Subduction Zone

Oceanic plates older than ~10 Myr are denser than mantle and subduct. As the slab descends, increasing pressure triggers dehydration reactions that release water into the overlying mantle wedge — lowering its solidus and generating arc magmas ~100 km above the slab. The Wadati-Benioff zone delineates intraslab seismicity from 0–700 km depth. The 660 km transition zone may stagnate or penetrate slabs. Back-arc basins form by extensional spreading behind the arc.