New clues about the Earth's movements


Synchrotron light has just revealed new clues about how the Earth moves. A team of scientists (Dubrovinsky et al.) from four different countries and different fields of expertise have come to the ESRF to study materials submitted to extreme conditions of pressure and temperature, similar to those found at the boundary between the core and the mantle of the Earth. The composition and properties of this interface (the so-called D'' layer) may explain some intriguing effects recently discovered concerning the Earth's nutation (small variations in the position of the Earth's axis of rotation). The article is published in Nature, volume 422, 6 March, under the title Iron-Silica interaction at extreme conditions and the electrically conducting layer at the base of Earth's mantle.

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The starting point of their research was to look deep inside the Earth. The boundary between the Earth's metallic core (mainly composed of iron), which is liquid, and the mantle (mainly composed of silica, SiO2), which is solid, is of major scientific interest. This interface is characterized by sharp changes in density, seismic wave velocities, electrical conductivity and chemical composition. earth
  Schematic view of the Earth's inner structure. Red star labels the D'' layer (pressure over 140 GPa and temperature of 3000°C).

What had to be done was to elucidate the composition and properties of the D'' layer at the core-mantle boundary. This requires an understanding of the chemical reactions between liquid iron and the complex Mg-Fe-Si-Al-oxides of the Earth's lower mantle. Experimental problems arise due to extremely high pressures (over 1 400 000 atmospheres, or 140 GPa) and temperatures (over 3000°C) typical of this region of the Earth's interior. At the ESRF, on beamlines ID30 and SNBL, this team of scientists studied an interaction between iron and silica (SiO2) at pressures up to 140 GPa and temperatures over 3500°C, simulating conditions down to the core-mantle boundary.

But first they reproduced the conditions existing when the Earth's core was formed, at pressures below 40 GPa and high temperatures. In these experiments they noticed that iron and silica react forming iron oxide and an iron-silicon alloy. However, at pressures of 85-140 Gpa (the current pressure of the Earth's core), iron and silica do not react and iron-silicon alloys dissociate into almost pure iron and the compound (B2) FeSi. This compound is electrically conducting under high pressure.

The presence of B2 FeSi at the base of the Earth's lower mantle could explain the anomalously high electrical conductivity of this region and provide a key to understanding some aspects of the Earth's movements. The rotation of the Earth is subjected to disturbances caused by the Sun and the Moon. Specifically, they cause a gravitational disturbance on Earth (lunisolar precession) and the nutation, or wobbling, of the Earth's rotation axis. It was recognized recently that the amplitude of the Earth's nutation is out of phase with the tidal force produced by the Sun and the Moon. This surprising observation could be due to the high electrical conductivity of the D'' layer, produced by the B2 FeSi compound.


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