First author

Chile's Chaitén volcano, in northern Patagonia, erupted suddenly on 1 May 2008. Soon after, scientists captured the first ever data from a volcano erupting rhyolite magma — the source of light-coloured igneous rock — which has fuelled some of Earth's most explosive eruptions. On page 780, Jonathan Castro at the French National Centre for Scientific Research in Orléans and Donald Dingwell of the Ludwig Maximillians University in Munich, Germany, detail the rapid ascent of rhyolite magma at Chaitén, which continues to erupt today. Castro tells Nature why rhyolite volcanoes merit monitoring.

Was an eruption expected at Chaitén?

No. It hadn't erupted in about 9,000 years, and was shrouded in dense forest. Most people considered it to be extinct.

How quickly did you get to the scene?

As soon as I saw pictures on the Internet, I thought the ash looked like a silica-rich magma, one that was charged with bubbles, and therefore water. Both of these attributes are consistent with a rhyolitic composition. The last rhyolite eruption occurred in Alaska in 1912. No active rhyolite system had previously been observed scientifically. I was in Chile by early June, and convinced the authorities to let me conduct scientific studies in real time, while the volcano was still erupting.

What did your analyses tell you?

We found that, prior to an eruption, rhyolite magma can rise at a surprising rate: 1 metre per second, which translates to very little pre-eruptive warning. Magma in most volcanoes in analogous terrain — such as Mount St Helens in Washington state — seems to move slowly before eruption, so these eruptions are typically preceded by weeks to months of seismic unrest. Our findings suggest that because magma moves quickly from where it has been stored, in some cases for hundreds or thousands of years, it doesn't have time to adjust chemically or physically to the new shallower environment. As a result, it generates powerful explosions due to its rapid decompression and the release of dissolved water.

Are other rhyolite volcanoes currently being monitored?

Only a small fraction of all active volcanoes on Earth are being monitored because of the costly equipment and human resources needed. I think this work underscores the importance of monitoring the calderas of Yellowstone in Wyoming and the Long Valley in eastern California, where many prehistoric rhyolite eruptions occurred. An eruption this close to San Francisco could be devastating.