El Misti is a towering, snow-dusted Andean peak dominating the skyline of southern Peru. Its symmetrical cone is positioned immediately northeast of Arequipa, Peru’s second-most populous city. The city center is only about 17 kilometers from the summit, highlighting a significant geological risk. El Misti is an active volcano that poses a long-term threat to the nearly one million residents who live within its shadow.
Identifying El Misti’s Volcanic Type
El Misti is classified as a stratovolcano, also known as a composite cone volcano. This type is characterized by its steep-sided, conical shape and high elevation, rising to 5,822 meters above sea level. The symmetrical shape of El Misti is a strong visual indicator of this type, implying a history of explosive eruptions rather than gentle, effusive flows.
The structure is built up over thousands of years through alternating layers of solidified lava flows and fragmented rock, ash, and cinders. The material erupted from El Misti is primarily andesitic, a thick and viscous type of magma. This viscosity prevents easy flow, contributing to the explosive nature of its eruptions and the steepness of the volcano’s flanks.
Defining the Stratovolcano Structure
The formation of a stratovolcano begins with the accumulation of highly viscous, silica-rich magma beneath the surface. This magma resists flow, trapping gas within the chamber. When pressure exceeds the strength of the overlying rock, the resulting eruption is violent, ejecting a large volume of pyroclastic material like ash, pumice, and scoria.
The steep-sided profile results from the alternating deposition of slow-moving lava flows and layers of explosive debris. Lava flows solidify quickly before traveling far, while pyroclastic material forms loose, sloping layers. This layering distinguishes stratovolcanoes from shield volcanoes, which are built from low-viscosity, fluid basaltic lava. The steep slopes of El Misti, averaging between 25 and 35 degrees, are a consequence of this alternating construction process. The composite structure includes two nested summit craters, resulting from past explosive events and collapses at the peak.
The Subduction Zone Setting and Eruptive History
El Misti’s existence and explosive nature are linked to its location within the Central Andean Volcanic Zone. This volcanic arc results from the ongoing subduction of the oceanic Nazca Plate beneath the continental South American Plate. As the Nazca Plate plunges beneath the continent, it releases volatile compounds that lower the melting point of the mantle rock above it. This process generates the silica-rich, gas-charged magma characteristic of stratovolcanoes.
The volcano has a documented history of numerous explosive events over the last 50,000 years, including at least 20 significant tephra-fall deposits. A major eruption about 2,000 years ago reached a Volcanic Explosivity Index (VEI) of 4, demonstrating the volcano’s potential for large-scale activity. The last significant eruption occurred between 1440 and 1470 CE, during the Inca Empire, which involved vigorous tephra emission.
The primary hazards for Arequipa stem from pyroclastic flows, lahars, and heavy ashfall. Pyroclastic flows are fast-moving currents of hot gas and rock that have historically traveled up to 12 kilometers from the vent. Lahars are destructive mudflows created when volcanic debris mixes with water from snowmelt or heavy rain. Lahars are a concern during the rainy season between December and March and pose a threat to the city’s upper suburbs. Monitoring efforts are currently in place, detecting continuous low-level seismicity and fumarolic activity, which helps authorities assess the potential for renewed activity and associated risks.