The June-July 2019 Stromboli Activity and the July 3 Paroxysm: Field Data, Effects on Inhabitants and Settlements, Lessons for the Scientific Community

On July 3 2019, at 14.46 UTC, a paroxysm occurred on Stromboli Island (Italy), involving the entire craters terrace where the volcano feeding system generates its persistent activity. In the last two weeks of June, instrumental and direct observations of strombolian sequences have also recorded a major eruption (June 25). Even after that, a sparkling ordinary activity together with lava fountaining occurred, culminating in what we can consider the highest energy eruption since 1930. This paroxysm produced ≈4 km-high plume together with a double eruptive clouds flowing on the SW Sciara del Fuoco slope and plunging in that stretch of coast, with a partial planning over the sea surface. The paroxysm was preceded by some "precursors": a) inflation at INGV-SVO station ≈8 min before the explosion; b) strong ground deformation signal (i.e. 27 µrad at ≈800 m from LGS-OHO tilt meter) and small lava overflows from all the vents a couple of minutes before the paroxysmal sequence. Their significance is potentially associated with the quick supply of gas-rich and poorly crystalline magma batch from deeper magma storage into the shallow system, usually feeding the ordinary strombolian activity. Collected rock samples and several instrumental and field-based observations support this idea. The main areas affected by the eruptive products were Ginostra village, the summit and W-SW sectors of the volcano. Eyewitnesses in Ginostra reported of 35-45 minutes-long tephra fallout, with a final muddy rain (probably due to magma/seawater interaction when the pyroclastic flows entered the sea). Large fires quickly developed in many parts of Stromboli, due to arrival of incandescent ballistic products, threatening villages very closely. The paroxysm changed the craters morphology and was followed by many weeks of variable strombolian activity including spattering and discontinuously fed lava flows, mainly from SW crater, which affected the upper slope of the Sciara del Fuoco and generated glowing rocks rolling to the sea. Since the most recent studies have evidenced that both the energy released and the frequency of the explosions have a rather wide range of variability, this paroxysm highlights even more that hazards at Stromboli volcano are still not fully understood and eruptive dynamics should be monitored and studied even more thoroughly.