Understanding the SO2 Degassing Budget of Mt Etna's Paroxysms: First Clues From the December 2015 Sequence

Abstract

The persistent open-vent activity of basaltic volcanoes is periodically interrupted by spectacular but hazardous paroxysmal explosions. The rapid transition from quiescence to explosive eruption poses a significant challenge for volcanic hazard assessment and mitigation, and improving our understanding of the processes that trigger these paroxysmal events is critical. Although magmatic gas is unquestionably the driver, direct measurements of a paroxysm's gas flux budget have remained challenging, to date. A particularly violent paroxysmal sequence took place on Etna on December 2015, intermittently involving all summit craters, especially the Voragine (VOR) that had previously displayed no activity for several years. Here, we characterize the volcano's SO 2 degassing budget prior to, during and after this paroxysmal sequence, using ground-based (UV-Camera) and satellite (OMI) observations, complemented with ground-and space-borne thermal measurements. We make use of the high spatial resolution of UV-cameras to resolve SO 2 emissions from the erupting VOR crater for the first time, and to characterize temporal switches in degassing activity from VOR to the nearby New Southeast Crater (NSEC). Our data show that onset of paroxysmal activity on December 3-5 was marked by visible escalation in VOR SO 2 fluxes (4,700-8,900 tons/day), in satellite-derived thermal emissions (2,000 MW vs. ∼2-11 MW in July-November 2015), and in OMI-derived daily SO 2 masses (5.4 ± 0.7 to 10.0 ± 1.3 kilotonnes, kt; 0.5 kt was the average in the pre-eruptive period). Switch in volcanic activity from VOR to NSEC on December 6 was detected by increasing SO 2 fluxes at the NSEC crater, and by decaying SO 2 emissions at VOR, until activity termination on December 19. Taken together, our observations infer the total degassed SO 2 mass for the entire VOR paroxysmal sequence at 21,000 ± 2,730 t, corresponding to complete degassing of ∼1.9 ± 0.3 Mm 3 of magma, or significantly less than the measured erupted magma volumes (5.1-12 Mm 3). From this mismatch we propose D'Aleo et al. SO 2 Degassing During Etna's Paroxysms that only a small fraction of the erupted magma was actually emplaced in the shallow plumbing system during (or shortly prior) the paroxysmal sequence. Rather, the majority of the erupted magma was likely stored conduit magma, having gone through extensive degassing for days to weeks prior to the paroxysm.

Changes in SO2 Flux Regime at Mt. Etna Captured by Automatically Processed Ultraviolet Camera Data

“…In particular, overflowing from the BN crater rim formed a large lava field that extended downslope up to 3 km. The cumulative volume of lava flows and pyroclastic fall deposits was preliminary estimated at 7 to 10 Mm 3 [56], which is similar to what erupted during the December 2015 paroxysmal sequence [30]. Lastly, the summit crater's area was affected by intense deformation with fracturing, subsidence, and a formation of a~1 km-long and nearly NS oriented graben-like structure (Figure 1b).…”

“…We reported below on the SO 2 data automatically acquired and processed during 2016, which is a period characterized by substantial temporal changes in activity styles, including a phase of reduced degassing in the aftermaths of the December 2015 eruption [30,67]. This was followed by gradual activity escalation culminating into the May 2016 eruptive phase, and in a new vent opening episode on the eastern VOR crater rim in August [68][69][70].…”

“…Reduced degassing emissions (<2000 t/d) were measured after the eruptive phase, from May 26 th until the end of June (~1 month), which they could interpret as the aftermath of voluminous gas/magma release during the previous December 2015 [30,39,67,72] and May 2016 eruptive sequences. Field observations indicated that the summit craters were weakly fuming and occluded by lavas and pyroclasts.…”

“…While satellites become invaluable during paroxysmal explosive eruptions, when measurements from ground are complicated by volcanic ash within the plume. UV-camera measurements are more effective in monitoring more sluggish quiescent emissions in the low troposphere, and perhaps more useful to capture the early phases of unrest with escalating degassing activity [30].…”

Volcanic Hot-Spot Detection Using SENTINEL-2: A Comparison with MODIS–MIROVA Thermal Data Series

“…This role arises from several pioneering studies demonstrating how volcanogenic heat, measured in the InfraRed wavelengths (IR; 0.7-20 µm), is strictly related to the activity of volcano itself (i.e., [1][2][3][4]). In the last decades, space-based thermal data have been used to investigate a wide spectrum of volcanic phenomena, in particular lava-flow-forming eruptions [5,6], lava lakes' dynamic [7,8], extrusion of lava domes [9,10], mechanisms driving effusive dynamics [11,12] and magma budgets [13,14], as well as to track high-temperature fumaroles [15][16][17]. Nowadays, thermal signals from volcanoes can be investigated by using new sensors and processing algorithms, and such advancements will continue to make thermal remote sensing an expanding field, whose techniques represent a safe and low-cost tool for improving volcano comprehension [18].…”