Description/Abstract There is a consensus that volcanism in the Andean back-arc has largely been controlled by time-varying angle of subduction. Recent long period magnetotelluric (MT) studies have revealed electrically conducting plumes rising from near the top of the Mantle Transition Zone (MTZ) at 400 km depth. These plumes lead to a different explanation for back-arc volcanism that is related to instability of the MTZ and not subduction angle. This type of volcanism probably occurs worldwide and represents a new paradigm for back-arc volcanism.