Stratigraphy, sedimentology, and eruptive dynamics of the 2-ka eruption of Misti Volcano, southern Peru

Abstract

The 2-ka eruption of Misti volcano produced voluminous flowage deposits and a wide-spread tephra-fall deposit. The flowage deposits form large terraces within channels draining the south side of the volcano. All the channels drain into the city of Arequipa. Arequipa's city center is ~16 km from the summit of the volcano. The large proximal population and historical unrest make volcano hazards assessment critical at Misti. The 2-ka flowage deposits are previously identified as pyroclastic-flow deposits. Abundant sedimentological and textural evidence suggests that 0.04 km³ (~80%) of these deposits are lahar deposits. Pyroclastic flows melted ~0.06 km³ of snow and ice on the volcano triggering ≤0.02-km³ lahars that inundated the southern channels and some interfluves. The downstream evolution of the lahars is represented by four facies. Small, bulking debris flows from the upper flanks of the volcano emplaced the proximal facies. Several large debris flows both bulking and debulking resulted in the terrace facies. Deposition upstream progressively thinned and diluted the flows resulting in the medial facies. Debulking and dilution continued until the flows became hyperconcentrated flows and deposited the distal facies. The 2-ka eruption was a VEI 4 that produced a 1.4-km³ tephra-fall deposit and 0.01 km³ of pyroclastic-flow deposits in ~3–5 h. Pyroclastic flows descended the southern flanks of the volcano. Column heights ≤27 km and winds dispersed the tephra fall southwest, resulting in ~20 cm of tephra in Arequipa. Pyroclastic flows and tephra fall of the same magnitude as the 2-ka eruption could occur again. Few people live in the high pyroclastic-flow hazard area but a large population live within the low hazard zone. Significant tephra fall could occur in Arequipa and would severely affect the city. There is not enough water available under modern climate conditions to generate lahars as voluminous as the 2-ka lahars. Water available under modern conditions suggests that lahars with volumes ≤1x10⁵–3x10⁶ m³ are possible. Lahars ≤1x10⁷ m³ would be possible if the Rio Chili were dammed during an eruption. Lahar hazards zones evaluated on the basis of these volumes, suggest that the largest of these lahars could enter Arequipa.