Quiescently active volcanoes are enigmatic due to their restlessness but lack of eruptive activity. I present a model of coupled conduit convection and foam accumulation to explain degassing behavior of Masaya, an active volcano in Nicaragua that is currently emitting large amounts of gas but not erupting. Gas-rich magma is transported through a conduit 2–6 m in radius and then released into a shallow reservoir. The magma is degassed in the reservoir and forms a foam 1–3 m thick at the top with bubbles 20–60 µm in diameter. The foam layer is stable because the input of gas into the foam is balanced by gas release through a vent to the surface. If the foam layer is destabilized, the volcano can erupt explosively. The most likely cause of foam destabilization is a large injection of volatile-rich magma from deeper levels into the shallow conduit-reservoir system, thereby increasing magma fluxes and gas fluxes, exsolving large gas bubbles, and reducing surface tension of the magma.