Adventive hydrothermal circulation on Stromboli volcano (Aeolian Islands, Italy) revealed by geophysical and geochemical approaches: Implications for general fluid flow models on volcanoes

Abstract

On March 15th 2007 a paroxysmal explosion occurred at the Stromboli volcano. This event generated a large amount of products,mostly lithic blocks, someofwhich impacted the ground as far as down to 200 m a.s.l., about 1.5 kmfaraway fromthe active vents. Two days after the explosion, a newvapouremissionwas discovered on the north-eastern flank of the volcanic edifice, at 560 m a.s.l., just above the area called “Nel Cannestrà”. This new vapour emission was due to a block impact. In order to investigate the block impact area to understand the appearance of the vapour emission, we conducted on May 2008 a multidisciplinary study involving Electrical Resistivity Tomography (ERT), Ground Penetrating Radar (GPR), Self-Potential (SP), CO2 soil diffuse degassing and soil temperature surveys. This complementary data set revealed the presence of an anomalous conductive body, probably related to a shallow hydrothermal level, at about 10–15 m depth, more or less parallel to the topography. It is the first time that such a hydrothermal fluid flow,with a temperature close to thewater boiling point (76 °C) has been evidenced at Stromboli at this low elevation on the flank of the edifice. The ERT results suggest a possible link between (1) the main central hydrothermal system of Stromboli, located just above the plumbing system feeding the active vents, with a maximum of subsurface soil temperature close to 90 °C and limited by the NeoStromboli summit crater boundary and (2) the investigated area of Nel Cannestrà, at ~500 m a.s.l., a buried eruptive fissure active 9 ka ago. In parallel, SP and CO2 soil diffuse degassingmeasurements suggest in this sector at slightly lower elevation fromthe block impact crater a magmatic and hydrothermal fluid rising system along the N41° regional fault. A complementary ERT profile, on May 2009, carried out from the NeoStromboli crater boundary downto the block impact crater displayed a flank fluid flowapparently connected to a deeper system. The concept of shallow hydrothermal level have been compared to similar ERT results recently obtained onMount Etna and La Fossa cone of Vulcano. This information needs to be taken into account in general fluid flow models on volcanoes. In particular, peripheral thermal waters (as those bordering the northeastern coast of Stromboli) could be contaminated by hydrothermal and magmatic fluids coming from regional faults but also from the summit.