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where’s alfred when you need him?

August 7, 2010

[Economist] At the centre of the Earth, below the mountains and the oceans and the thin, brittle crust, below the stony, slow-flowing mantle and the roiling outer core of liquid iron, is a solid inner core. If anything about the planet looked unlikely to partake in a process of endless recycling, you might think this ball of metal, 1,200 kilometres across, squeezed from every direction by a planet’s worth of weight, would be it—a dense static hub about which all else turns.

Scientists have known for some time that this inner core is not unchanging. But they had thought that it changed in only one direction—that it simply grew bigger. The Earth is growing cooler as it loses the heat trapped in its creation and generated by radioactive elements within it. It is in fact this cooling which powers the slow circulation of the mantle, and through that the endless remaking of the surface through plate tectonics. As things cool down, the liquid outer core freezes into the solid inner core. It is thought that this process leads the inner core to grow larger at a rate of roughly 30 centimetres a century.

The remarkable new idea floated by Thierry Alboussière, Renaud Deguen and Mickaël Melzani of the Joseph Fourier University in Grenoble is that this slow growth is a net effect, the residual left over when a greater rate of freezing is offset by a rate of melting almost as large. This notion follows from the hitherto unexplored idea that the spherical inner core is very slightly offset from the planet’s centre of mass, so that one side—the western side, as it happens—is slightly lower than the other. On the lower side the pressure is greater, and liquid iron freezes solid. On the higher side the pressure is less, and solid iron melts. (read)

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