Abstract

Between 38.5 ka cal BP and 32.4 ka cal BP, a dacitic Volcanic Explosivity Index 5 eruption at Misti volcano emplaced the Sacarosa tephra-fall deposit. Its biotite phenocrysts, fine grain size, scarce lithics, and abundant loose crystals characterize the deposit at locations sampled. The eruption’s ~ 800 °C magma rose rapidly from ~ 10 km depth, culminating in a Plinian eruption which reached a mass eruption rate of 7.7 × 106–4.1 × 107 kg/s and emplaced about 3 km3 of tephra within tens of hours. The unit comprises two layers of subequal thickness separated by a diffuse contact with the upper distinguished by being slightly coarser and less well sorted than the lower. The deposit’s coarser upper layer indicates either climactic conditions or a lesser degree of fragmentation during the latter half of the eruption. Strong winds distributed the deposit southwest of Misti, where it crops out over at least 800 km2 and drapes the present site of Arequipa with up to 100 cm of tephra. The Sacarosa deposit is the first among the Cayma stage deposits, a distinctive group of felsic, biotite-bearing units, to be carefully described and its eruption characterized. Several Cayma stage deposits were emplaced by voluminous explosive eruptions similar to the Sacarosa eruption, representing a ~ 8.9–15.5 ky interval of powerful eruptions. Such an explosive eruption today would threaten Arequipa’s over 1,100,000 residents, many of whom live within the Sacarosa deposit’s distribution.

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