Tephrostratigraphy of Misti Volcano, Arequipa, Peru: A Chronicle of Multiple VEI 25 Eruptions from the Late Pleistocene to the Mid-15th Century

Misti volcano, Peru, has an extensive history of explosive eruptions since the late Pleistocene. The city of Arequipa (pop. ca. 1,200,000) is near the volcano, emphasizing the need to understand Mistis explosive behavior. Early mapping efforts by Peruvian and international groups resulted in identification of a handful of tephra-fall deposits, but only the Autopista and 2-ka deposit were thoroughly investigated. As an international partnership, we are carrying out an on-going, multi-year project to identify, characterize, map the distribution of, and date the tephra-fall deposits emplaced by Mistis explosive eruptions. In doing so, we are developing a comprehensive tephrostratigraphy for Misti. Thematic investigations of individual deposits physical volcanology and the entire suites petrologic details are occurring in parallel. Such investigations provide insights into the eruptions, their triggers, and Mistis pre-eruptive magmatic conditions and evolution. Through our efforts, we now identify and incorporate at least 24 distinct tephra-fall deposits within our tephrostratigraphy. Detailed distribution, thickness, and maximum grain-size data are documented for 17 units. Glass and phenocryst compositional data from each unit facilitates their correlation throughout the field area. We further recognize new key marker beds, such as the Zebra, Rosada, Sandwich Couplet, and Pampa de los Huesos. The newly documented Pampa de los Huesos and Apo deposits also have distribution axes to the east-southeast and possibly northeast, confirming for the first time the potential of tephra fall in these areas. Whole-rock and electron microprobe analyses reveal that most of the units are amphibole/pyroxene-bearing andesite. The four lowest units in our tephrostratigraphy, however, are biotite-bearing rhyolites/dacites. Published ages, >20 new 14C ages, and on-going 40Ar/39Ar dating suggest that all 24 units were emplaced since about 45 ka. We infer from deposit characteristics that many, including the Cogollo, Sacarosa, Autopista, Pampa de los Huesos, and Sandwich Inferior among others, were likely emplaced by VEI 5 eruptions with multiple VEI 24 eruptions represented by other deposits. Such a history of powerful explosive eruptions from a volcano near such a large population represents a clear hazard.