Geophysics: Core

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Geophysics: Core
Geophysics: Core

Core-mantle interface not liquid after all?

A high proportion of iron in the rock layer between the earth's mantle and earth's core is possibly responsible for the fact that shear waves - waves that oscillate transversely to the direction of propagation of the wave - only move extremely slowly in this layer or are even no longer transmitted at all.

Wendy Mao of Los Alamos National Laboratory and Ho-kwang Mao of the Carnegie Institution Geophysical Laboratory tested how shear waves propagate in so-called post-perovskite. This iron-magnesium silicate, only known for two years, occurs at the core-mantle boundary and is a special variant of perovskite, the main component of mantle rocks.

In laboratory experiments, the two geoscientists simulated the environmental conditions prevailing in the mantle and core at a depth of around 2900 kilometers. At about 1.5 million times the air pressure and a temperature of 1750 degrees Celsius, they determined how fast shear waves moved through post-perovskite different iron content. Although the iron content in the mineral could be experimentally increased to eighty percent, the waves slowed down drastically halfway through.

Accordingly, the irregular layer between the core and the mantle could consist largely of solid post-perovskite with a high iron content. The latter could possibly be explained by reactions between the silicate shell and the iron core. Until now it was assumed that the material between the core and the mantle was partially melted, since shear waves cannot travel in liquids.

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