Large composite volcanoes can collapse into debris avalanches even if alteration is absent and an eruption is not in progress or imminent. At San Francisco Mountain, Arizona, a large valley that penetrates the core of the volcano is at the apex of a large fan of bouldery volcaniclastic deposits. A 115-m-thick layer sampled in a borehole that penetrates the fan is composed of dense and unaltered lithic clasts in proportions similar to the bulk composition of the andesitic volcano; pumiceous clasts are absent. Within this layer, a 20-m-thick deposit consists mostly of clasts from two dacite extrusions that predate the valley: a flank flow and an inferred summit dome. This dacite-rich deposit may be from a debris avalanche that originated by failure of the summit and northeast flank of the volcano in a repose interval during the last eruptive stage, which was about 0.43 to 0.22 Ma. Preconditions for slope failure include top loaded by massive lava flows and summit dome and cone inflated by intrusions. Only lower-flank and base extrusions followed the initial breach, which was enlarged by mass wasting, erosion, and glaciation.

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