Stress pinch points from glacial loading modulate magma ascent and storage in continental arcs

Growing evidence indicates that glacial cycles influence volcanic activity, yet the physical mechanisms linking glaciation to magmatic processes in continental arcs remain poorly understood. Here, we integrate realistic ice and topographic loads with a mechanical model of dike propagation to evaluate how glaciation modulated magma ascent and storage beneath Mocho-Choshuenco, Chile, a continental arc volcano impacted by the growth of the Patagonian Ice Sheet during the Last Glacial Maximum (LGM) 35-18 ka, followed by rapid ice loss between 18 and 16 ka. We find that during peak ice thickness, dikes ascending from lower crustal reservoirs and initiated below an ice load-induced stress “pinch point” stall several kilometers deeper than in ice-free conditions, effectively cutting off recharge to shallow reservoirs. This mechanism offers a straightforward explanation for the ~2-3 km increase in magma storage depths and the marked reduction in eruption rates observed during the LGM, without requiring changes in mantle melt supply or reservoir strength. By shutting off shallow recharge, glacial loading also favors prolonged magma differentiation, setting the stage for potentially explosive silicic eruptions once deglaciation again permits dikes to reach upper crustal levels. Our results thus identify a robust mechanism by which modest glacially driven stress changes regulate dike arrest depths, offering a unified framework to explain observed shifts in magma composition, storage depth, and eruption rate at Mocho-Choshuenco and potentially other arc volcanoes worldwide.