Geophysical and geological observations document that beneath the submerged forearc, processes of sediment subduction and subduction erosion move large volumes of material toward the mantle. The conveying system is the subduction channel separating the upper plate from the underthrusting ocean plate. Globally, the zero-porosity or solid-volume rate at which continental debris is shuttled toward the mantle is estimated to be ∼2.5 km³/yr. To deliver this volume, the average thickness of the subduction channel is ∼1.0 km. Some deeply subducted material is returned to the surface of Earth as a component of arc magma or as tracks of high-...