Cartoon cross section of a subduction zone showing the principal mineralogy of the upper 800 km of Earth's mantle. Arc volcanism at the surface is depicted, as are upper mantle earthquakes (red dots and white diamonds) thought to be the result of dehydration of water-bearing minerals, principally serpentine, and transition zone earthquakes thought to be caused by progressive breakdown of metastable olivine as the down-going slab heats up.
