Characteristics of the beginning of the 2019 eruptive crisis at Ubinas volcano (Peru)

Ubinas volcano has produced moderate explosive eruptions during the last ~500 years. With 26 eruptive periods, this composite cone is the most active volcano in Peru. The 2006–2009 and 2013–2017 eruptions impacted people, agriculture, and livestock within 15 km of the vent. On 24 June 2019 a new eruptive cycle started with minor emissions of tephra and aerosols. Activity increased on July 19 with an explosion beginning at 2:30 AM (local time). At that time, seismicity also increased with a predominance of LP-type signals. Two clearly differentiated and wind-controlled volcanic plumes were observed. Initially, the plume reached 6500 m above the summit and the main dispersion axis was ESE, dispersing ash as far as 300 km away to the villages of Jesús de Machaca and Catacora, Bolivia, where ash fall disrupted people’s daily activities. While the first plume was still active, a 1200-m-high secondary plume developed and was dispersed to the SE, reaching more than 200 km away into the Tacna region (Peru). After July 19, the SO2 emission rate increased reaching 9600 TN/day on July 23. The tephra fall on July 19 and gas emissions forced Civil Defense authorities to subsequently evacuate residents living in the valleys around Ubinas within 15 km of the volcano. Just after the tephra fall on July 19, deposit thickness was measured along the secondary dispersion axis, where the nearest populated and most impacted areas are located. The accumulation of lapilli and ash during the July 19 eruption reached 7 mm in the village of Ubinas, 5 mm in Tonohaya, 4 mm in San Miguel, 3 mm in Escacha and Huatagua, 2 mm in Huarina and 1 mm in Matalaque, ~20 km away. Fine ash accumulation was also reported at the Quellaveco mine, 90 km to the SE. Samples collected at 3.2 and 6 km from the vent allowed three types of juvenile clasts to be differentiated; dark- and light-gray scoria and dense, dark-gray lithics. Some juvenile clasts have bands of dark and light material, suggesting a partial mixing (mingling) of compositionally different magmas, which might have triggered the eruption. Based on the variety of juvenile clasts and unprecedented SO2 emission rates compared to the two past eruptive periods, we expect stronger eruptive activity or at least a long-lasting eruptive cycle.