The eruption of Chaitén volcano took everyone by surprise because it was judged to be dormant, but how well-founded was that judgement? In a timely post over at Highly Allocthonous, Chris Rowan takes a look at how volcanoes are classed active, dormant or extinct.

His key point is that such classifications are in the end only as good as the information on which they are based: without knowing how long ago a volcano last erupted it’s impossible to judge whether it is dead or merely sleeping, and even that information is only part of the story – you need to have some idea of the cyclical pattern of the volcano’s eruptive history over the long term. For Chaitén, as for many of the world’s volcanoes, such information simply isn’t available. Meanwhile, the authorities charged with volcano monitoring can’t keep a watch on everything and have to decide how best to use their limited resources. If a volcano is judged to be ‘dormant’ the seismometers and tiltmeters can be put to use elsewhere … but if that judgement is a negative one based on an absence of information, rather than a positive one based on sound knowledge of the volcano’s past behaviour, there’s always the potential for a nasty surprise.

Whatever the obscurities and difficulties of the issue itself, Chris Rowan’s explanation is a model of informative clarity. Highly recommended: ‘Active, dormant, and extinct volcanoes’.