Effusive-explosive transitions are observed in volcanoes erupting water-rich magmas that lack mineralogical evidence of degassing, suggesting that gas loss occurred rapidly and ascent was fast despite the high viscosity of rhyodacitic to rhyolitic melts. Here, we show that pre-eruptive heating of water-rich, silicic magma by mixing with hot recharge in subvolcanic chambers controls the effusive-explosive transition. Volcán Quizapu (Chile) emitted a 5 km3 mingled dacite-andesite lava flow in A.D. 1846–1847 that was ∼130 °C hotter than the 5 km3 of the same dacite that lead to a Plinian eruption in A.D. 1932. We explain the suppression of explosive fragmentation in the 1846–1847 lavas by enhanced syneruptive magma degassing as a consequence of late reheating following mixing with andesite recharge. Higher magma temperatures led to the transitioning into effusive eruptive behavior by significantly accelerating volatile diffusion and lowering melt viscosity, which facilitated bubble nucleation, growth, and coalescence, while also inhibiting brittle fragmentation. Thus, recharge by mafic magmas in subvolcanic magma chambers may reduce the risk of explosive eruptions in cases where heat can be efficiently transferred to cold, volatile-rich silicic magmas.