Abstract
Ash-rich eruptions represent a serious risk to the population living nearby as well as at thousands of kilometers from a volcano. Volcanic ash is the result of extensive magma fragmentation during an eruption, and it depends upon a combination of magma properties such as rheology, vesicularity and permeability, gas overpressure and the possible involvement of external fluids during magma ascent. The explosive process generates infrasonic waves which are directly linked to the outflow of the gas-particle mixture in the atmosphere. The higher the overpressure in the magma, the higher should be the exit velocity of the ejected material and the acoustic pressure related to this process. During violent eruptions, fragmentation becomes more efficient and is responsible for the extensive production of ash which is dispersed in the atmosphere. We show that the phase of intense ash emission that occurred during March 2016 at Copahue volcano (Argentina) generated a very low (0.1 Pa) infrasonic amplitude at 13 km, raising a number of questions concerning the links among acoustic pressure, gas overpressure and efficiency of magma fragmentation. Infrasound and direct observations of the eruptive plume indicate that the large quantity of ash erupted at Copahue was ejected with a low exit velocity. Thus, it was associated with eruptive dynamics driven by a low magma overpressure. This is more evident when infrasonic activity at Copahue is compared to the moderate explosive activity of Villarrica (Chile), recorded by the same array, at a distance of 193 km. Our data suggest a process of rigid fragmentation under a low magma overpressure which was nearly completely dissipated during the passage of the erupting mixture through the granular, ash-bearing crater infilling. We conclude that ash released into the atmosphere during low-energy fragmentation dynamics can be difficult to monitor, with direct consequences for the assessment of the related hazard and management of eruptive crises.
Highlights
Scientific attention regarding eruptions dominated by ash emission has been increasing since the paradigmatic 2010 Eyjafjallajokull eruption in Iceland (Dellino et al, 2012; Gudmundsson et al, 2012; Cioni et al, 2014)
Eruptions from Copahue volcano are generally a source of a large quantity of ash, so that understanding the processes of magma fragmentation, eruption, atmospheric injection and dispersal assumes a fundamental role in the assessment of the volcanic hazards posed to the population living around the volcano or to the air traffic
In spite of the large ash plume dispersed in the atmosphere, infrasound signals recorded during the explosive phases of March 2016 appear to be very weak, likely indicating low overpressures involved in the eruptive process
Summary
Scientific attention regarding eruptions dominated by ash emission has been increasing since the paradigmatic 2010 Eyjafjallajokull eruption in Iceland (Dellino et al, 2012; Gudmundsson et al, 2012; Cioni et al, 2014). On March 21, 2016 at 6:09 UTC, array processing reveals coherent small acoustic signals with amplitude between 0.05 and 0.2 Pa propagating with a back-azimuth of ∼266°N, corresponding to the direction of the active crater (Figure 5) These coherent acoustic signals were characterized by impulsive transients with a mean duration of 1.5 s and frequency of 1.12 Hz, typical of moderate explosive activity, or overpressurized magma degassing (Figure 4). Infrasonic activity with a back-azimuth of ∼200°N points to the position of Villarrica volcano, located 193 km from CPH array (Figure 8) and showing a frequency content ∼0.8 Hz (Figure 7) which falls into the infrasound frequency range of 0.5–2 Hz previously recorded at Villarrica (Ripepe et al, 2010; Goto and Johnson 2011) These detections coincide with moderate explosive activity occurring on april 3, characterized by an ash column ∼300 m high and thermal anomalies detected by satellite (OVDASSERNAGEOMIN, 2016b), abril vol 04).
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