Processes culminating in the 2015 phreatic explosion at Lascar volcano, Chile, monitored 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 October 30, 2015, which was thoroughly monitored by cameras, a seismic network, and gas (SO2 and CO2) 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 augmented level of activity at the volcano. Additionally, SO2 flux and thermal anomalies were detected before the eruption. Then, our weather station reported a precipitation event, followed by changes in the brightness of the permanent volcanic plume and (10 days later) by the sudden volcanic explosion. 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-grey eruption column in camera images, and (3) a pronounced spike in sulfur dioxide (SO2) emission rates reaching 55 kg sec−1 during the main pulse of the eruption as measured by a mini-DOAS scanner. Continuous CO2 gas 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 carbon dioxide (CO2) mixing ratio and the gas outlet temperature; we believe that this change was associated with the prior precipitation event. An increased thermal anomaly inside the active crater observed through Sentinel-2 images and drone overflights performed after the steam-driven explosion revealed the presence of a fracture ~ 50 metres in diameter truncating the dome and located deep inside the active crater, which coincides well with the location of the thermal anomaly. Altogether, these observations lead us to infer that a lava dome was present and subjected to cooling and inhibited degassing. We conjecture that a precipitation event led to the short-term build-up of pressure inside the shallow dome that eventually triggered a vent-clearing phreatic explosion. This study shows the chronology of events culminating in a steam-driven explosion but also demonstrates that phreatic explosions are difficult to forecast, even if the volcano is thoroughly monitored; these findings also emphasize why ascending to the summits of Lascar and similar volcanoes is hazardous, particularly after considerable rainfall.

A quest for unrest in multiparameter observations at Whakaari/White Island volcano, New Zealand 2007–2018

“…Following the pioneering reviews of Barberi et al (1992) and Browne and Lawless (2001), Stix and de Moor (2018) recently compiled and examined existing data associated with various phreatic eruptions and highlighted two endmembers: eruptions fed by deep hydrothermal systems, where magmatic gases are sealed creating overpressure (type 1) and open-vent shallow degassing systems, where vaporized liquid water drive eruptions (type 2), with both systems potentially undergoing sealing at various timescales and both type of eruptions being possible at the same system (e.g., Poás volcano). Phreatic eruptions can be triggered by magma intrusions (Nakamichi et al 2009), injection of gas (Christenson et al 2017) or infiltration of meteoric water (Gaete et al 2019). Although not restricted to these processes, hydrothermal sealing and failure has recently been recognized as pivotal in driving phreatic eruptions as it can lead to decreased vent porosity and permeability, and pressurization followed by sudden decompression (Christenson et al 2010;Mayer et al 2015;Rodgers et al 2015).…”

“…Yet Kawah Ijen and Whakaari/ White Island DSAR patterns suggest sealing over timescales of weeks to months, which may relate to rapid mineral precipitation (sulphur, sulphates, silica) or sufficient pore space to accommodate precipitation (Christenson et al 2017). A future comprehensive study should be performed, since sealing is largely influenced by pH, temperature, redox conditions, pressure, pore structure, and crystal and glass content of original lithology (Christenson et al 2010;de Moor et al 2019;Gaete et al 2019;Mordensky et al 2018;Roman et al 2019;Stix and Moor 2018). Little is known about the timescales of these processes that may vary from decades down to days (Stix and de Moor 2018).…”