Oggi 15/10/2019 ore 03:40

The most important eruptions of Mount Etna.

Among the more than hundred historical eruptions of Mount Etna happened in the past of which we have precise information there have been chosen only some of them because of their importance and/or originality.
Here, it follows little information for every chosen eruption, whilst in a table there have been collected the most interesting volcanological data.
The eruption of 1614-24 is the longest eruptive event verified on the volcano Etna in historical times, having lasted for more than 10 years.
It is a typical slow eruption that gave place to a vast lava field rich of lava tunnels, among which the famous “Grotta del Gelo”, and it was formed of almost exclusively rope lava (“lava Pahoehoe”).
The eruption of March 1669 must be considered as the most important eruption occurred in the Etna area in the last centuries. I
t took place in the South-oriental side of the volcano that, also, in those times was the most populated.
The lava flows of that eruption destroyed 9 villages and reached, after having travelled for 15 kilometres, the city of Catania covering it partially. The lava flows penetrated into the sea for around a Kilometre and a half, with a head front of two kilometres length.
The explosive structure, the so called “Monti Rossi”, is one of the secondary cones or parasites most important of Etna. During the eruption of June 1763 it was formed a majestic explosive cone, the so called "Montagnola", that dominates half side of the Southern Etna Slope.
The eruption of the "Montagnola" is abnormal among the Etna eruptions because of its high explosiveness and because of the viscosity of its lava that gave place to the formation of squat and thick lava flow.
The eruptive event of 1811-12, during which Monte Simone was formed, is very similar to the eruption of October 1986-February 1987.
The lava flows of this eruptions have formed a large lava field within the Valle del Bove and only in one occasion one lava flow has managed to cross the threshold of the Valley, through the torrent Fontanell, coming near the village Fornazzo.
The Eruption of 1832 (Monte Nunziata) and 1843 have taken place, within a brief distance from one another, in the North-occidental slope of the volcano and they have both come near the inhabited area of Bronte.
The eruption of 1843 is also remembered as the only eruption during which there have been victims.
A lava flow of this eruption, in fact, reached and covered a full tank of rain water not yet emptied resulting in an explosion which hit many farmers that were trying to keep save from the lava the wood of a nearby forest. Over 24 years were verified in the middle Southern side of Etna four eruptions (1883, 1886, 1892 and 1910) ever higher and of increasing intensity.
The lava flows of these eruptions, particularly those of 1886 (Monte Gemmellaro) and 1892 (Monti Silvestri), threatened closely Nicolosi.
The eruption of 1883 must be considered an abnormal eruption due to its brief length and because of the small quantity of thrown material, for this reason it was considered by the Scholars of the time as an aborted eruption.
The 1910 eruption is remembered by its fluidity and the high temperature presented by its lava flows: fortunately the main lava flow of this eruption stopped by the fields between the towns of Nicolosi and Belpasso.
On the side North-oriental of the volcano were verified two classic paroxysmal lateral eruptions in 1911 and 1923; the lava flows of the last one threatened the town of Linguaglossa. The eruption of 1928 has been the most disastrous eruption in history. In fact, its lava flows have completely covered the city of Mascali and many vineyards and citrus groves of the low oriental side of Etna.
The eruption of 1950-51 is without a doubt one of the most important eruptions of Mount Etna of the XX century. Despite of having the characteristics of a quiet eruption (formation of a vast lava field) its lava flows have reached considerable lengths (10 km) nearly threatening the towns of Milo and Fornazzo.
This has been possible due to the formation of the lava-tunnels that have allowed the lava to remain fluid for a long time. This eruption has taken place in the oriental side and most part of its lava flows had spilled over the Valle del Bove.
After the eruption of 1971 that has occurred in the first place the higher side of the southern side of Etna and then the oriental one, it started one of the most active periods of the volcano.
The lava flows of the eruption of 1971 destroyed some of the housing of the north periphery of Fornazzo and covered very fertile agriculture fields, with vineyards, hazelnuts trees and orchards.
During the first part of the eruption, on the higher southern side, the lava flow covered the Volcanological Observatory of the University of Catania and the arrival station of the cableway.
In the eruption of 1974 there were two different eruptive phases, separated one from the other by a complete eruptive stasis 22 days long. During these two eruptive phases, essentially explosive, there were formed two parasitic cones, the mounts De Fiore I and II, of discrete dimensions.
A classic paroxysmal eruption was that of 1981, whose lava flows threatened the nearby of the city of Randazzo (North-occidental side). In the first 24 hours of this eruption was thrown the 70% of the lava, valued around 30 millions of cubic metres.
A characteristic of this eruption is the length of its eruptive fissures or cracks: 7,5 kilometres, going from 2.600 m height to 1.100 m height.
Even during this eruption were destroyed many hectares of fertile agriculture fields, in particular vineyards. One of the best examples of slow eruptions is that of the eruption of 1983.
This eruption has been the eruption of world records.
During this eruption, in fact, it has been done for the first time in the history of Mount Etna an intervention for the deviation of the main lava flow and the construction of containment barriers, authorised and financed by the National Government.
For the first time it was active and functioned 24 hours over 24 hours, an operative room of the Civil Protection at the Prefecture of Catania. In the end this eruption was the most followed, studied and photographed one.
Among the most recent eruptions we must mention two typical slow eruptions: the eruption of 1985 and that one of 1986-87.
The first one, apart from its length (123 days), will be remembered for having covered some of the pylons of the Etna cableway, after rebuilt on a different route.
During the second one, lasting four months (from the 30th of October 1986 until the end of February 1987), it was formed a cone of ash and slag named Monte Rittmann in memory of the famous Swiss volcanologist Prof. Alfredo Rittmann.
The lava flows of this eruption have given place to a wide lava field in the northern part of the Valle del Bove, in the same area of the eruption of 1811, and its lava flows have managed to exceed the eastern boundary of the valley, stopping at 1.300 m height, five kilometres away from the point of emission.
In September 1989, after a series of violent explosive-effusive episodes the crater of SE, had a paroxysmal eruption in the high North-occidental sector of the Valley del Bove, at the edge of the Valley del Leone.
This eruption, lasted only 11 days, gave life to lava flows that covered the northern and central part of the Valle del Bove, stopping at an altitude of 1.100 m after having run for seven kilometres.
The Valle del Bove was the place, in its southern portion, of another eruption, started the 14 December 1991 and ended the 30 March 1993, lasting 473 days full of activity; an eruption that could be said to be the most important one of this century, at least for what regards the volume of lava flows: more than three hundred millions cubic metres in more than fifteen months of activity. During this eruption have been undertaken other experiments of lava flows deviation.
The last one of this deviations has unfortunately cover the Trifoglietto and destroyed the Refuge Menza and the beech trees that were still present along the southern part of the Valley.
All this happened when it did not exist, at that moment (May 1992), any danger for the town of Zafferana.


Conclusions

The Etna that, until today it was known as the highest (3.345 m) and the largest (1.250 km) volcano in Europe, has shown in the last years to be among the most active volcanoes in the world.
In fact apart from presenting an almost continuous activity on the depths of the chasms of its summit craters, it has had around 50 eruptions, among its flanks, terminals, sub terminals and laterals.
The volcanic areas are generally very fertile and very welcoming due to their natural beauties, this makes so that these areas are densely populated and grown.
The population density poses problems to the volcanic risk more or less dramatic and easily solved depending on the area under consideration.
Regarding the area of Mount Etna the biggest danger is posed by the lava flow invasions that could be considerably consistent, as it was the one in 1669, and reach areas densely urbanised: it is necessary, then, to identify either the areas where the most probability of openness of eruptive cracks could be, or the areas more likely to be covered by lava flows.
For the Etna area these studies have already been undertaken and, with a good approximation, the preferential ways of the magma and the most easily reached places by the lava flows (Graphic of p.54).
It should be then initiated by the appropriate authorities a study program that could allow the realization of particular defensive plans or prevention works in order to avoid the invasion of lava flows in the areas most at risk.


Types of eruptions

Every eruption has its own characteristics that distinguish one from the others. Bearing in mind this concept, it is still possible to classify the eruptions of Mount Etna in at least four different fundamental types, according to the place where the eruptive activity manifests and also according to the way it evolves. a) Terminal eruptions [Table. 2.1]: when the explosive and effusive activity is concentrated in the Central Crater and/or North-East Craters or South-East Craters. Generally, during these eruptions, we assist to violent explosive manifestations (lava fountains, drops of lava shreds, bombs, incandescent slag and lapilli or tephra) accompanied, sometimes, by lava overflow, mostly little consistent.
Recent examples of this type of eruptions are those of 1964 at the Central Crater, in September 1986 at the NE Crater and in September 1989 at the SE Crater. Apart from the paroxysmal eruptive activities, it is necessary to take into account that at the summit craters (Voragine West or “Bocca Nuova” and Voragine East or “Grande Voragine” of the Central Crater, Crater of NE and Crater of SL) there is an impressive and continuous activity of gas and steam expulsions, more or less violent, of black and reddish ashes.
Expulsions of black ashes (fresh lava material finely crushed) take place when the magmatic column is very close to the surface; on the contrary, the reddish ashes (altered lava material), appear when there are internal collapses, due to the movements, often negative of the same magmatic column at the inside of the principal conducts. b) The sub terminal eruptions [Tav.2.2]: c) The terminal eruptions [Tav. 2.3] : d) The eccentric eruptions [Tav.2.4]: The study of recent eruptions in comparison with the volcanological data of the previous eruptive activities of Etna have given evidence of the fact that the eruptions could be further distinguished in paroxysmal or slow eruptions. The paroxysmal eruptions are of short duration (days or weeks) and they have their maxim emission within the first 24 hours and after having reached the peak (generally after 3-4 days).
The lava flows of these eruptions manage to travel long distances (7-15 km), they are predominantly formed of a single a long lava flow, not too long and relatively not thick, where it is possible to notice the lava channels, generally one, of different dimensions.
The slow eruptions are a long length (months or years) with a reduced and constant emission of products, with brief oscillations in positive and negative sense, during the whole period of the eruption.
These eruptions cover vast areas and they become considerably thicker, but the lava flows do not reach long distances (max. 6-8 km) from the point of leakage of the main lava flow; generally, a long and varied lava field is formed, sometimes it is some kilometres long, where it is possible to note a network, rather complex, of tunnels, lava channels and numerous ephemeral and effusive mouths.
Generally, slow eruptions are the typical sub terminal ones, whilst the paroxysmal are the classic lateral ones.
Other differences could be found on their physical properties, in fact, the temperature of the lava emitted during slow eruptions is generally lower (under 1.100°C) than that of the paroxysmal eruptions (at around 1.200 °C).
Whilst it is exactly the opposite for the viscosity of the lava: it is higher in slow eruptions.
It is clear, though, that there exist eruptions, rare ones, with intermediate characteristics.