Title: Rapid ascent of rhyolitic magma at Chaitén volcano, Chile
Abstract: The Chaitén volcano in Chile erupted unexpectedly and explosively on 1 May 2008, and it is still erupting. The eruption has displaced over 5,000 people, and resulted in millions of dollars of lost revenue in Chile. It has also provided geophysicists the rare opportunity of directly observing a rhyolite magma fuelled eruption — the cause of some of Earth's largest explosive volcanic eruptions. Jonathan Castro and Donald Dingwell present petrological and experimental evidence to show that the hydrous rhyolite magma at Chaitén ascended very rapidly, with velocities of the order of a metre per second. Such rapid ascent, contrasting markedly with the behaviour of most silicic magmas, implies a transit time from storage depths greater than 5 km to the near surface of only 4 hours, leaving little warning time for such eruptions. This work suggests that rhyolitic volcanoes that have been active during the Holocene — the past 10,000 years or so — should be closely monitored, especially those near major centres of population. Although rhyolite magma has fuelled some of the Earth's largest explosive volcanic eruptions, our understanding of these events is incomplete due to the previous lack of direct observation of these eruptions. On 1 May 2008, Chaitén volcano in Chile erupted rhyolite magma unexpectedly and explosively. Here, petrological and experimental data are presented that indicate that the hydrous rhyolite magma at Chaitén ascended very rapidly from storage depth to near-surface, with velocities of the order of one metre per second. 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.