Description: From Waythomas and Nye (2002): "The present caldera formed during a major flank collapse of Mount Spurr volcano that either caused or resulted from a major eruption in late Pleistocene or early Holocene time. A discontinuous mantle of pyroclastic debris composed of high-silica andesite rests directly on the debris-avalanche deposits on the proximal south flank of the volcano. The debris-avalanche and pyroclastic-flow deposits and the caldera structure itself are the primary evidence that the eruption and flank collapse were initiated by magmatic activity (fig. 4 [in original text])."
From Waythomas (2007): "Radiocarbon-dated volcanic mass-flow deposits on the southeast flank of Mount Spurr in south-central Alaska provide strong evidence for the timing of large-scale destruction of the south flank of the volcano by sector collapse at 4,769-4,610 yr B.P. The sector collapse created an avalanche caldera and produced an ~1-km3-volume clay-rich debris avalanche that flowed into the glacially scoured Chakachatna River valley, where it transformed into a lahar that extended an unknown distance beyond the debris avalanche. Hydrothermal alteration, an unbuttressed south flank of the volcano, and local structure have been identified as plausible factors contributing to the instability of the edifice. The sector collapse at Mount Spurr is one of the later known large-volume (>1 km3) flank failures recognized in the Aleutian Arc and one of the few known Alaskan examples of transformation of a debris avalanche into a lahar."
"Cryptodome intrusion, which is also a plausible mechanism for the sector collapse at Mount Spurr, could have initiated a deep-seated failure of the volcano (Siebert, 1984; Siebert and others, 1987; Donnadieu and others, 2001). At other volcanoes, deep-seated failures associated with cryptodome intrusion commonly have a substantial magmatic component, though apparently not at Mount Spurr because extensive pyroclastic flow and fall deposits associated with this event have not been recognized."
"Radiocarbon-dated wood from the clay-rich lahar deposit that evolved from the debris-avalanche deposit now provides direct evidence for the timing of flank collapse at Mount Spurr. Six samples of wood from the lahar deposit that were dated by conventional radiocarbon scintillation-counting techniques yielded raw 14C ages of 4,040+/-70, 4,060+/-60, 4,180+/-70, 4,210+/-50, and 4,230+/-50 yr B.P. (table 1 [in original text]). Using the radiocarbon calibration program of Stuiver and others (1998), calibrated 2 sigma age ranges for the wood samples were determined (table 1 [in original text]). If the anomalous age is excluded, the average age of the wood samples is 4,159+/-26 yr B.P., and the calibrated 2 sigma age range is 4,610-4,769 yr B.P., which constrains the timing of emplacement of the lahar, the associated debris avalanche, and sector collapse to this interval."