A model is proposed to understand the magma genesis beneath the volcanic front which overlies the dipping seismic zone with a constant depth of about 110 km in most subduction zones. It is suggested that the constant depth of 100 km is governed by the decomposition of amphibole in the mantle wedge. Hydrous phases in the subducted slab decompose at levels shallower than 100 km, that is, beneath the fore-arc region. The slab-derived H2O, enriched in incompatible elements with larger ionic radii, reacts with the overlying mantle wedge materials to form polluted amphibole peridotite. The polluted peridotite is transported downward on the slab by the induced convection in the mantle wedge. Amphibole in the dragged peridotite decomposes at a depth of about 110 km, just beneath the volcanic front. H2O released migrates upward to reform amphibole peridotite in the higher-temperature and lower-pressure region. When the front of amphibolization reaches a level at which the solidus temperature of amphibole peridotite is distributed, initial magmas are produced and rise as a form of mantle diapir. The mantle diapir stops rising to segregate a primary magma for lavas on the volcanic front.