Slope failures resulting from structural instability of andesitic volcanic edifices can generate mobile debris avalanches that travel long distances down or beyond the flanks of volcanoes. More than 20 major slope failures have occurred worldwide over the past 500 years, a rate exceeding that of caldera collapse. Hazards derive from the debris avalanches themselves, from associated explosive activity that ranges from vertical eruptions (often accompanied by pyroclastic currents) to devastating directed blasts, from associated lahars, and from tsunamis. Collapses of growing lava domes are more frequent, are similar, in many ways, to edifice collapse, and can directly generate devastating pyroclastic currents.
This paper examines some aspects of current understanding of edifice and lavadome instability. The primary focus of the presentation is on mechanisms and factors associated with collapse, the geometric factors, augmented loading by magma, localized strength reduction by physical and chemical changes (the latter commonly associated with hydrothermal processes), strain weakening, pore-fluid (water or gas) pressure enhancement, retrogressive failure, time-dependent failure, and seismic shaking. Some aspects of material property evaluation, analysis procedures, and implications on monitoring are also discussed. Case examples discussed include edifice instability at Mt St Helens, USA, and Soufriere Hills volcano, Montserrat, the stability of lava spines at Mont Pelee, Martinique, and Lamington, Papua New Guinea, and lavadome stability at Soufriere Hills. The topics bear on understanding hazardous edifice and dome failures, and the measures to anticipate such failures.