Filippo Mure

Spectroscopic evidence for a lava fountain driven by previously accumulated magmatic gas

Lava fountains are spectacular continuous gas jets, propelling lava fragments to heights of several hundred metres, which occasionally occur during eruptions of low-viscosity magmas. Whether they are generated by the effervescent disruption of fast-rising bubbly melt or by the separate ascent of a bubble foam layer accumulated at depth still remains a matter of debate. No field measurement has yet allowed firm discrimination between these two models. A key insight into the origin of lava fountains may be gained by measuring the chemical composition of the driving gas phase. This composition should differ markedly depending on whether the magma degassing occurs before or during eruption. Here we report the analysis of magmatic gas during a powerful (250–600 m high) lava fountain, measured with Fourier transform infrared spectroscopy on Mount Etna, Sicily. The abundances of volcanic gas species, determined from absorption spectra of lava radiation, reveal a fountain gas having higher CO2/S and S/Cl ratios than other etnean emissions, and which cannot derive from syn-eruptive bulk degassing of Etna basalt. Instead, its composition suggests violent emptying of a gas bubble layer previously accumulated at about 1.5 km depth below the erupting crater.

Volcanic gas emissions from the summit craters and flanks of Mt. Etna, 1987-2000

In the last 13 years gas emissions from both the summit and the flanks of Mount Etna volcano have been monitored using remote sensing techniques (COSPEC, and FTIR since 2000) and on-site monitoring devices. The SO 2 flux variations (600 to 25,000 Mg/day) indicated: (i) low values coinciding with deep seismicity prior to eruptions or/and preceding increases in summit volcanic activity; (ii) increasing trends tracking the ascent of fresh magma within the shallow feeding system and whose rate seems proportional to the speed of magma rise; (iii) decreasing trends related to progressive degassing of magma batches; (iv) an imbalance between the amount of magma erupted and that which contributed the SO 2 emission (∼ 13 % of the degassing magma having been erupted during the studied period), implying that magma degassing is dominantly intrusive; (v) a seasonal component, probably due to variations in solar zenith angle, meteorological parameters and, possibly, tidal forces.FTIR monitoring allowed to recognize significant variations of SO 2 /HCl and SO 2 /HF ratios in the volcanic plume which, combined with COSPEC data, provided new insight into the dynamics of ascent and degassing of discrete magma bodies. Strong variations in CO 2 -rich soil degassing are interpreted as markers of gradual magma ascent from great depth (>10 km) to the upper (<5 km) feeding system of Mt. Etna. These changes appear to precede increases in SO 2 plume flux at the craters and, so, provide additional constraints upon the interpretation of COSPEC data and the modeling of magma rise at that volcano.

FTIR remote sensing of fractional magma degassing at Mount Etna, Sicily

Abstract The chemical composition of volcanic gas emissions from each of the four summit craters of Mount Etna was measured remotely in May 2001, using a Fourier transform infrared (FTIR) spectrometer operated on the upper flanks of the volcano. The results reveal constant HCl/HF ratio but distinct SO2/HCl and SO2/HF ratios in the emitted gases, which, in the light of melt inclusion data for Etna basalts, can be interpreted in terms of escape of gases from partially, and variably, degassed magma at different depths beneath the summit. Gases released from the three main summit craters (Bocca Nuova, Voragine, and Northeast) had an identical composition, controlled by bulk degassing of a single magma body that had previously lost c. 25% of its original sulphur. The similar gas composition at all three main craters suggests that these are connected to a central conduit system that branches at relatively shallow depth. Measurements of the bulk volcanic plume on the same day, c. 7 km downwind, show that degassing from these craters dominated the total gas output of the volcano, and that no significant chemical evolution occurred within the plume over a time-scale of c. 12 min. Weaker gas emissions from the Southeast crater were comparatively depleted in SO2 (SO2/HCl and SO2/HF ratios a factor of two lower), implying that this crater is fed either by a separate conduit or by a branch of the central conduit whose geometry favours solubility-controlled volatile fractionation. Still lower SO2/HCl and SO2/HF ratios measured for residual degassing of a lava flow erupted from the Southeast crater verify the lower solubility and earlier escape of sulphur compared to halogens at Etna. Fractional magma degassing is also implied by strong chemical contrasts between the bulk volcanic plume and fissure gas emissions measured during the July-August 2001 flank eruption. These results highlight the ability of FTIR spectrometry to detect fine spatial and temporal variations in magma degassing processes, and thereby constrain models of shallow plumbing systems.

Volcanic and Seismic Activity at Stromboli Preceding the 2002-2003 Flank Eruption

Regular surveys with a thermal camera from both ground- and helicopter-based surveys have been carried out on Stromboli since October 2001. This data set allowed us to detect morphological changes in Strombolis summit craters produced by major explosions and to track an increase in volcanic activity associated with a heightened magma level within the main conduit that preceded the 2002―2003 effusive eruption. Together with thermal measurements, geophysical surveys performed in May and September/October 2002 highlighted clear increases in the amplitude of very long period (VLP) events, consistent with the ascent of the magma column above the VLP source region. The increased magma level was probably induced by elevated pressure in the deep feeding system, controlled by regional tectonic stress. This, in turn, pressurized the uppermost part of the crater terrace, producing greater soil permeability and soil degassing. Eventually, the magma loading caused the NW flank of the summit craters to fracture, allowing lava to flood out at high effusion rates on 28 December 2002, starting an approximately 6-month-long effusive eruption.