In contrast to earlier activity, stage III volcanism was in part explosive. Events of this period included: dome growthy, catastrophic edifice collapse and debris avalanche; a laterally directed blast and associated pyroclastic flows; and emplacement of small debris flows. The presence of non-vesiculated, prismatic, juvenile dome rock in late stage III volcaniclastic deposits suggests that this stage began with dome growth along the western flank of Cerro Quemado. An edifice-collapse event and debris avalanche either immediately following or concomitant with dome growth, however, destroyed the early stage III dome. Edifice collapse at Cerro Quemado was followed by a laterally directed blast and an associated pyroclastic flow that covered the LLano del Pinal valley and overwhelmed the east flank of Siete Orejas. Stage III ended with reneewed dome growth and emplacement of a small Peleéan dome in the upper-central part of the scarp.
The timing of edifice collapse is constrained by radiocarbon ages from charcoal admixed with associated lateral blast and pyroclastic flow deposits. Calibrated ages for five charcoal samples give a range of 797 to 1409 years B.P. Sample #2 consists of carbonized twigs and stems, presumably from a juvenile plant, and has a calibrated age of approximately 1150 yr B.P. This age is similar to that of sample #3 and is intermiediate to those of samples # 1, 4, and 5. A weighted average of all five samples yields a calibrated age of about 1150 years B.P. Apparently, both edifice collapse and subsequent pyroclastic activity occurred at about this time.
