A volcano reawakens after more than 100,000 years of "silent" magma reservoir growth

Magmatic systems can remain dormant for tens of thousands of years, creating a misleading perception of extinction that complicates hazard forecasting. To identify drivers of protracted quiescence, we integrate geochemical, isotopic, and zircon geochronological data comprising over 1250 crystallization ages from 31 eruptions at Methana, an active volcano near Athens, Greece. This record allows us to link eruptive activity, magma reservoir evolution, and mantle source variations over 700,000 years. Here, extended repose correlates with increased metasomatism of the mantle wedge by slab-derived components. The longest quiescence at Methana (>100,000 years) coincides with substantial magma production that was preferentially trapped in the crust. We attribute this trapping to the generation of superhydrous melts (>6 wt % H₂O) from a highly metasomatized mantle. These volatile-rich magmas undergo water saturation and crystallize during ascent, preventing eruption. Such trapping mechanisms can grow large magma reservoirs and may enable transitions from small stratovolcanoes to highly hazardous, caldera-forming systems.