Search

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 Stromboli's 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.

On 28 December 2002, an effusive flank eruption started at Stromboli volcano (Aeolian Islands, Italy). This lasted until 22 July 2003 and produced two lava flow fields that were emplaced onto the steep slopes of Sciara del Fuoco. The first flow field was fed by a vent that opened at 500 m elevation and was active between 30 December 2002 and 15 February 2003. The second was supplied by a vent at 670 m and was emplaced mainly between 15 February and 22 July 2003. Here we review the lava flow field emplacement based on daily thermal and visual surveys. The variable slopes on which the lava flowed yielded an uncommon flow field morphology. This resulted in a lava shield in the proximal area where flow stacking and inflation caused piling up of lava due to the relatively flat ground. The proximal area was characterized by a complex network of tumuli and associated tube-fed flows. The medial-distal lava flow field was emplaced on an extremely steep zone. This area showed persistent flow front crumbling, producing a debris field on which emplaced lava flows formed lava channels with excavated debris levees. This eruption provided an exceptional opportunity to examine the evolution of lava flow fields emplaced on steep slopes and proved the usefulness of thermal imagers for safe and efficient monitoring of the active lava flows. In addition, thermal monitoring allowed calculation of quantitative parameters, such as effusion rate, allowing constraint of the time varying nature of supply to this eruption.

Stromboli's 5 April 2003 explosion sent an ash plume to 4 km and blocks to 2 km, representing one of the most powerful events over the past 100 years. A thermal sensor 450 m east of the vent and a helicopter-flown thermal camera captured the event dynamics allowing detailed reconstruction. This review links previous studies providing a complete collation and clarification of the actual event chronology, while showing how relatively inexpensive thermal sensors can be used to provide great insight into processes that cannot be observed from locations outside the eruption cloud. The eruption progressed through four phases, comprised 29 discrete explosions, and lasted 373 s. The opening phase (phase 1) comprised ∼30 s of precursory ash emission, with stronger emission beginning after 17 s. This was abruptly terminated by the main blast of phase 2 that comprised emission of a rapidly expanding ash cloud followed, after 0.4 s, by a powerful jet with velocities of up to 320 m/s. A second explosive phase (phase 3) began 38 s later and involved ascent of a phoenix cloud and explosive emission above a lateral vent lasting 75 s. This was followed by a 175-s-long phase of weaker, pulsed emission. The eruption was terminated by a series of three explosions (phase 4) sending ash to ∼600 m at velocities of 27―45 m/s and lasting 87 s. Together, these results have shown that a low-energy opening phase was followed by the highest-energy phase. Each phase itself comprised groups of discrete explosions, with energy of the explosions diminishing during the two final phases.

In this paper we integrate stratigraphic and sedimentological analyses of the volcaniclastic deposits, emplaced during initial opening and later widening of the Valle del Bove depression, with the available stratigraphy of the inner walls, marine offshore data, structural data, and magnetic surveys to develop a comprehensive model for the opening of the Valle del Bove depression. The resulting model adds new insight into the triggering mechanisms of the flank collapse. Additionally, it suggests a three-stage evolution of the eastern flank of Etna. (1) About 10 Kyr ago, the extinct Ellittico volcano (80 to 15 Kyr) collapsed, forming the early Valle del Bove. The collapse produced an avalanche deposit that spread ESE and formed the base of the Milo Lahar and the Chiancone deposits. (2) The second stage involved instability-related minor collapses within the valley, causing southward and westward enlargement of the depression and the emplacement of the debris flow sequence that comprises the upper part of the Milo Lahar deposit. (3) Available debris that accumulated within the Valle del Bove from smaller subsequent collapses was deposited at the mouth of the Valle del Bove in the fluvial sequence that forms most of the exposed part of the Chiancone deposit. The emplacement of the whole volcaniclastic sequence occurred between 10 and 2 Kyr ago. Since then, the Valle del Bove has acted as a basin protecting the lower eastern flank of Etna from lava flows or inundations of volcaniclastic debris.