How continents persist

    Earth scientists have explained why Canada and South Africa are still here.

    Plate tectonics paints a picture of Earth's mantle as a constantly changing sea of geological violence. Processes such as subduction and mantle convection mock the political boundaries imagined on the surface, as the ground we stand on is inevitably dragged back into the planet to be recycled as new territory elsewhere.

    Yet amid this slow-motion destruction, blocks of Earth's crust have survived relatively unscathed for billions of years. Located at the core of continents such as North America and Australia, these cratons have been constants for much of the planet's history. Cratons are not new to science — their name can be traced back to 1921. But no one, until now, has been able to confirm how they remain in place for so long.

    On page 78 of this issue, Anne Peslier and her colleagues offer a solution. After studying the southern African Kaapvaal craton, they conclude that water is the key. The deep roots of cratons are protected by a layer of unusually dry minerals, which resist the surrounding swirl of more fluid mantle. The findings are not completely unexpected. Water alters the viscosity of olivine, the most common mineral of the upper mantle. Geologists saw this as a likely way to preserve cratons.

    This new study shows that they were correct. It was not easy work. To measure the water content of tiny grains of mineral demanded tremendous analytical skills. To retrieve the samples from deep underground in a pristine state took painstaking efforts. To combine the two to make a compelling scientific case marks a triumph for those involved.

    The work may not make headlines, but it deserves recognition. Working in diamond country, they have established how some parts of our planet are forever.

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    How continents persist. Nature 467, 7 (2010).

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