Preliminary Tephrostratigraphy and Eruption History of Misti Volcano, Southern Peru, Since the Late Pleistocene

Since the late Pleistocene, numerous voluminous explosive eruptions at Misti have produced widespread tephra-fall deposits (TFD), many of which are present in the city of Arequipa, Peru. Arequipa has a population of about 1 million and development is rapidly encroaching into known high hazard zones. Pyroclastic density current (PDC) and lahar deposits from some of Misti's eruptions, including its most recent Plinian (VEI 5) eruption, ca. 2 ka, are present in now-populated areas, illustrating the hazard. The Autopista TFD (deposits herein are informally named) forms the base of our tephrostratigraphy. PDC deposits associated with the Autopista eruption are present at the foot of Misti's southwest flank. Two unnamed TFDs overlie the Autopista and underlie the Sandwich Couplet, which consists of two voluminous TFDs separated by a thin, reworked layer. The limited thickness of the intervening layer suggests that the two eruptions may have been separated by only a short time period. An unnamed TFD overlies the Sandwich Couplet and underlies the La Rosada TFD. The La Rosada has a high proportion of lithics and a pink color. Two unnamed TFDs overlie La Rosada, one of which has associated PDC deposits at the foot of Misti's southwest flank. The TFD from Misti's 2 ka eruption overlies all other deposits except a poorly preserved ash layer from a small-volume explosive eruption in the mid-15^th century. The poor preservation of such a youthful ash layer suggests that other such eruptions may have occurred since the Autopista was emplaced, but their deposits are cryptic. At least five of the TFDs we document, including the 2 ka, La Rosada, and both TFDs of the Sandwich Couplet contain compositionally banded pumice suggesting that magma mingling is a common process during Misti's eruptions. All TFDs except the 2 ka have defied dating attempts. We, nonetheless, infer that the Autopista was emplaced during or just after the Last Glacial Maximum (LGM; 18-14 ka) based on new 14C ages from paleosols in the pre-Autopista stratigraphy. The composite tephrostratigraphy suggests that at least 11 explosive eruptions have occurred at Misti since about the LGM and the next eruption of Misti could severely disrupt Arequipa.