by:L. Giacomelli group national volcanology

Web www.costierabarocca.it

 

The Etna is the greater active volcano of Europe and one among the greatest of the world. Its base has an almost oval form of around 1600 km (2), with the greatest axle in direction long North-south around 60 km and that smaller in direction East-west of around 40 km. Its volume is around the 500 km (3). 

The perimeter of 150 km is marked by the rivers Simeto and Alcantara and, for around 30 km, from the Ionian Sea. To South, the plain one of Catania separates the volcano from the Iblei mountains, more ancient and of volcanic origin, to North it confines with the Peloritani Mountains, primarily constituted by granite granite rocks. 

The whole building can be considered as the succession of cones developed him one above the other, around a same point of gone up again of the magma. Gives the succession of different cones one on the other, built by different eruptive phases, the inside structure of the crater of North - East it is particularly complex.

 

 

Genesis of the Etna.

 

 

Around 130 million years ago Africa, pushed by the expansion of the southern Atlantic, it drew near to Europe slowly closing the ocean Tetide that existed among the two continents. 

Around 80 million years ago, while the Atlantic widened and Africa kept on stirring toward Northeast, the ocean Tetide was practically disappeared and the irregular edges of the two continents were seen one the other. 

The northern profile of Africa introduced a big prominence or a series of islands, that interposed him among the two borders. These islets or this peninsula, compressed among the two continental plates, they are partly future Italy. 

In base to the theory of the sod tectonics, the volcanos are primarily formed when the terrestrial lithosphere suffers phenomena of compression or relaxation. In the first case, a sod is inclined up to wedge under another and sinks in the mantle, while in the relaxation a sod breaks him along a fracture and they is formed two pieces of lithosphere that estranges one from the other. 

The movements of the sod provoke physical variations (pressure, temperature, etc.) what they favor in depth the formation of magmi and theirs gone up again toward the surface. The magmis have different characteristics and, particularly, those erupted in the zones in relaxation are of predominantly basaltic composition. 

The Sicilian eruptions both those more ancient than the Ibleis (you begin ago around 10 million years), that those of the Etna, have produced magmi with near characteristics to those that are formed in the areas in relaxation, although Sicily is found in a zone in compression. 

This particularity could be justified with the formation of fractures, perpendicular to the compressive push, that you/they extend to widen and to create zones in relaxation inserted in an ampler area of compression. 

The zone of the oriental border of Sicily responds to the push of Africa fracturing himself/herself/itself and favoring in this way gone up again her/it of stagnant magmi in depth that has formed the volcanos of the Ibleis and the Etna. The volcanic islands of the arc of the Eolies as the lifting that it forms the Peloritani Mountains, are instead consequence of phenomena of compression and subduzione. 

The volcanic activity in the area etnea relatively begins in epochs recent geologiche, between 700.000 and 500.000 years or are and it develop him through various phases.

 

1 - Activity pre-Etna  

700.000 years ago the volcanic activity around whose beginning is dated happens inside a gulf (gulf pre-etneo) that extended him from the Peloritani Mountains to the Ibleis. The whole Sicilian area, included the gulf pre-etneo, you/he/she had started to rise beginning from around 1 million and 700 thousand years ago. 

The first eruptions are predominantly submarine and they happen inside the gulf pre-etneo already reorganized by the lifting and by the emersione of part of the clays that you/they covered the fund of it. The lifting of the continuous area up to that the eruptions happen above the level of the sea and they expand him above the clays deposits. 

The said eruptions pre-etnees are those happened before the construction of the building of the real Etna, in an arc of inclusive time among 700.000 and 200.000 years ago. It deals with basaltic eruptions, whose produced they are still seen partly along the edges of the actual southeast and southwest building. These eruptions, sporadic and alternated by phases of stasis, you/they have formed small volcanos and datum place to castings of very fluid lave. The submarine eruptions form accumulations of blocks of it washes roundish and fractured that you/they are called pillow lave or pillow it washes. Together with these deposits of consequential glassy (ialoclastiti) material are found by the magma quickly cooled to the contact with the water or accumulations of pieces of fractured (breaches) pillow. 

The first eruptions pre-etnee on dry land is testified in the zone of Aci Castello by layers of ash interstratificati to the clays of the seabed. The eruptions have succeeded with castings of it washes, visible for over 15 southwest km of the volcano, where I/you/they have been cut and eroded by the river Simeto. 

Some eruptions pre-etnee has happened from isolated volcanos, of which two are still visible to Motta S. Anastasia and to Paternò. Their rests, put in evidence by the erosion, they form small reliefs.

2 - Primordial or ancient Etna. 

The skeleton of the Etna is formed in an inclusive period among around 150.000 and 80.000 years ago. This first apparatus, denominated ancient or primordial Etna it is formed from castings of it washes, from products of explosive eruptions and from rehandled deposits. And' an a little known phase of the life of the Etna, being the products to a large extent covered by those of the following activity. 

After the eruptions of the primordial Etna and up to around 25.000, years ago numerous eruptive centers are formed one above raising and widening more always the form of the volcano. The eruptive centers of this long phase, whose reconstruction is possible graces to the scarp of the Valley of the Ox that has sectioned a side of the Etna, are divided in ancient, intermediary and of the Mongibello.

The Mongibello

 

The Mongibello is a big cone that covers the central zone of the actual thick of the Etna and he/she understands more than a bystander of the volume of it. It deals with the more recent eruptive center and it is center of the mouths actual sommitali. The life of this apparatus can be divided in more phases: Ancient Mongibello and recent Mongibello, separated by the formation of the caldera of the Elliptic Crater and modern Mongibello, represented by the historical eruptions.

 

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