Etna, the mythical Forge of the Gods, is a volcano with a rather complex composition. It originates from the overlap and juxtaposition of products from eruptions over different periods and through different systems of surfacing magma.
The thousands of lava flows, immense quantities of slag, sand, ash and tuff that have emerged as a result of the ceaseless volcanic activity of this extraordinary, natural thermodynamic machine have destroyed and, in some cases, sealed or simply covered the remains of various pre-existing eruptions in stratigraphic layers.
Etna cannot be considered a single volcanic edifice: at least two, Trifoglietto and Mongibello, have succeeded and covered one another over time.
Through the painstaking work of geological surveys and testing rock samples, and with information obtained from earthquakes and eruptions, geologists have been able to write the story of this polygenic volcanic complex, even if much remains to be written.
Geography e Geomorphology
Etna is a particular stratovolcano, around 3,340 m high and located in the central-northern part of Sicily’s eastern coast, in the district of Catania; it is crossed by the 15th meridian, known as the Etna Meridian (2.517.600 E 4.179.925 N Gauss Boaga East zone Datum Roma 40), and it covers a surface of more than 1,250 km2, and a perimeter of over 135 km.
It is bordered by the Peloritani mountains in the north, the Nebrodi in the north-west, and by Simeto and the flood plain (Piana di Catania) in the south and south-west.
Etna is a special “asthenospheric window”, in an area dominated by the tectonic processes of lithospheric convergence, which probably took place during the Mesozoic period, due to the effect of the different spreading velocities along the mid-Atlantic ridge.
The higher spreading velocity along the southern segment of the ridge, compared to the speed of the northern segment caused the African plate to accelerate compared to the Eurasian plate, causing Africa to rotate counterclockwise and squeeze up against Eurasia.
This collision developed through a series of events that occurred in different times (diachronic). The evolution of the convergence between the African and Eurasian plates has involved various collision stages, causing extended subduction and distension in the oceanic crust (Tethys), leading to intense disarticulation and fragmentation in the original lithospheric structure, creating a mosaic of more or less stable micro plates, moving against one another, as well as with regard to the more extended African and Eurasian plates.
In terms of complexity, the evolution of volcanic activity as tracking the geo-dynamic processes in the Mediterranean basin reflects the more general complexity that characterises the neo-tectonic system in the area.
The volcanic events that have occurred within the Mediterranean basin are mainly represented by silica-rich magma caused by lithospheric convergence processes. The tectonic distensions that have developed locally have led to the opening of deep cracks in the crust, allowing asthenosphere to rise, with the result of the emplacement of anorogenici basaltic magma from the upper mantle.
One of these areas is represented by the eastern margin of Sicily, where intense and continued eruption events of a basic nature have occurred since the end of the Miocene, around 10 million years ago. These events also involved a strip extending inland by some 30-40 km from the Ionian coast and moving further north, they also reached the area of Etna. The lithospheric discontinuity in a NNW-SSE direction along which the islands of Vulcano, Lipari and Salina are located, extends southwards, crossing the north-east of Sicily, from Capo Tindari to Giardini on the Ionic coast.
This structural discontinuity, known as the “Tindari-Letojanni Line”, together with the system of lithospheric faults bordering southern Sicily on the east and which is usually known as the “Ibleo-Maltese escarpment”, is the location of the most frequent and often disastrous earthquakes that have struck eastern Sicily. Etna’s volcanic activity is strongly conditioned by the tectonic system of the region.
Hundreds of cones and secondary systems, sand, gravel and volcanic slag, sometimes of imposing size, isolated or aligned along eruptive rifts, represent emission points for the pyroclastic materials generated during intense explosive activities in the peripheral mouths during a lateral eruption and they are one of the particular features in the overall physical geography of Etna.
The sides of the volcano have seen the activities of countless generations of people who, learning to live with the ‘Muntagna’, have moulded the surroundings to the extent of creating new rural landscapes, developed around farming and livestock breeding, and leaving their indelible mark in the unmistakable, significant signs found in the structure of the area. In terms of type, Etna’s lava flows are mostly aa (an onomatopoeic Hawaiian term used to describe rugged lava on which it is very difficult to walk barefoot) or pahoehoe (an onomatopoeic Hawaiian term used to described cordate lava on which it is easy to walk barefoot), or lava in irregular slabs in set out in different ways. Geological conditions have been a determining factor in these lava fields, creating lava tubes that, thanks to thermal insulation, allow the lava to flow over large distances, feeding lava fronts up to 10 km or more from the mouths, as well as volcanic caves created by expansion activity or fractures (over 25 recorded). Inside the caves are various particular and rare mineralogical concretions: windows, striations, shelves, lava rolls and lava stalactites. The Grotta del Gelo, the Grotta degli Archi, Grotta delle Palombe, Grotta dei Tre Livelli, and Grotta dell’Abisso del Profondo Nero, are some of the best-known lava caves on Etna. They have been used by the people in the area since ancient times, as holy or burial grounds, shelters and even places in which to store snow (ice houses) for use during the summer in an age in which refrigerators did not exist.