planet

Subduction

The process by which one tectonic plate sinks beneath its neighbour and returns to the mantle. This is the machine's recycling step, and it is why nowhere on Earth is there ocean floor older than about 200 million years while the cores of continents reach nearly four billion. Subduction leaves marks that cannot be mistaken: a dipping sheet of earthquakes reaching 670 km down, a volcanic arc above it, and water carried down with the sinking slab — and it is that water which makes granite, and the planet's richest ore deposits.