1. Centre National de la Recherche Scientifique (CNRS), Institut National des Sciences de l'Univers (INSU), Université d'Orléans, Université François Rabelais–Tours, Institut des Sciences de la Terre d'Orléans, UMR 6113, Campus Géosciences, 1A Rue de la Férollerie, 45071 Orléans cedex 2, France
2. Department of Earth and Environmental Sciences, Ludwig-Maximillians Universitaet, 80333 Muenchen, Germany

Correspondence to: Jonathan M. Castro¹ Correspondence and requests for materials should be addressed to J.M.C. (Email: jonathan.castro@cnrs-orleans.fr).

Rhyolite magma has fuelled some of the Earth's largest explosive volcanic eruptions¹. Our understanding of these events is incomplete, however, owing to the previous lack of directly observed eruptions. Chaitén volcano, in Chile's northern Patagonia, erupted rhyolite magma unexpectedly and explosively on 1 May 2008 (ref. 2). Chaitén residents felt earthquakes about 24 hours before ash fell in their town and the eruption escalated into a Plinian column. Although such brief seismic forewarning of a major explosive basaltic eruption has been documented³, it is unprecedented for silicic magmas. As precursory volcanic unrest relates to magma migration from the storage region to the surface, the very short pre-eruptive warning at Chaitén probably reflects very rapid magma ascent through the sub-volcanic system. Here we present petrological and experimental data that indicate that the hydrous rhyolite magma at Chaitén ascended very rapidly, with velocities of the order of one metre per second. Such rapid ascent implies a transit time from storage depths greater than five kilometres to the near surface in about four hours. This result has implications for hazard mitigation because the rapidity of ascending rhyolite means that future eruptions may provide little warning.

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