planet

Eclogite

An ultra-high-pressure metamorphic rock — red garnet set in green pyroxene — formed when basalt is compressed at depths of tens of kilometres. It matters for two reasons. First, it is markedly denser than its parent rock, by some 400 kilograms per cubic metre, so the very reaction that makes it adds weight to a sinking slab and drags it deeper. Second, it is one of the few rocks that can only form along the distinctive cool geotherm of a subduction zone; finding it is petrological proof that plate tectonics operated there.