The last 1000 years of eruptive activity at the Fossa Cone (Island of Vulcano, Southern Italy)

The last 1000 years of eruptive activity of the Fossa cone have been characterised by a wide spectrum of eruptive styles, from mafic ash eruptions (violent Strombolian), to sub-Plianian, toveffussive, to steam-blast explosions and eruptions related to the ascent of viscous plug, asvVulcanian explosions. Tephra and lava sequences on the cone-flanks have covered almost completely the older products, forming an “onion-like” structures of this small composite cone. Given the nature of the eruptions of the Fossa cone, being mainly of small- to medium intensity, the importance of the ring plain, as a record of the chronostratigraphy of volcanic and sedimentary activity is crucial to a correct reconstruction of the evolution of the Fossa cone. In general, the first activities of the investigated period (XIII sec.), namely Palizzi eruption (Chapter 3), were characterised by frequent eruptions of small intensity (mafic ash eruptions, sensu Ono 1995) as reported by historical accounts (De Fiore 1922). These eruptions have been spaced by rest period, during which erosion occurred, producing a re-distribution of the volcanic material from the cone to the neighbouring areas. The mafic ash eruptions were punctuated by two more violent, sub-Plinian, eruptions affecting a wide area of the Island of Vulcano with a large volume (3.75x106-6.40x106 m3 and 3.65x106-3.67x106 m3 for the first and the second, respectively) of tephra fall deposits. After the second sub-Plinian eruption, a phase of effusive eruptions occurred, signing the end of this period characterised by magmatic activity only. The style of the eruptions drastically changed and a large, probably long-lasting, steam-blast eruption, called Commenda eruption (Chapter 4), occurred, ejecting ballistic blocks in a large range (at least 2.2 km from the summit crater area) and producing wide-spread PDCs (>4x105 m3) and non-juvenile ash emission (>1x107 m3 m3). The climactic phase of the eruption consumed a large quantity of energy (3,75X1012J), forming the current crater and ejecting lithic material with a overpressure of ~2 MPa. This eruption took place simultaneously with the Mt. Pilato-Roche Rosse rhyolitic eruption in the Island of Lipari, triggered by a mafic injection. The Commenda eruption was driven by the sudden uprising of a large amount of magmatic gas (2,8x106 m3), produced by a degassing shoshonitic melt (~10-1 km3). After a rest period (at least one century, based on the analysis of the historical chronicles), the activity renewed (Chapter 5) in the AD1444 with a second, high-energetic (1.25x1013 kJ) steamblast eruption (Forgia 1), that occurred eccentrically with respect to the summit crater area of the Fossa cone, in its northern flank and affecting an area currently occupied by the Vulcano Porto town. This eruption “opened” a new stage of the activity of the Fossa cone, characterised by the ascent of viscous plugs of magma that powered at least 8 discrete eruption lasted from days to years. These eruptions were mainly Vulcanian in styles, including the first described Vulcanian eruption (AD1888-1890), but frequent steam-blast explosions occurred, generally before the onset of the main magmatic phase. One effusive phase characterised the long-lasting, complex eruption that occurred on AD1731-39. The erupted volume ranged between 106 and 107 m3, affecting an wide area that comprised at least the northern Sicilian coast and probably reached the Mt. Etna (De Fiore et al 1922), more or less 70 km far from the island. Comparing the pre- and post-AD1888-90 topography of the summit crater area, a minimum value (16.5x106 m3) of deposited material was produced. This volume approximately corresponds the 88% of the associated tephra volume (18.5x106 m3 calculated by the methods of Bonadonna and Houghton 2005), confirming the evidence of high-deposited volume in the in the summit crater area during these stage of eruptions. During this eruptive crise, the summit crater area of the volcano growth at an average rate of 0,34 m3s-1 in a period of ~20 months. This value is something less but in the same order of magnitude of the rate of cone grew evaluated for the AD1986 eruption at Izu-Oshima (Sumner 1998) and the AD1954 eruption at Ngaurouhoe (Hobden 2002) volcanoes, while is one order of magnitude greater than the value calculated for the AD1783 eruption of the Asama volcano (Yasui and Koiaguchi 2004). The data on emitted tephra volumes during the investigated period suggested that the Fossa cone experienced eruptions ranging bewteen 2 and 3 in the VEI classification (Volcanic Explosivity Index, Newhall and Self 1982), without a clear evidence of waning. These data are useful for future “short-term” hazard evaluations for the Island of Vulcano, that need to consider an eruption of VEI 3 as the most likely scenario (Chapter 6). Diluted PDCs and ballistic showers have been considered so far as the most recurrent hazardous events of La Fossa volcano (e.g. Dellino et al. 2010). Nonetheless, this work suggests that widespread and long-lasting fallout events as well as lahars represent a frequent phenomenon and, therefore, fundamental processes to be considered for a comprehensive hazard assessment of the island of Vulcano.