If we were to draw a schematic cartoon diagram of a subduction zone, it would include a diving oceanic plate, represented by a uniform slab. As the plate dove deeper, water driven off by the increasing heat might be shown with a blue arrow. And, of course, that water will create some blobs of red magma in the mantle between the two tectonic plates, as adding water lowers the melting point of the rock.
But in reality, the subducting plate is not a uniform slab. An oceanic plate can be divided into a number of layers. On the top, there’s the ocean mud that slowly accumulated as the plate traveled from the mid-ocean ridge toward the subduction zone. Beneath that, you’ve got the basalt (and basalt’s larger-crystalled sibling, gabbro) that makes up the oceanic crust. At the bottom, there’s a layer of mantle rock that stuck to the plate as it slowly cooled over the course its long life beneath an ocean of water. Water is everywhere in this process; it soaks into the sediment, and fills cracks in the rock, and it also works its way into the minerals themselves, becoming a part of them chemically.
When an oceanic plate is subducting, the gradual warming as it sinks deeper into the hot Earth can drive off the water within the sediments and fractures, but the minerals can transform and give up their store of water as well.
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