Historical and morphological evidence for multi-stage growth of El Volcancito, Volcán de Colima

We present a multidisciplinary study of an important 1869 eruption of Volcán de Colima, Mexico. This eruption created a parasitic cone, known as El Volcancito, which has traditionally been attributed to a small flank eruption. However, new analysis of historical records suggests that the size, explosivity, and duration of this eruption have been seriously underestimated. While previous reports suggest that activity ceased after ~3 years, our evidence shows that the eruption was highly energetic and constituted multiple eruptive phases over an 8-year period. There was a substantial emission of volcanic material into the atmosphere and the event directly affected communities up to 150 km from the volcano. A new estimate of the volume of material emitted (0.8 km3) is almost four times higher than the previous estimate (0.21 km3), and represents the largest historical andesite lava deposit at Volcán de Colima. At least 10 large explosions with eruptive columns of >6 km occurred. The eruption differed significantly to activity from the central summit cone in terms of eruption dynamics, evolution of activity over time, and petrology; as such, it cannot be explained using the widely accepted eruptive scheme for this volcano. Based on seismic and petrological evidence, we suggest that the highly energetic behavior can be explained by changes in the local stress field following two large regional earthquakes (M > 8.0) and subsequent magma-groundwater interaction. This study challenges the common assumption that most monogenetic cone-building flank eruptions at andesitic volcanoes are low energy compared with edifice-building summit activity, which has important implications for risk analysis at similar volcanoes worldwide.