there are slow convection movements that explain the movement of the continents. In addition, the basalt of the asthenosphere flows by extrusion along the mid-ocean ridges, which constantly renews and expands the ocean floor. On the other hand, where the expansion encounters an obstacle represented by a continent, it sinks under it, thus returning the bottom matter to melt within the asthenosphere and deeper mantle, a phenomenon known as subduction.
The athenosphere increases its rigidity as you go deeper: after 350 kilometers it loses its ‘soft’ property until it merges with the lower mantle, at about 850 kilometers. However, the reason for this ductility is not well known. Scientists thought that molten rock might be a factor. But this work shows that melting does not, in fact, appear 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 a significant role in the viscosity of the material,” he explains. Junlin Hua, PhD 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, convection of heat 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 a less complicated variable for computer models.
A diagram of the asthenosphere, which aids plate tectonics, where UT Austin Jackson School of Geosciences researchers say they detected a global layer of partial melting (shown in dappled red).
“We can’t rule out that local melt doesn’t matter,” he says. Thorsten Becker, who designs geodynamic models of the Earth. “But I think it prompts us to see these melt observations as a marker of what’s happening on Earth, and not necessarily as an active contribution to anything.”
They are not isolated places; it’s a whole layer
The idea to search for a new layer in the interior of the Earth occurred to Hua while studying seismic images of the mantle under Turkey during his doctoral research. Intrigued by the 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.
The next surprise came when he compared his melt map with seismic measurements of tectonic movement and found no correlation, despite the fact that the melt layer covered almost half of the Earth. “This work is important because understanding the properties of the asthenosphere and the origins of why it is weak is critical to understanding plate tectonics,” he explains. Karen Fischerseismologist and professor at Brown University.