our knowledge of the Earth is restricted to only about 1/1820 of its total radius. Understanding the remaining mass--estimated through seismology--is critical to predict natural phenomenon like earthquakes and volcanic activity. A study has shown our methods for reading the deep mantle need refining.
Seismology determines the composition and density of the planet's interior by sending sound waves through it. The velocity of a sound wave depends on the density of the material through which it passes. The faster the wave travels, the denser the material.
Now comes the finding that velocity of sound waves in the lower mantle is affected by high temperature and pressure. A study has shown the sound wave speed readings may not be very reliable as the material is softer than present calculations suggest. This has come out in a recent paper by a group of geologists led by J C Crowhurst of Lawrence Livermore National Laboratory, California, usa (Science, January 25, 2008).
The research deals with the elasticity of ferropericlase, the second most abundant mineral in the lower mantle. In normal conditions, the outermost electrons in an iron atom travel in separate orb i ts; this helps the atom remain stable and in a non-reactive state. Under extreme temperature and pressure, electrons from different orbits pair up into a single orbit. The two electrons spin in opposite directions, hence cancelling out each other's spin.
This leads to a transition from a high-spin electronic state to a low-spin one. Knowing the effect of the spin transition on sound waves would help get more accurate data necessary for interpretation of seismic observations.
Does a change in the mantle density affect our prediction of earthquakes? Alexander Goncharov of the Carnegie Institution's Geophysical Laboratory, one of the authors of the paper, said, "In principle it is possible. Our study may change interpretation of seismic observations."
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