Melting by meteorite may have formed early silica-rich rocks

The enigmatic origins of the Earth's oldest known silica-rich rocks might be explained by meteorite impacts melting the planet's crust, suggests a paper published online this week in Nature Geoscience.

The early Earth of around four billion years ago sported a crust that was principally made up of dark-coloured, silica-poor - or ‘mafic’ - rocks. A metamorphic rock of Canada’s Northwest Territories - called Idiwhaa gneiss - however, contains pale, silica-rich - or ‘felsic’ - rocks. However, these felsic rocks formed from the mafic crust available at the time has been a mystery.

Tim Johnson and colleagues examine the chemical composition of the Idiwhaa gneisses, and use models to show that it was possible to make these special felsic rocks by melting the ancient mafic crust. Specific conditions would have been needed, however, with low pressure - within the top 3 km of the crust - and a high temperature. The most likely way to generate enough heat to melt rocks at such a shallow depth would be from a meteorite crashing into the crust.

Meteorite impacts were a common occurrence four billion years ago, and could have played an important role in changing the composition of the ancient Earth’s crust, the authors conclude.