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Rapid ascent of rhyolitic magma at Chaitén volcano, Chile


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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Figure 1: Backscattered electron micrographs of Chaitén pumice pyroclasts.
Figure 2: H 2 O-saturated phase relations in the Chaitén rhyolite.
Figure 3: Montage of backscattered electron images collected on decompression experiments on the Chaitén pumice.

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We are grateful for funding from the Alexander von Humboldt Stiftung, the Smithsonian Institution, and ERC grant 202844. N. La Penna provided an eyewitness account of the eruption sequence and critical field assistance. We thank T. Fehr, S. Bernstein and A. Logan for their analytical support. M. Rutherford and H. Tuffen provided comments that greatly improved the manuscript.

Author Contributions J.M.C. collected samples, performed the experiments and analytical work, and co-wrote the paper. D.B.D. analysed data and co-wrote the paper.

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Correspondence to Jonathan M. Castro.

Supplementary information

Supplementary Information

This file contains Supplementary Figures 1 and 2 with Legends and Legends for Supplementary Tables 1-4. (PDF 1282 kb)

Supplementary Table 1

This table indicates the experimental conditions (T,P, duration), and results of phase equilibrium and decompression experiments conducted on the Chaitén pumice (see file s1). (XLS 41 kb)

Supplementary Table 2

This table provides the compositions of experimentally produced plagioclase and orthopyroxene microlites (see file s1). (XLS 19 kb)

Supplementary Table 3

This table provides the spectral information and water concentrations in Chaitén obsidians and plagioclase-hosted glass inclusions as determined by FTIR (see file s1). (XLS 36 kb)

Supplementary Table 4

This table provides the compositions of coexisting titanomagnetite and ilmenite grains in the Chaitén pumice (see file s1). (XLS 19 kb)

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Castro, J., Dingwell, D. Rapid ascent of rhyolitic magma at Chaitén volcano, Chile. Nature 461, 780–783 (2009).

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