Acasta river gneiss. Coloured scanning electron micrograph (SEM) of gneiss a highly metamorphosised rock (formed from other metamorphic rock) of granular texture with a banded appearance. It is abundant around the Acasta river of Canada, which gives it its name. It has been dated at 3.96 billion years, making it the oldest known rock on the planet. It was dated by analysing the uranium content of crystals within the gneiss. Magnification: x5000 at 5x7cm size.
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