A model is proposed to explain the high heat flow and active volcanism above the descending plate. The slip zone between the descending plate and the overlying asthenosphere is assumed to be a low-viscosity and low-density fluid layer enriched with liquid. The flow rate and the layer thickness are estimated as functions of viscosity from simple model calculations of fluid motion and thermal and force balances. The flow tends to be upward when the viscosity is low. The amount of heat transported by the ascending flow is greater than that generated by viscous dissipation. Basalts and andesites in arc volcanism are considered to be derived from the ascending liquid fraction and remelting of the subducted oceanic crust, respectively. Seismic evidence supporting the existence of a low-viscosity layer between the sinking plate and the overlying mantle is discussed.