Earth’s lithosphere is subdivided into rigid plates known as tectonic or lithospheric plates. These move on a more fluid layer of the mantle, the asthenosphere. The asthenosphere’s lower viscosity allows the tectonic plates to move around on the underlying mantle, but until today, the origin of this low viscosity remained unknown.
Seismic tomography is a technique for imaging the subsurface of the Earth with seismic waves. The method produces three-dimensional images of the Earth’s interior by analyzing millions of seismic waves recorded at seismological stations spread across the globe’s surface.
Since the 1970s, seismologists have analyzed these waves to identify a single parameter: their propagation speed. This parameter varies with temperature (the colder the medium, the faster the waves arrive), composition and the possible presence of molten rocks in the medium the waves pass through. Seismologists instead studied another parameter, wave attenuation, alongside the variation in wave propagation speeds.
In a new study, scientists from the Laboratoire de géologie de Lyon: Terre, planètes et Environnement (CNRS/ENS de Lyon/Université Claude Bernard Lyon 1) have created a new model that considers not only the velocity of seismic waves but also how the medium attenuates them they pass through. The velocity of tectonic plates near the surface is thus directly correlated with the quantity of magma present.
The model shows that deep magma facilitates the movement of tectonic plates.
Using a model, scientists mapped the amount of molten rock under tectonic plates for the first time. Their analysis reveals that a small amount of molten rock (less than 0.7% by volume) is present in the asthenosphere under the oceans, not only where this was expected, i.e., under ocean ridges and some volcanoes such as Tahiti, Hawaii, or Reunion, but also under all oceanic plates.
The low percentage of molten rock observed is enough to reduce the viscosity by one or two orders of magnitude underneath the tectonic plates, thus “decoupling” them from the underlying mantle.
Besides, Lyon’s seismologists observed that the amount of molten rock is higher under the faster-moving plates, for example, the Pacific plate. This recommends that the rocks’ melting encourages the plates to move and the deformation at their bases. This exploration improves our comprehension of plate tectonics and how it works.
- Debayle, E., Bodin, T., Durand, S. et al. Seismic evidence for partial melt below tectonic plates. Nature 586, 555–559 (2020). DOI: 10.1038/s41586-020-2809-4