Scientists from the University of Texas at Austin have discovered a new layer of partially molten rock in the Earth’s interior that plays an important role in plate tectonics. This is reported in an article published in the journal Nature Geoscience.

The layer is located more than a hundred kilometers from the surface of the planet and is part of the asthenosphere, which is located under the tectonic plates in the upper part of the mantle. The asthenosphere is more plastic than neighboring layers, meaning it has a low viscosity, which allows the plates of the lithosphere to move above it. The reason for the plasticity was thought to be the melting of the rock, but a new study has shown that melting does not significantly affect the viscosity of mantle rocks.

The low viscosity of the asthenosphere causes the velocity of seismic waves in it to decrease, but the lower limit of this region has not been adequately studied. By analyzing seismic images of the mantle, the researchers created a global map of the asthenosphere and found that there is a positive seismic velocity gradient (the velocity starts to increase with depth) at a depth of 150 kilometers. This layer is the bottom of the asthenosphere containing the partial melt.

The scientists also compared the melt-layer map with seismic measurements of tectonic movement, but found no correlation, even though the melt-layer covers almost half of Earth. In other words, the low viscosity of the asthenosphere floor did not show a link with tectonic movements, and the presence of this layer did not affect the large-scale viscosity of the entire asthenosphere.