Misti volcano in southern Peru has a record of explosive eruptions and a nearby population of over 810,000, making it a hazardous volcano. The city center of Arequipa, Peru's second most populous city, is 15 km from the summit of Misti, and many neighborhoods are closer. As the population increases yearly, the urban boundary continues to move up the south side of the volcano. Many parts of the city are built upon the deposits from Misti's most recent Plinian eruption at ca. 2 ka. The 2 ka Plinian eruption (Volcanic Explosivity Index [VEI] 5) produced a 1.4 km 3 tephra-fall deposit and 0.01 km 3 of pyroclastic-flow deposits in ~2–5 h. Column height varied during the eruption but ascended up to 29 km. Pyroclastic flows descended only the south side of the volcano. The tephra fall spread southwest, resulting in ~20 cm of tephra accumulation in the area now occupied by the city center. The flowage deposits were previously identified as pyroclastic-flow deposits, but new sedimentologic and textural evidence suggests that ~80% (by volume) of the deposits were emplaced wet and relatively cold. As such, they are lahar deposits. A Neoglacial advance concurrent with the eruption supports evidence for voluminous snow and ice on the edifice. Pyroclastic flows melted between 0.01 km 3 and 0.04 km 3 of ice and snow on the volcano, triggering lahars that descended the volcano and inundated channels and some interfluves on the south flank. The lahars evolved downstream from proximal debris flows to distal hyperconcentrated flows, emplacing ~0.04 km 3 of deposits. Four facies of lahar deposits are present in the channels and another facies occurs on the interfluves. Such a comprehensive understanding of the 2 ka eruption will help to inform future volcanic hazards assessments. Pyroclastic-flow and tephra-fall deposits of the same magnitude could occur again and are useful in hazards assessment. The 2 ka lahars required voluminous water, which is no longer available on the volcano, and, within modern climate conditions, these deposits are not representative of possible future events. Estimations of water available from modern rain and snow suggest that lahars with volumes between 1 × 10 5 m 3 and 3 × 10 6 m 3 are possible. Lahars are more likely if an eruption occurs during a period of high snow accumulation or during subsequent heavy rainfall. Lahars up to 1 × 10 7 m 3 are possible if the Río Chili is dammed during an eruption. Lahar hazard zones generated using these volumes suggest the largest lahars could enter Arequipa.

Studies of the type, extent, and volume of Holocene pyroclastic and lahar deposits have concluded that future eruptions of El Misti volcano, even if moderate in magnitude, will pose a serious threat to the city of Arequipa, Peru. After describing the most probable volcanic scenarios at El Misti, this paper concentrates on lahar and flood risk assessment. Scenarios were derived with the help of the simulation codes LAHARZ and TITAN2D. The lahar risk assessment varies significantly depending on the method selected. LAHARZ simulations indicate that a considerable part of the urban areas and infrastructure could be severely affected. Losses due to impacts inflicted by lahars in three selected parts of the urban area are estimated to be in the order of 40–100 million U.S. dollars. In the case of TITAN2D, the resulting lahar-affected area only includes infrastructure assets mainly located along the Río Chili. Results indicate that although simulation codes could be useful tools in the analysis of lahar hazard scenarios, it is still premature to regard them as accurate sources of information for actual decision making related to risk mitigation at the local level. More research is required to further adjust simulation codes and refine risk scenarios. The first priority for the mitigation of the volcanic hazard faced by the city of Arequipa should be improvement of the risk map (a hazard map has already been drawn and is under scrutiny) and the preparation of contingency plans.