Andesitic arcs are sited over convergent lithospheric plate junctions. Aqueous fluids driven off the downgoing slab are responsible for partial fusion of the warming subducted plate and/or the overlying mantle wedge, and the ascent of calc-alkaline melts. Various hydrous minerals have been proposed to be the source of this H2O. Experimental equilibrium studies show that, under subduction-zone geothermal gradients of 5–7 °C/km, clinoamphibole constitutes a major phase in deep-seated (>75 km) metabasalts; other hydrous minerals are absent or are of very minor abundance. Clinoamphiboles dehydrate at pressures of ∼2.2–2.4 GPa under equilibrium conditions (pressure overstepping is probable), so mafic blueschists and amphibolites expel H2O at magmagenic depths. Partly serpentinized mantle beneath the oceanic crust devolatilizes at comparable to slightly higher pressures. However, micas remain stable in pelitic and granitic gneisses to pressures far exceeding 4.0 GPa, so at subduction depths >100 km, micaceous lithologies characterizing the upper and middle sialic crust fail to evolve significant H2O. Deep underflow of hydrated oceanic lithosphere thus generates most of the volatile flux along and/or above a subduction zone prior to continental collision. As large masses of quartzofeldspathic material enter a suture zone, volatile evolution at deep levels nearly ceases. While small amounts of peraluminous, S-type anatectic melts may be produced, I-type calc-alkaline arcs—and the continents—owe their formation over geologic time to the sustained underflow of oceanic lithosphere.