The immersed volcanoes
In the last few decades a systematic bathymetric survey has been done in the lower Tyrranean to study the geological-morphological arrangement of the submerged sections of the Aeolian volcanic range. From that study it has emerged that the Aeolian volcanic arc extends north completing an annular structure, along which at least seven other submerged ranges have been counted, called “seamount” to whom each one has been given a name. Departing from west of Alicudi turning clockwise one encounters: Eolo, Enarete, Sisifo, Marsili (the 2000 m giant still fully active), Plainuro, Alcione and Lametini (north east of Stromboli). None of these ranges is still immersed under the sea, even though some of the secondary volcanic centres, like the sandbanks off the coast of Filicudi, of Salina and of Panarea, have brought their summit to just a few metres from the sea surface.
VULCANO
Vulcano is the southern most island in the Aeolian archipelago, the third on the surface. As a result of the intense volcanic activity the island doesn’t seem to have been inhabited until recent times, which is proven by the lack of archaeological testimonials. Before the last eruption, 1888-1890, only a few farmers lived in the Piano and Gelso districts, but then, thanks to the state of quiescence of the crater “La Fossa”, the island became completely colonized. Currently the volcanic activity on the island is represented by only a few secondary manifestations due to gas rising both under water and underground. Sacred in ancient times to the god of the Vulcans, Efesto for the Greeks, Vulcano for he Romans, the island became known as Hierà (the Sacred one), or Thermessa (the hot one), but also Tèrasia (hot earth), and surely due to the constant volcanic activity that characterizes it since it’s formation about 120,000years ago. The current appearance of the island is extremely different from it’s origins, when the primordial volcano dominated it’s form. The geological reading of the island is made easier by the fact that there were only four main volcanic structures which made up it’s form,, and they didn’t overlap. They are easily identifiable, whole or partially destroyed, aligned from south to north. The primordial volcano, constituted by alternating lavic streams and piroclastic deposits (volcano-layer), was a perfect cone, with a diameter of a base of about 5 km and a height of about 1000 m . But it’s activity concluded with a paroxysm of exceptional violence which determined it’s collapse of the summit which was higher than the cone : 600 metres of mountain sank into the abyss forming a caldera (which means cauldron or kettle in Spanish) with a diameter of 2.5 km, the Caldera of Piano. As usual in these cases, the volcanic activity resumed in the caldera after a long silence. Various eruptive centres formed which filled the crater with all their volcanic waste, even though not totally. North-west of the primordial volcano, meanwhile, about 15.000 years ago, a complex ryolitic of Lentià was formed which became involved and semi-swallowed by the Caldera of the Fossa during a series of collapses which characterize the third period, concluding about 13.000 years ago. The fourth period began about 10.000 years ago with the resuming of volcanic activity in the centre of the Caldera of the Fossa The cone of the Fossa began to form, alternating with the pyroclastic products and lavic streams (trachytes and ryolitics). The cone reached a height of 391 metres and it’s activity continues up until today. The last eruption occurring in 1888-1890 was widely studied and described by Mercalli and Silvestri, who defined that type of eruption as “vulcaniano.” Ultimately in the fifth period another autonomous structure was formed north of the Island between Lipari and Vulcano: Vulcanello, the latest addition to the Aeolian volcanoes.
STROMBOLI
Stromboli has been known since ancient times for it’s persistent explosive activity which, in relation to it’s peculiar characteristics, has been defined as ”stromboliana” and in the volcanology field it is considered an important reference point. Its constant activity in moderate bursts, up to 100-200 metres in height, of gas and fragments of shimmering magma which consolidate during the ballistic trajectories in the air. The dynamic of such explosions is linked to the accumulation of gas in the superficial part of the column of magma present in the conduit: when the internal pressure of the gas, which accumulates in a superficial “bubble” expanding , overtakes the force of cohesion of the magma, the “bubble” of gas explodes giving origin to the burst of lavic shreds which usually fall into the funnel of the crater again. The rhythmic pattern to these bursts varies in order of ten minute periods, just like the number of active openings, according to the greater or lesser activity of the volcano. When this mechanism of natural release becomes blocked, paroxysmic events take place: resulting in strong explosions accompanied by sysmic quakes, with bursts at a distance of quite large lavic fragments and a dense cloud of fiery gas and ashes. The volcanic mass of Stromboli rises from the Tyrranean seabed placed approximately at 1.500-1.800 metres in depth and reaching a height of 924 metres it is the second highest peak of the Aeolian Islands.
The geological history of the subarea began 200.000 years ago with an eruptive centre of Strombolicchio, which is the only remainder of the island which no longer exists. In fact after it’s formation it was completely demolished by marine erosion, except for the “neck”, that is the lavic andestic-basaltic pylon solidifying on the inside of the conduit. After a long period of stasis , around 100.000 years ago, five kilometres south-west of Strombolicchio a new eruptive centre rose from the sea, around which the island of Stromboli was formed. The geological evolution of Stromboli is distinguished by volcanologists in at least five evolutionary stages which can be grouped in two main periods. During the first period, which concluded about 20.000 years ago, an island was formed, paleostromboli (ancient Stromboli), maybe a bit bigger and higher than the present one, of which today remains the south-east part which alone forms two thirds of the island. In the last period, with the eruptive centre moved further north , neostromboli (new Stromboli) was formed, which reconstructed in part that which had sunk in the north-west sector of the island and, thanks to the activity of the minor centres, the promontories of Saint Bartolo and Ginostra to the west were formed. But this first neostromboli underwent a serious diminution and all that slice including the sections between the Filo del Fuoco(Thread of Fire) and Fili di Baraona (Threads of Baraona) slid and sank up to 500 metres under the sea.That’s how the current Sciara del Fuoco(Slope of Fire), very appropriately named ,developed, down which roll masses of rock or the lava emerged from the current eruptive centres placed at 750 metres flows, on a crater terrace north of Vancori. For more than 6000 years these eruptive centres, with their basaltic products, continue to slowly fill the depression left by the last collapse.