Processes culminating in the 2015 phreatic explosion at Lascar volcano, Chile, evidenced by multiparametric data

Abstract

Abstract. Small steam-driven volcanic explosions are common at volcanoes worldwide but are rarely documented or monitored; therefore, these events still put residents and tourists at risk every year. Steam-driven explosions also occur frequently (once every 2–5 years on average) at Lascar volcano, Chile, where they are often spontaneous and lack any identifiable precursor activity. Here, for the first time at Lascar, we describe the processes culminating in such a sudden volcanic explosion that occurred on 30 October 2015, which was thoroughly monitored by cameras, a seismic network, and gas and temperature sensors. Prior to the eruption, we retrospectively identified unrest manifesting as a gradual increase in the number of long-period (LP) seismic events in 2014, indicating an enhanced level of activity at the volcano. Additionally, sulfur dioxide (SO2) flux and thermal anomalies were detected before the eruption. Then, our weather station reported a precipitation event, followed by an increase in steaming and a sudden volcanic explosion at Lascar. The multidisciplinary data exhibited short-term variations associated with the explosion, including (1) an abrupt eruption onset that was seismically identified in the 1–10 Hz frequency band, (2) the detection of a 1.7 km high white-gray eruption column in camera images, and (3) a pronounced spike in SO2 emission rates reaching 55 kg s−1 during the main pulse of the eruption as measured by a mini-differential optical absorption spectroscopy (DOAS) scanner. Continuous carbon dioxide (CO2) and temperature measurements conducted at a fumarole on the southern rim of the Lascar crater revealed a pronounced change in the trend of the relationship between the CO2 mixing ratio and the gas outlet temperature; we speculate that this change was associated with the prior precipitation event. An increased thermal anomaly inside the active crater as observed in Sentinel-2 images and drone overflights performed after the steam-driven explosion revealed the presence of a ∼50 m long fracture truncating the floor of the active crater, which coincides well with the location of the thermal anomaly. This study presents the chronology of events culminating in a steam-driven explosion but also demonstrates that phreatic explosions are difficult to predict, even if the volcano is thoroughly monitored; these findings emphasize why ascending to the summits of Lascar and similar volcanoes is hazardous, particularly after considerable precipitation.

Reactivating and Calming Volcanoes: The 2015 MW 8.3 Illapel Megathrust Strike

“…Another volcano that had a period of unrest after the earthquake was Láscar, in North Chile. Phreatic explosions were registered in October, which have been linked to the occurrence of heavy rains and not the earthquake (Gaete et al., 2020).…”

Phreatic and Hydrothermal Eruptions: From Overlooked to Looking Over

“…2, Table 1). Aquifers can be "primed" for eruption by sealing via hydrothermal alteration and mineralisation, including build-up of pore/fracture-filling sulphates, clays, sulphur minerals and silica (Gurioli et al 2012;Sutawidjaja et al 2013;Heap et al 2019;Gaete et al 2020;Mick et al 2021). However, sealing can be localised or affect extensive areas in these diverse geological environments and occur progressively or suddenly, as well as can be contrasted by rapid permeability increase (e.g.…”

The Transition from MODIS to VIIRS for Global Volcano Thermal Monitoring

“…The last major eruption (VEI 4) occurred between January and August 1993 and was characterized by a series of phreatic to Plinian explosions that produced columns up to 25 km high and more than 0.1 km 3 of ash [62]. Since then, Láscar had experienced 10 minor eruptions (VEI 2-3), the most recent in 2013 and 2015 [60,63,64]. The activity of the last decade shows a thermal pattern defining three yearly-long cycles having a similar trend.…”

“…Because of its geographical conditions, the activity of Láscar volcano has been investigated mainly using satellite remote sensing data [9,10,63,[67][68][69][70]. According to these authors, the source of thermal emission is located mainly at the bottom of the Active Crater (800 m wide, 400 m deep) where extensive fumarolic areas (having average temperature estimated between 300 • C [59,71,72] and 600 • C [73]) have been observed, eventually associated to the presence of permeable andesitic lava dome(s).…”

“…Based on MIROVA VIIRS data analysis, we identify "Cycle 1" between 5 April 2013 and 29 October 2015, "Cycle 2" between 30 October 2015 and 22 November 2018, and "Cycle 3" between 23 November 2018 and 31 December 2020 (still ongoing). The highest VRP values were recorded during Cycle 2, after a phreatic eruption occurred on 30 October 2015 [63], and produced VRP values between 13 MW and 16 MW (see Figure 4 for details). Notably, the time gap between the thermal onset of MIROVA VIIRS and MIROVA MODIS (i.e., a sharp increase of VRP) characterizing each cycle was within 24 h, with a difference in the VRP value reached in the subsequent peak phases of less than 2 MW (Figure 4).…”