Deep magma facilitates the movement of tectonic plates

Date:
October 21, 2020
Source:
CNRS
Summary:
A small amount of molten rock located under tectonic plates
encourages them to move. This is what scientists have recently
discovered. Their new model takes into account not only the velocity
of seismic waves but also the way in which they are attenuated
by the medium they pass through. The velocity of tectonic plates
near the surface is thus directly correlated with the quantity of
magma present.



FULL STORY ==========================================================================
A small amount of molten rock located under tectonic plates encourages
them to move. This is what scientists from the Laboratoire de ge'ologie
de Lyon: Terre, plane`tes et environnement (CNRS/ENS de Lyon/Universite'
Claude Bernard Lyon 1) have recently discovered. Their new model takes
into account not only the velocity of seismic waves but also the way in
which they are attenuated by the medium they pass through. The velocity
of tectonic plates near the surface is thus directly correlated with
the quantity of magma present. This research is published on October 21,
2020 in Nature.


==========================================================================
The lithosphere, the outer part of the Earth, is made up of the crust
and part of the upper mantle. It is subdivided into rigid plates, known
as tectonic or lithospheric plates. These move on a more fluid layer of
the mantle, the asthenosphere. The lower viscosity of the asthenosphere
allows the tectonic plates to move around on the underlying mantle,
but until today the origin of this low viscosity remained unknown.

Seismic tomography produces three-dimensional images of the Earth's
interior by analysing millions of seismic waves recorded at seismological stations spread across the surface of the globe. Since the 1970s,
seismologists have analysed these waves with a view to identifying 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 from the Laboratoire de
ge'ologie de Lyon: Terre, plane`tes et environnement (CNRS/ENS de Lyon/ Universite' Claude Bernard Lyon 1) instead studied another parameter,
wave attenuation, alongside the variation in wave propagation speeds. This analysis, which provides new information on the temperature of the medium traversed by the waves, makes it possible to ascertain the quantity of
molten rock in the medium the waves pass through.

Their new model made it possible, for the first time, to map the
amount of molten rock under tectonic plates. This work 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. Moreover,
the seismologists from Lyon observed that the amount of molten rock is
higher under the fastest-moving plates, such as the Pacific plate. This suggests that the melting of the rocks encourages the plates to move and
the deformation at their bases. This research improves our understanding
of plate tectonics and how it works.


========================================================================== Story Source: Materials provided by CNRS. Note: Content may be edited
for style and length.


========================================================================== Journal Reference:
1. Eric Debayle, Thomas Bodin, Ste'phanie Durand, et Yanick
Ricard. Seismic
evidence for partial melt below tectonic plates. Nature, October
21, 2020 DOI: 10.1038/s41586-020-2809-4 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/10/201021112348.htm

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