The mobilization of large rock masses at the surface is felt beneath the volcano: at depth, unloading forces cause changes in magma reservoir pressure 10, its location and geometry 11, and are associated with compositional changes of the erupted products 12. The consequences of sector collapses are often dramatic and associated with catastrophic directed blasts and pyroclastic flows, devastation over distances as far as tens of kilometers, and potential for triggering far-reaching secondary hazards such as lahars and tsunamis, with an estimated 20,000 death toll caused by historic volcano flank collapses and associated hazards 9. The volumes mobilized by sector collapses vary considerably, from small scale 6 to large edifice collapses involving 10 8−10 11 m 3 7, with the larger collapses being rarer 8.
The collapses may occur slowly and creep-like 2 or fast and catastrophic 3, some with precursory eruptive activity 4 and others without any apparent change in behavior 5. Active volcanoes are subject to flank instability that may cause large-scale sector collapses 1.
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