planet

Blueschist

A metamorphic rock carrying the blue of the mineral glaucophane, formed at high pressure but low temperature — an odd combination, since rock buried deep normally gets hot. There is only one natural way to make it: take rock down faster than it can warm up, which means dragging it down with a subducting slab. So blueschist is an index. Its near-absence from the geological record before about 800 million years ago is one of the strongest arguments that modern-style plate tectonics began late.