Vent temperature trends at the Vulcano Fossa fumarole field: the role of permeability

Abstract

International audienceBetween 1994 and 2010, we completed 16 thermal surveys of Vulcano's Fossa fumarole field (Aeolian Islands, Italy). In each survey, between 400 and 1,200 vent temperatures were collected using a thermal infrared thermometer from distances of ∼1 m. The results show a general decrease in average vent temperature during 1994-2003, with the average for the entire field falling from ∼220°C in 1994 to ∼150°C by 2003. However, between 2004 and 2010, we witnessed heating, with the average increasing to ∼190°C by 2010. Alongside these annual-scale field-wide trends, we record a spatial re-organisation of the fumarole field, characterised by shut down of vent zones towards the crater floor, matched by rejuvenation of zones located towards the crater rim. Heating may be expected to be associated with deflation because increased amounts of vaporisation will remove volume from the hydrothermal system Gambino and Guglielmino (J Geophys Res 113:B07402, 2008). However, over the 2004-2010 heating period, no ground deformation was observed. Instead, the number of seismic events increased from a typical rate of 37 events per month during 1994-2000 to 195 events per month during 2004-2010. As part of this increase, we noticed a much greater number of high-frequency events associated with rock fracturing. We thus suggest that the heating event of 2004-2010 was the result of changed permeability conditions, rather than change in the heat supply from the deeper magmatic source. Within this scenario, cooling causes shut down of lower sectors and re-establishment of pathways located towards the crater rim, causing fracturing, increased seismicity and heat flow in these regions. This is consistent with the zone of rejuvenation (which lies towards and at the rim) being the most favourable location for fracturing given the stress field of the Fossa cone Schöpa et al. (J Volcanol Geotherm Res 203:133-145, 2011); it is also the most established zone, having been active at least since the early twentieth century. Our data show the value of deploying multi-disciplinary geophysical campaigns at degassing (fumarolic) hydrothermal systems. This allows more complete and constrained understanding of the true heat loss dynamics of the system. In the case study presented here, it allows us to distinguish true heating from apparent heating phases. While the former are triggered from the bottom-up, i.e. they are driven by increases in heat supply from the magmatic source, the latter are triggered from the top-down, i.e. by changing permeability conditions in the uppermost portion of the system to allow more efficient heat flow over zones predisposed to fracturing

Remote Sensing and Geodetic Measurements for Volcanic Slope Monitoring: Surface Variations Measured at Northern Flank of La Fossa Cone (Vulcano Island, Italy)

“…The results show different trends regarding both vertical and horizontal displacements of points distributed along the rim of the order of some centimeters, thus revealing a complex kinematic, in agreement with the results of the EDM time series [36]. These time series, related to the 1994-1998 time span, show a progressive lengthening of the VVUL-V3 baseline (VVUL is a benchmark in the N part of Vulcano Island and V3 is placed on the La Fossa crater rim; see Figure 1).…”

“…Results are shown in Figure 4, where a potential instability of the studied area appears, as highlighted by the EDM time series studied by [36]. Significant ground deformation affects mainly the upper northern crater rim.…”

“…More recent studies [35,36] pointed out critical zones of La Fossa crater rim characterized by potential landslide hazard, since they are located above an unstable portion of the northern volcano flank (Figure 1). The northern section of the crater is just above Vulcano's harbor and main village; an accurate check of surface variation, therefore, is required.…”

“…A further geodetic network, again consisting in poles driven into the ground, lies along the northern rim of the La Fossa crater and is aimed to monitor the stability of this slope. Like the Lipari-Vulcano network, it was measured by EDM techniques from the northern promontory of the island [35,36] and is currently used for EGPS measurements. In particular, the last geodetic measurements over this portion of La Fossa crater rim were performed in 2010 (13 points) and 2012 (10 points) by means of semi-kinematic measurements after a 1 hour static initialization at the westernmost benchmark (Figures 1 and 4).…”

Combining Ground‐ and ASTER‐Based Thermal Measurements to Constrain Fumarole Field Heat Budgets: The Case of Vulcano Fossa 2000–2019

“…This, coupled with an increasingly magmatic isotopic signature (Capasso et al, 1997), was defined as a magmatic "crisis" (Granieri et al, 2006). Since 2000, however, maximum temperatures have declined to typical values of around 360-430°C (Harris et al, 2012).…”