Most explosive, silicic volcanoes spend thousands of years in repose between eruptive events. The timing of the switch from repose to eruption is key to interpreting monitoring signals and improving the safety of people living close to active volcanoes. We addressed this question using a novel technique based on lithium isotopic (δ7Li) and elemental concentration profiles within plagioclase crystals from the Mesa Falls Tuff of the Yellowstone volcanic system (Idaho and Wyoming, USA), constraining volatile degassing to occur on minimum time scales of tens of minutes prior to eruption. During this ephemeral time, Li abundances drop by a factor of four to 10 from crystal cores to rims, accompanied by an increase in δ7Li of as much as 10‰, reflecting diffusion-driven equilibration between plagioclase cores and outgassed, Li-poor melt. New times scales obtained in this study show the potential for rapid syneruptive changes in the volatile inventory of magmas.