Ars Technica: Subduction zones occur where oceanic crust is forced down under continental crust. As the subducted material is pushed toward the mantle, it melts into magma. The amount of water present in the different layers of the rock affects how quickly pockets of magma form. Rock that formed more recently is still warm, which means that it doesn't have to descend as far down into the mantle to start heating up and become dehydrated. Kristina Walowski of the University of Oregon and her colleagues, who have collected minerals from volcanos in the southern region of the Cascades, have confirmed that the subducted plate there is quite young. However, a simulation based on the isotopes in the minerals suggests that the water escaped the rock well before it reached the depth at which magma forms. That led Walowski's team to adjust the model to incorporate water trapped at the bottom layer of the crust. That reservoir of water leads to the formation of magma, and the ratios of other elements in the minerals in the Cascades support the idea that a young oceanic plate is being turned into magma there.
