Subduction processes are central to plate tectonics and to crust-mantle recycling and differentiation. Here we present a study of lavas from the Tonga-Kermadec Island arc which places important constraints on the processes and rates involved. The mantle wedge overlying the subducting oceanic plate is dynamically coupled to the descending plate, but may convect more slowly than expected. Fluid and sediment fluxes from the ocean plate enrich the wedge but differ in their location, mechanisms and rates. After partial melting, magma extraction occurs rapidly via channelled flow through the wedge.