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Model of a dislocation in the post-perovskite phase
of the D" layer. This type of dislocation is responsible for the
deformation of this layer, affecting convection movements within
the mantle perceived via plate tectonics. © Patrick Cordier - CNRS
2007
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Direct access to the Earth's interior is
impossible: even the deepest bore holes are only scratches on the
surface. Our knowledge of the Earth's interior comes from studying the
seismic waves which propagate through the Earth from the focus of an
earthquake. We know today that the Earth is divided into layers. The
crust on which we live only represents a thin skin. The main shell is
called the mantle, a layer made up of solid rock which extends to a
depth of up to 2900 kilometres. It surrounds the liquid core which in
turn shields the solid core, with a radius of 1200 kilometres. The
interface between the mantle and the core, called the D" layer, has
long intrigued geophysicists because they are unable to explain the
seismic data it generates.
From a mineralogical point of view, 80% of the
terrestrial mantle is made up of a silicate (MgSiO3) with a
crystalline, perovskite structure. This mineral accounts for half of
the Earth's mass. In 2004, several teams (notably from Japan) showed
that perovskite became unstable near the core-mantle interface to form
a new phase, or post-perovskite. Could post-perovskite deformation
explain the seismic signature of the D" layer?
Patrick Cordier and his colleagues based themselves
on this hypothesis. But how could a crystalline solid be deformed? The
answer lies at the atomic scale: the crystals contain defects called
dislocations, which are responsible for plastic deformation. Although
their structure is relatively well understood in simple materials such
as certain metals (copper, aluminium, etc.), the scientists had little
knowledge of the structure of dislocations in complex materials such
as minerals, particularly under extreme conditions of pressure. The
team in Lille employed a novel approach: instead of reproducing the
conditions prevailing inside the Earth in the laboratory, they used a
simulation method by injecting the results of quantum mechanics into a
numerical model to render it simpler. They are the first to have thus
modelled dislocations at the atomic scale for complex materials under
extremely high pressure.
The dislocations of which we now know the structure
move within the crystal and interact between each other. Scientists
thus have access to calculation codes which allow them to describe
these interactions. They now want to clarify the behaviour of each
grain of crystalline matter, then of the rock and beyond that, of the
mantle. A dream? Maybe not. The advances achieved in recent years
allow us to be optimistic. So perhaps our voyage to the centre of the
Earth will be numerical. |
