ABSTRACT
In 1907 Mercalli defined the term “Vulcanian Eruption” to describe the eruptions that occurred on the island of Vulcano (Archipelago of Aeolian Island) between 1888 august 3 and 1890 march 19 . The main
characteristics of this type of eruption are: the pulsatory nature of the activity, the short duration (from seconds to minutes), of each individual burst, the emission of a great quantity of ash and lapilli, and the impulsive ejection of bombs and blocks to distances from the vent.
The aim of this work was to identify and document the products of these eruptions from physical and compositional point of view, to reconstruct the eruptive activity as it can be deduced from the sequence of deposits and make a comparison with the chronicles.
The pyroclastic fall deposits of 1888-1890 are stratified and are separated from the more ancient deposits by an erosive and /or reworked unconformity. After identification in the field, the deposits were measured and described in a several sites distributed around the Fossa cone; field mapping indicates that the main values occur SE of the crater in accordance with the prevailing tropospheric winds of the region.
The study of the eruptive sequence was carried out by digging a 250 cm deep trench, in a plane site 500 m SE of the crater and along the dispersion axis of fall products. The succession consists of parallel layers of lapilli and ash with thickness of 1 to 12 cm; the sequence is topped by a layer of scattered
bombs and blocks.
The deposits of the trench were described in detail and systematically sampled in 33 stratigraphic intervals for granulometric and component
analyses; the latter were performed in the size interval between -5 φ and 0 φ. About 100 clasts were collected to make measurement of total density, on 6 samples of coarse lapilli, spaced at regular distance along the stratigraphic
succession. 3 of these samples were also characterized with DRE and vescicularity measurements.
The results were integrated with the petrographic observation and with the bulk-rock chemical XRF analyses. (Each powder was obtained after grinding several clasts, of total weight of about 70/80g, of about 1 cm in diameter). In addition to the work on the trench macroscopic observations on the blocks and bombs bed and assessment of the numerical percentage of bombs and blocks were conducted on three equivalent and equidistant square .
The results yielded important and innovative information, the sedimentation of fall pyroclastic material was only with fall processes from eruptive
tropospheric convective columns, and only sporadically we have observed deposits related to horizontal transportation (base surge deposits).
The emission of bombs and blocks of great dimensions is characteristic of the final period of the eruption only. This is not obvious in the Mercalli’s work that on contrary gave us the impression that the ejected material is typical of the entire duration of the eruption.
The first ejected materials are characterized by a lower density value and by a higher vescicularity respect to the typical values of the last eruptive events, where blocks are ejected in greater number.
The chemical analysis shows that during the eruption the composition of juvenile material gradually varied, from latitic (59,7 % SiO2) to trachitic (64,8% SiO2) composition. The abundance of the ejected lithic materials suggests that the formation of crater is due to continuus escavation and in asstated by Mercalli, at the end of the eruption, the crater was full of
accumulated debris forming a gentle slope towards the crater rim.
