Catastrophic collapse at stratovolcanoes induced by gradual volcano spreading

Unlike ordinary mountains, which are formed by slow uplift and erosion, volcanoes are constructed rapidly. As a consequence, many are liable to massive flank failures, leading to debris avalanches (for example, at Mount St Helens in 1980). Such failures occur worldwide about once every 25 years (ref. 1) and even small ones can present a major hazard-in particular if far-reaching tsunamis are generated, as at Mayu-yama in 1792 (ref. 2). Previous work has tended to emphasize differences in eruption style associated with flank failure(2), but here we focus on the fundamental structural causes of failure. Most volcanic failures are generated by magmatic intrusion and flank spreading(3). We present evidence, however, that Mombacho volcano in Nicaragua experienced a previously unrecognized type of failure, triggered by sub-volcanic basement spreading. Notably, collapses related to basement spreading do not require that the volcano be magmatically active, and thus flank failure may pose a significant risk even at inactive volcanoes, which are rarely monitored.