"During the summer solstice at dawn rose a column of sea and the waters raised high while the earth subsided." Thus in the first century BC the Roman historian Strabone mentions the event that should correspond according to the scholars, to a tsunami caused by an undersea eruption, one of the many geological changes of the Aeolian archipelago, known in geology as "Aeolian Arc."
It is a complex and articulated structure with a linear development of about 200 kilometres, which constitutes a superficial part (the islands) and a more complex structure(the submerged portion). Its formation is the result of a sequence of geological events that have involved the formation of Apennines and the Tyrrhenian Sea due to the phenomena of subduction (scroll of a lithospheric plaque below the edge of another active plaque above) of the oceanic crust, bringing to the approach of two areas of paleo-continent and African paleo-continent. The archipelago is made up of seven major islands: (Alicudi, Filicudi, Salina, Lipari, Vulcano, Panarea and Stromboli), and numerous submarines reliefs, some of which are of volcanic origin. The islands rise from depths of about 2000 metres and can reach a height of 1000 meters above sea level, as in the case of “Monte Fossa Felci” in Salina.
The first eruptions were both explosive and effusive and were caused by the release of an enormous amount of gas, but above all by the contact between the liquid hot magma (over 1,000 ° C) and sea water. The alternation of effusive and explosive eruptions had created the volcanic apparatus called "stratovolcano” consisting of dark lava flows and pyroclastic material produced by rocks launched into into the air by explosions.
Regarding the composition of the rocks of the archipelago, it can be distinguished into three classes:
A) lava flows
B) pyroclastic rocks
C) Steep dikes
The lavas consist of magma erupted to the surface of a crater and made cold quickly. They have a composition: basalt (dark, low percentage of silica, sodium and potassium and high percentage of iron, magnesium and calcium), andesitic (black or gray, rich in silica) and dacitic (similar to andesites and essentially glassy appearance).
The pyroclastic rocks are solid, launched in the air during explosive eruptions, that falling were scattered over vast areas forming an inconsistent rock composed with various elements chemically identical to lavas.
The dikes are magma cooled in deep through existing fractures in rocks, similar to lava, but with a different aspect from the surrounding rocks.