Volcanic hazards at Etna

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Etna and Man (3)


1981 eruption
Dramatic view of the town of Randazzo and eruptive fissures emitting lava flows that move towards the town, on 18 March 1981. This was one of the most vigorous eruptions of Etna in recent decades, but fortunately the activity weakened significantly after only two days, and the lava flows stopped before reaching Randazzo. The main flows produced by this eruption passed a few hundred m east of the town, destroying farmland, fruit gardens, and dozens of isolated buildings.
1992 lava flows
Night view of lava flows running down Val Calanna, and toward inhabited areas. The threatened town of Zafferana lies out of the photo towards the right. 20 March 1992.

Although generally considered a rather "harmless" volcano because devastatingly explosive eruptions are very rare (with the strongest explosive activity being commonly confined to the summit craters), Mt. Etna is a potentially very hazardous volcano. The hazard is primarily from lava flows which do not present a significant threat to human lives but a serious one to property. The amount of damage to be expected from invations by lava flows is strongly related to several variables among which the mass eruption rate and the location of eruptive vents are the chief ones. However, the unusually explosive flank eruptions in 2001 and 2002-2003 have revealed a previously unrecognized hazard: tephra falls, which may affect much larger areas than lava flows. Less frequent events with a much higher hazard potential are strongly explosive summit eruptions and gravitationally induced sector collapse, evidence for both of which has been found in recent years. All volcanic hazards at Etna are minor if compared to the seismic risk in eastern Sicily.

Here volcanic hazards are described according to different types of volcanic activity and from highest to lowest degree of hazard of each eruption type, starting with the most common types of activity (which are therefore the most probable to occur in the future) and concluding with very rare, but potentially very hazardous events. Note that there are gradations among all types of activity.

I. High-frequency, low-hazard events

I.1. Summit eruptions

I.1.1. Persistent, low-level Strombolian and/or effusive activity
The most frequent type of eruptive activity at Etna during the past century has been the so-called "persistent" (that is, more or less continuous and regular) activity at the summit craters, in particular at the Northeast (NE) and Southeast (SE) Craters. Activity of this kind is generally characterized by relatively small, discrete explosions that eject incandescent fragments of lava to a few tens to a few hundreds of meters high; this is called Strombolian activity. The main products of Strombolian activity are bombs and scoriae which accumulate around the eruptive vent(s) and build pyroclastic cones (which are also frequently called "cinder" cones, a somewhat misleading term). Minor ash emissions at times accompany the Strombolian bursts. This activity does not constitute a significant hazard to human property since the summit is not inhabited and not suitable for agricultural activity. However, visitors to the summit area who watch the activity at close range are exposed to the risk of falling bombs and scoriae, which might cause serious injury or death.
The Strombolian explosions may or may not be accompanied by the emission of fluid lava, usually from vents on the flanks or at the base of a pyroclastic cone, at a low rate (0.1-1 cubic meters per second). Lava emission may also occur without being accompanied by explosive activity as in the spring of 1999 and in early 2001. Lava flows fed by the effusive activity extend a few hundred meters to a few kilometers from their source vents. They may interrupt the dirt roads in the summit area and destroy monitoring equipment but are otherwise harmless since no lava flow produced by this type of activity have ever extended into areas of forest or cultivated land.

I.1.2. Paroxysmal summit eruptions
Since the late 1970s much of the summit activity at Mt. Etna has been characterized by short-lived but violent episodes of lava fountaining, tephra emission and generation of fast-moving lava flows at high mass eruption rates (however, not all of these events produced lava flows). Since 1977, more than 170 such events have occurred, of which more than 120 took place between 1995 and 2001. These events are often called paroxysms, paroxysmal eruptions or paroxysmal eruptive episodes. In comparison, less than 50 paroxysms were documented during the period between 1900 and 1977, so it seems obvious that this type of activity has become much more common in recent years, and therefore it must be expected that summit activity in the near future will be characterized by frequent paroxysms.
The main hazard from paroxysmal eruptive episodes is constituted by heavy falls of pyroclastics, also named tephra. These products encompass all types of fragmented volcanic rock (in contrast to coherent lava flows), from fine-grained material (ash) over medium-sized (lapilli) to large fragments (bombs and blocks). If these ejecta have a high degree of porosity they are also called scoriae (or scoria), whatever their grain size. Large fragments as a rule fall close to the erupting vent(s) and therefore may affect only people who are too close (mostly because they do not recognize the danger or are surprised by the sudden onset of a paroxysm). Lapilli-sized scoriae and finer material is frequently carried toward inhabited areas located downwind of the volcano. Heavy tephra falls in those areas can disrupt traffic, damage cars (especially windshields) and window panes, and even cause injuries since scoria fragments are highly abrasive. Furthermore plants may suffer from such rains of scoriae, although in the long term they have a powerfully fertilizing effect. In the case of particularly strong paroxysms occurring during dry periods, forest fires may be caused by the fall of very hot pyroclastics, as happened during a powerful explosive episode at the Voragine in July 1960.
The most memorable sequence of tephra falls in recent time occurred in the spring of 2000 when the SE Crater generated more than 60 paroxysmal eruptive episodes. Heavy showers of millimeter to centimeter-sized scoriae repeatedly occurred in a sector encompassing the NE, E and SE flanks of Etna, and cleaning efforts became an obsession to the inhabitants of the affected towns and villages. Abundant quantities of lapilli and ash not only left continuous deposits on roads making driving an adventure, but they also clogged drainage systems and entered homes.
The effects of strongly explosive paroxysmal eruptive episodes can be even more far-reaching and endanger air traffic. Since Etna lies in one of the areas of major business and touristic interest, civil aviation may be affected by tephra falls and eruption plumes; furthermore a large U.S. military air base (Sigonella) lies about 40 km S of the volcano. The existence of this risk, which had previously not been recognized at this volcano, was dramatically demonstrated on 26 April 2000 when an airplane with more than 100 passengers and staff encountered falling lapilli while flying underneath an eruption plume emitted from the SE Crater shortly before. The pilot made an emergency landing at Catania airport and no one was injured, but the aircraft suffered significant damage from abrasion by impacting scoriae; it was pure luck that this indicent did not develop into a tragic accident. As a direct consequence, flights were re-routed from the E side of Etna (which is most commonly used but also the sector most exposed to the passage of tephra plumes) to the safer W side as long as paroxysmal eruptive episodes continued at the SE Crater.

I.1.3. Phreatic or phreatomagmatic activity
Etna is not particularly well known for a type of activity that consists of the explosive interaction of magma (or hot rock in general) with external water, but it seems that this is more common than previously thought, especially at the summit craters. Water may seep into the conduits of Etna during heavy rain falls or due to the melting of ice and snow (which occurs after each snow fall near the summit craters because the ground there is hot) and come into contact with hot rock at depth. If the conduit is blocked, vapor generated by this encounter will generate increased pressure, until the material blocking the conduit is explosively removed. Alternatively, the uprise of fresh magma through a water-soaked conduit will lead to similarly explosive interactions. If all the ejected material is composed of old rock without any fresh magma fragments, the activity is called phreatic; if a mixture of old rock and fragments derived from fresh magma is ejected, it is called phreatomagmatic. A further variation on the theme is the obstruction of one of the summit craters by a pond of dense, solidifying lava; gas pressure beneath this "cork" may accumulate and lead to its explosive disruption without fresh magma being involved, leading to phreatic activity. The products are blocks of old rock that may reach dimensions of several meters, lapilli-sized fragments of old rock and ash and minor amounts of fresh magmatic ejecta whose proportion may vary from zero to about 50%. What makes phreatic or phreatomagmatic explosions particularly treacherous is their sudden onset, the almost complete lack of noise, and their unpredictability. Another evil fact is that at night many blocks ejected by such explosions are virtually invisible because they are not incandescent. It is for these reasons that all of the 13 deaths directly attributable to eruptive activity of Etna during the past century were caused by phreatic or phreatomagmatic explosions.
This type of activity thus represents one of the main hazards to human lives at this volcano, but fortunately it is limited to the immediate surroundings of the summit craters. Large clasts ejected by phreatic or phreatomagmatic eruptions as those seen during the past few decades fall in a range of a few tens to a few hundreds of meters from the source vent and are therefore only dangerous for those who are too close.
If phreatic or phreatomagmatic summit activity generates significant amounts of ash, this may disrupt car and air traffic and cause drainage problems in inhabited areas.

I.2. High-altitude subterminal or flank eruptions

Eruptive activity from vents (normally fissures) away from the summit craters and their immediate surroundings is called subterminal if it is very closely related to the activity of one or more of the summit craters. Subterminal can also be termed "quasi-summit" activity, and in recent years a trend toward considering it part of the summit activity is evident in the volcanological literature. This is mostly based on the fact that in the case of the opening of subterminal vents the activity may shift back and forth between these vents and the summit craters. If new eruptive fissures open at a certain distance from the summit craters and their activity fully replaces that of those craters, it becomes a flank eruption. Although there is no clearly fixed transition, in terms of distance from the summit craters, between subterminal and flank eruptions, the latter commonly are followed by a period of repose at the summit craters, while the earlier may be immediately followed by renewed activity at the summit craters.
The activity of these types of eruption normally consists of quiet lava emission with minor Strombolian activity or spattering (weak explosive activity that ejects clots of fluid lava to a distance of a few meters from the vents) and does not endanger human lives except in cases when people get extremely close to the vents or move on top of lava tubes. Effusion rates may be higher than during persistent summit activity and allow the emplacement of longer lava flows. These, as in the case of persistent summit activity, might bury the dirt roads in the summit area and threaten, damage or destroy one of the few manmade structurers in the upper parts of the volcano, such as the "Torre del Filosofo" mountain hut which stands only 1 km south of the SE Crater. Since little explosive activity is associated with these eruptions they do not have any far-reaching effects. If the eruptive vents lie at elevations of about 2900 m or less, the effusion rates are high enough and the activity lasts for a certain period, lava flows might advance as far as several kilometers and affect the tourist facilities on the southern or northern flanks. In fact, during the July-August 2001 eruption lavas fed by an eruptive fissure at about 2700 m elevation came close to the "Rifugio Sapienza", a large mountain hut at about 1900 m altitude, and the nearby departure station of the cable car. Much of the hazard potential of such eruptions depends on where they take place - if they occur on the W or E side of the volcano, they would affect areas without any man-made structures whereas the S side is highly vulnerable.

II. Medium-frequency, medium to high-hazard events

II.1. Mainly effusive flank eruptions

Flank eruptions occur at irregular intervals, which may last from less than one year to several decades. The frequency of flank eruptions since 1971 has been unusually high - between 1971 and 1993 the average interval between the beginning of one flank eruption and that of the next one was about 1.5 years. The July-August 2001 eruption occurred more than 8 years after the end of the preceding flank eruption, which is probably due to a cyclic eruptive behavior of Mt. Etna that leads to flank eruptions being clustered in time rather than occurring at more or less random intervals. However, the 2001 eruption was atypical for some reasons and should not be taken as strictly representative of the eruptive behavior of the volcano.
The hazards from eruptions of this kind are definitively higher than those associated with summit eruptions but depend on the location of the eruptive vents - both in the sense of elevation and geographical location - and from the mass eruption rates and the eruption's duration. The lower the eruptive vents, the greater the probability that lava flows arrive in populated and agricultural areas. The higher the mass eruption rates, the more rapid and longer the resulting lava flows and thus the risk of invasion of populated and cultivated areas. While a distinction according to the physical eruption parameters like elevation of the eruptive vents, eruption duration and mass eruption rates is often applied for the evaluation of the volcanic hazard at Etna, it seems more useful to make this distinction relative to the vulnerability of the areas on the various sides of the mountain and consider the various eruptive parameters in this framework.
It has to be noted that while material damage resulting from flank eruptions can be overwhelmingly high, such events do not threaten human lives because lava flows move quite slowly once they have reached a certain distance from their source vents. All sources attributing human deaths to lava flows at Etna (such as the striking number of up to 20,000 deaths in the 1669 eruption) are pure inventions or misconceptions based on confusing eruptions with earthquakes - the latter in fact have caused more than 200,000 deaths in Sicily during the past 1000 years, while Etna's volcanism can only be blamed for less than 100 deaths during the past 2000 years (see "Etna and Man").

II.1.1. Eruptions on the E flank
12 out of the 23 flank eruptions during the 20th century occurred in the eastern sector of Mt. Etna and were at least partially confined to the Valle del Bove, a large collapse structure on the E flank of the volcano. If the eruptive fissure(s) are located within that depression there is a good chance that the lava flows they generate will accumulate on its floor, a vast, desert-like area covered by numerous lava flows of previous eruptions. Usually flank eruptions occurring in the Valle del Bove are greeted by the local population with a sense of relief, since it acts like a giant natural catchment basin. In the Valle del Bove there is little to destroy; only a few patches of forest have survived the numerous invasions by lava flows especially since 1950, which have led to the resurfacing of about 80% of the Valle del Bove floor in this period.
However, some eruptions that occurred in this area did cause damage and seriously threatened population centers lying below the lower (eastern) end of the Valle del Bove, such as in 1950-1951, 1979 and 1991-1993. In 1950 the lava flows from the initial phase of the 1950-1951 eruption, fed by high effusion rates, advanced close to the village of Fornazzo and damaged nearby fruit gardens. In 1979 high effusion rates generated a fast-moving lava flow that advanced on top of the 1950 flow and again seriously threatened Fornazzo, which was evacuated for a few days but fortunately was not touched by the lava flow; however, fruit gardens and a road were buried by the lava. A different situation developed during the first months of the 1991-1993 eruption. In that case a constant, moderately high effusion rate led to the establishment of an efficient system of lava tubes, which allowed the transport of lava without significant heat loss (lava flowing through tubes is protected from the chilling effect of the outside air) over many kilometers. As a result the lava flow field attained a length of more than 8 km and extended very close to the town of Zafferana on the SE flank of Etna (Calvari and Pinkerton, 1998). This eruption is a striking example of how the various factors determining the hazard potential of an eruption may interact with each other and produce unexpected effects. Eruptions like those of 1979 and 1991-1993 will inevitably lead to a repetition of the threat to the villages below the Valle del Bove, and since this sector of the volcano has been particularly active in recent decades, many of the eruptions in the foreseeable future can be expected to take place there.

II.1.2. Eruptions on the S flank
Although less frequent than E flank eruptions, eruptions that occur on the S flank have a high hazard potential since this is the side of the mountain where man-made structures extend to the highest altitudes. A large complex of tourist facilites such as hotels, restaurants, souvenir shops and the Etna cable car (partially destroyed during the 1983, 1985 and July-August 2001 eruptions) lie at an altitude of about 1900 m, so that even small eruptions occurring in this area might cause significant damage. Both the 1983 and 2001 eruptions were highly destructive, although in both cases the threat to towns lying further downslope was vastly exaggerated by the press and local administrations. Any future eruption in this area will threaten or damage the tourist facilities, but rebuilding and maintenance of these structures is of paramount interest since this is the main access route for tourists to the higher regions of Etna, in spite of the presence of a beautiful alternative route on the N flank (Linguaglossa-Piano Provenzana).

II.1.3. Eruptions on the W and N flanks
During the 20th century, the W and N flanks of Etna have been affected by fewer eruptions than the E and S flanks, but two dramatic eruptions on the NE flank in 1911 and 1923 caused significant damage and threatened the town of Linguaglossa, and the 1981 eruption on the NNW flank led to the destruction of hundreds of buildings, roads and railway lines and nearly consumed the scenic town of Randazzo. The complex of tourist facilities (hotels, restaurants, souvenir shops, and ski lifts) of Piano Provenzana, located at about 1800 m elevation on the NE flank, was virtually wiped out and buried by lava flows at the beginning of the 2002-2003 eruption. On the other hand, the somewhat atypical 1974 eruption on the W flank was very small and affected the least densely populated and developed sector of the volcano. As a rule, eruptions with high effusion rates are more likely to produce lava flows long enough to extend into vulnerable areas, while low-effusion rate events may even take place at relatively low elevations without representing a significant threat. The 1974 vents opened at 1500-1600 m elevation, which is fairly low, and in 1981 the most destructive lava flows were emitted at exceptionally high effusion rates from vents at 1300-1700 m elevation while vents at only 1150 m altitude generated small, slow flows because the effusion rates there were low. The areas where flank eruptions are most likely to occur in the future are the so-called Northeast Rift (a zone where eruptive fissures and pyroclastic cones are densely clustered along a line of structural weakness) and the much less distinct so-called West Rift Zone, in which numerous sizeable pyroclastic cones occur. Many Northeast Rift eruptions in the past were characterized by high effusion rates and similar events in the future might endanger the town of Linguaglossa or one of its neighboring villages, and the extensive areas of wine and hazelnut production on the N and NE sides of the mountain as well as the largest contiguous natural forest on Etna, the "Pineta Ragabo", which was indeed severely damaged by lava flows at the beginning of the 2002-2003 eruption.

II.2. Explosive activity during flank eruptions

The first two flank eruptions of the new millennium, in July-August 2001 and October 2002-January 2003, have revealed a hazard that had until then been largely ignored: tephra falls over large areas around the volcano, generated by strongly explosive activity. Until 2001, explosive activity was substantially believed to be a privilege of the summit craters, but large amounts of airborne ash were produced from vents on the southern flank in 2001 and, even more significantly, in 2002-2003. Ash fell heavily in downwind sectors, amounting to several centimeters up to 30 km away, interrupting air traffic, and causing enormous logistical and economic problems to the Catania region. Fine ash fell up to several hundred kilometers distant, in Greece to the east, and northern Africa to the south. Near the eruptive vents, pyroclastic deposits reached thicknesses of more than 50 m, and sizeable pyroclastic cones were built around the vents, reaching heights of 100 m in 2001 and more than 200 m in 2002-2003.
The explosive eruptions of 2001 and 2002-2003 marked the end of a fairly peaceful period of essentially non-explosive flank eruptions that had lasted for more than a century. The common belief that Etna was not capable of producing strongly explosive eruptions was largely a result of this period, even though flank eruptions had become more frequent during the last 30 years of the 20th century, and brief episodes of highly explosive activity had characterized much of the summit activity since the 1960s. In fact, the most recent strongly explosive flank eruption occurred in 1892, in a time when the Etna region was strongly underdeveloped, and much less densely populated than it is now. However, a simple look at all the large pyroclastic cones (many of them being of historical age) that dot large areas on the flanks of the volcano should have served as an indicator of potentially explosive flank eruptions. One of the main reasons why explosive flank eruptions have been largely neglected until recently lies in the fact that such events in the past (that is, before the end of the 19th century) have caused much less consternation and problems to the population at that time, because the society and its infrastructures were considerably less vulnerable. A fall of ash surely meant a nuisance for those living in the fallout sector, but it could also be used to fill the holes in the dirt roads that made up much of the lifeline network then. Airplanes had yet to be invented, and sophisticated techniques now occupying broad space in everybody's everyday life were not even dreamt of. This is why the 2001 and 2002-2003 eruptions came as a very bad surprise, affecting a socienty that had advanced close to mid-European standards.
The long break in strongly explosive flank eruption between 1892 and 2001 was very probably an exceptional period in Etna's recent history, and it is unlikely that another such period of considerable length will be seen in the near future. Given the current dynamics of the volcano (with a certain proportion of magma being supplied into a new, "eccentric" reservoir that has formed next to the central conduit system), it must be rather expected that there will be more flank eruptions with similar degrees of explosivity as those of 2001 and 2002-2003 in the near future.
As seen during these events, widespread and voluminous tephra falls create a variety of problems in the affected sectors. Closer to the volcano, thick tephra deposits on roads represent a significant obstacle to road traffic and render car driving dangerous especially if mixed with rainfall water, which renders pavements quite slippery. Crops may temporarily suffer damage, although in the long term the ash is a powerful fertilizer (one of the main reasons why the Etna region is so densely populated). The ash enters every home, and thus affects the quality of life of those living in the tephra fall sectors. A hazard to health (mostly from inhaling the ash) has not been recognized, because Etna's ash is generally to heavy and coarse-grained to remain suspended in the air - it simply falls to the ground and thus cannot be breathed. In areas of intense road traffic (foremost the city of Catania with its dense traffic, which is unbearable already when there is no ash), however, ash grains are likely to be crushed and pulverized by frequently passing vehicles and thus transform into fine dust that may be suspended in the air every time there is wind. In that case it cannot be excluded that it is inhaled by people and affect their health. No detailed study has been made public so far in the wake of the recent eruptions, although the subject has been intensely discussed in local mass media, and local administrations have categorically excluded any risk to public health (and done little to remove the enormous quantities of black ash still persisting all over the Catania area, although much of it has been removed by heavy rainfalls between January and May 2003).
The sector that is most sensible to tephra falls is aviation, and with it tourism, and thus a large business sector in the Catania area. During the 2001 and 2002-2003 eruptions, the international airport of Catania, ominously named "Fontanarossa" (red spring), had to be closed and flights were rerouted to Palermo (200 km distant) or Reggio Calabria (90 km distant, and on the opposite side of the Strait of Messina), or simply cancelled. In the case of the latter eruption the problems came essentially after the end of the tourist season (which had strongly suffered from the worldwide effects of the terrorism attacks of 11 September 2001) but persisted over a period of several months, compared to about two weeks in 2001.
Following the 2002-2003 experience, Italian civil aviation experts have shown eager activity to mitigate the effects from future explosive volcanism at Etna, including a recent meeting with representatives from the Japanese city of Kagoshima, which has a decades-long experience in dealing with frequent ash falls from nearby Sakurajima volcano, and seems to deal with them pretty well. Catania will have much to learn from that example. And this is warranted. There is a good possibility that future eruptions of Etna will show similar degrees of explosivity as the most recent ones in 2001 and 2002-2003.

III. Low-frequency, high-hazard events

III.1. Low-altitude flank eruptions

Throughout the historical period, flank eruptions at Etna have been most frequent at medium to high elevations - that is, between 1500 and 2500 m altitude. However, flank eruptions may also occur at much lower elevations, and these represent the most hazardous type of activity seen during the past 2000 years. Two historical eruptions from the S flank may serve as the possible extremes of what may occur in the case of a flank eruption - that of March-July 1985 whose vents lay above 2500 m elevation, and an eruption generally ascribed to the year 1381 (evidence presented by Tanguy, 1981 and Tanguy and Patanè, 1996, indicates that this eruption occurred about 200 years earlier, i.e. during the 12th century) when an eruptive fissure opened at about 400 m elevation. In the earlier case - an eruption characterized by low effusion rates - the eruptive fissure cut through the Piccolo Rifugio which had already been damaged by the 1983 eruption and abandoned; lava flows extended below 2000 m but did not threaten the tourist facilities around the Rifugio Sapienza which had suffered extensive destruction in the 1983 eruption. Both the low mass eruption rates and the high elevation of the eruptive vents contributed to a fairly tranquil course of events (from a human standpoint). The "1381" eruption, on the other hand, was characterized by higher effusion rates and occurred in an area which is now practically a part of Catania's suburbs. Lava flows from that event reached the sea to the south of Porto d'Ulisse, after overrunning an area now occupied by Ognina and Picanello, parts of present-day Catania.
The historical record shows that eruptions from low-lying vents on the S and SE flank were clustered in time, such as various eruptions during the Roman age and again between the 10th and 15th centuries. The large 1669 eruption, though, appears to have been an isolated event, but it entered the historical record as the most devastating and violent known flank eruption of Mt. Etna. All these eruptions were characterized by high eruption rates and their lava flows frequently reached the sea; in 1669 nearly one cubic kilometer of lava (about 40 times the volume of the July-August 2001 lava) was emitted within four months, which corresponds to an average effusion rate of 80-100 cubic meters per second (compared to an average rate of 12-13 cubic meters during the 2001 eruption). While no eruption at Etna since 1669 has occurred from vents lying below 1000 m elevation, there is absolutely no reason to assume that low-altitude eruptions will not occur in the future. Considering the long period elapsed since 1669 one could actually reason that such an eruption is overdue.
Low-altitude flank eruptions are relatively well documented for the southern and southeastern flanks of Mt. Etna but only sketchy, confusing or erroneous information exists for such eruptions in all other sectors of the volcano, since these areas were much less densely populated and virtually no people with a certain level of culture and education dwelled there to witness and describe those events. Yet the abundance of numerous large pyroclastic cones associated with extensive lava flow fields on the W and N to NE flanks testifies to major eruptions from vents at low elevations in the past.
A resumption of eruptions from vents below 1000 m elevation would cause widespread destruction today, especially if they occurred on the S or SE side of the volcano. The vent areas and lava flows of the eruptions of Roman age, of the 10th to 15th century and of 1669 all lie in areas that are now densely populated, including the city of Catania. While lava flows may be (partially) diverted in the case of eruptions characterized by low effusion rates and vents at high elevations (such as in the 1983, 1991-93 and 2001 eruptions), such measures will not be possible if the vents open in populated areas and lava flows could only be diverted from one town to another because there is practically no area left to where a flow might be diverted to without causing damage.
The only possibility would be a protection at least of Catania or other coastal towns, which in any case would be relatively far from an eruptive vent. The 1669 eruption serves as a good example of how Catania might be protected from being invaded by a lava flow. In 1669, the lava was kept from reaching and destroying the center of Catania by simply constructing thick barriers across the main streets of the city. Similarly, gaps between building complexes in the marginal areas of the town could be closed by concrete walls that could be erected after the onset of an eruption at low elevations, once the probable course of the lava flows is more or less predictable (e.g. with the help of computer simulations).
Furthermore it should be expected that the incredibly huge accumulation of buildings, with many of them being contiguous, would exert some resistance to a lava flow by itself. During modern history, no large city of Catania's dimensions in the industrialized part of the world has been reached or invaded by lava flows, and the effect of large building complexes on a slowly approaching, but thick lava flow is not known. The only precedent, which the world witnessed via the mass media as recently as in January 2002, was the invasion by lava flows of the city of Goma in the Democratic Republic of Congo. Even though that city cannot be compared in the least sense with Catania, it was fascinating to see how the lavas erupted from Nyiragongo volcano were channelized by roads and buildings, of which many were burned but only few were crushed.
Even in the case of a low-altitude flank eruption in a more remote and less densely populated area, devastating consequences are to be expected. The next eruption of this type needs not necessaritly affect the highly urbanized SE flank but might equally strike the N flank and the valley of the Alcantara river. A major eruption in that area would lead to the destruction of the town of Francavilla and several smaller villages and cause the loss of the world-famous Alcantara gorge which attracts numerous tourists and local residents. Furthermore, immediately before reaching the sea the lava flow would cut a highway, various important roads and the Messina-Catania railway. The damming of the Alcantara river would cause flooding over a wide area. While much of the recent hazard evaluations at Etna are concerned with the S and SE flanks and the Catania area, the possibility of a disastrous low-altitude flank eruption in other sectors of the volcano has been mostly neglected so far.

III.2. Major explosive summit eruptions

Until a few years ago, explosive activity at Etna was considered a less common and thus less significant type of activity than lava emission, and generally it was believed to be quite modest. This view has changed since a group of researchers began to publish the results of their search for evidence for large-scale explosive events at the volcano, an evolution that has received further stimulus from the frequent, vigorously explosive events at the summit craters in the past decade. Documented explosive summit eruptions during the past 2000 years were exclusively paroxysmal episodes as described above, but as a matter of fact the historical records of the past 400 years describe no period of similar explosive activity as that between 1995 and 2001.
Detailed field work by Coltelli et al. (1998, 2000) has led to the discovery and study of numerous large-scale explosive events over the past 100,000 years. The most recent - and apparently one of the most violent - of these occurred in 122 BC. That eruption had until recently been attributed to a flank vent (M. Trigona), located at 425 m elevation on the SE flank near the village of Trecastagni, which is associated with a lava flow. Historical records report strong tephra falls that caused most of the houses in Catania to collapse. Damage was in fact so widespread that the inhabitants of Catania were exempted from paying taxes to the government of Rome for ten years. Coltelli et al.'s research indicates that the eruption occurred from the summit area rather than from a flank vent, and therefore must have been of exceptional violence. Furthermore the study shows that pyroclastic flows occurred in the summit area, but surges rushed much farther downslope to reach areas that are now densely populated. A similar event today would certainly cause much damage and, due to its suddenness, there would be a direct risk to human lives. It is of paramount interest to understand the dynamics that could lead to an event of such proportions, and Coltelli et al. (1998) propose that the 122 BC eruption was caused by a sudden decompression in the central conduit system, which in turn was a result of major fracturing and extension in the upper part of the volcano. The magic word in this context is volcano instability (see the next section, III.3.), which opens the stage to a spectrum of worrisome scenarios, none of us would desire to see become reality.
Evidence of still larger, and more devastating, eruptions in the past is readily accessible in spectacular outcrops near the town of Biancavilla on the lower SW flank of Mt. Etna. This evidence consists of thick deposits of pyroclastic flows, also known as "nuées ardentes" or glowing avalanches, which rushed down the flanks of an ancestral Etna volcano (named "Ellittico" by geologists) some 13,000 to 15,000 years ago. The composition of the magma from which these pyroclastic flows were derived is different from that of the magmas erupted since then and indicates that before that eruption a large quantity of magma accumulated in a shallow reservoir below the volcano. This process is assumed to be the main reason for the exceptionally explosive and voluminous activity at the end of the "Ellittico" volcanism at Etna, and nothing indicates that a similar process is occurring today. For an eruption of this magnitude and character to occur thousands of years of magma accumulation and chemical evolution would be needed; at present it seems that Etna is erupting most of the magma it receives from a mysterious source at depth (see "The storage and transport of magma: The search for the hidden magma chamber") and there is no evidence of magma accumulation in a large shallow reservoir.

III.3. Volcano instability and sector collapse

One of the most hazardous processes that can occur at a volcano, active or not, is the collapse of one of its flanks leading to a huge avalanche of volcanic debris, a process that is generally known as sector collapse. The famous eruption of Mount St. Helens in Washington (U.S.A.) on 18 May 1980 was triggered by such a sector collapse, which generated a series of devastating events, starting with a debris avalanche immediately followed by a laterally directed explosion (or blast) and a Plinian eruption. During the years following that eruption it was discovered that a surprising quantity among the volcanoes on Earth had experienced similar events, but many of these had been of far greater magnitude than the 1980 collapse, debris avalanche and eruption at Mount St. Helens. Sector collapse soon became known to leave peculiar morphological features in the sides of the affected volcanoes, so-called collapse amphitheaters, and distinct deposits showing a hummocky surface were interpreted to be the remainders of the collapsed volcano flanks, which had advanced at awesome speed for up to tens of kilometers from their original positions. It was logical that the concept of sector collapse would eventually be tested in the case of the Valle del Bove on the E flank of Etna.
Since the mid 1980s and especially since the early 1990s various groups of researchers have proposed an origin of the Valle del Bove by one or more sector collapse events (Guest et al., 1984; Borgia et al., 1992), the later hypotheses associated such events with the gravitational spreading of the E flank of the volcano which is open to the sea. Gravitational spreading was seen as a possible cause - or a possible result - of repeated intrusions of magma in a set of fracture systems extending from the summit area to ENE and SE, actually these fracture systems have been particularly active since 1971. A major problem in painting the picture was the apparent lack of a deposit that could be clearly interpreted as a debris avalanche deposit, but in 1998 Calvari et al. presented evidence for such a deposit which they dated at no less than 8400 years before present. More recently, a group of researchers from the U.K. has begun a study of rocks on the S rim of the Valle del Bove, which led them to the preliminary conclusion that the latest collapse event in the Valle del Bove occurred only some 3500 years ago. The data presented by Calvari et al. (1998) and by Deeming et al. (2001) indicate that catastrophic sector collapse occurred at least twice during the Holocene (that is, during the past 10,000 years) at Etna. This is not a happy bit of news. Geological events that happened in the past are very likely to be repeated in the future. Sector collapse is the most dramatic scenario to be envisaged at Etna, and various processes observed at the volcano in the past few decades are interpreted by some geologists to point to continued volcano instability which might eventually lead to further collapse in the upper parts of the Valle del Bove.
One of these is the frequent intrusion of magma into the fracture systems that bound the W wall of the Valle del Bove. Each time such an intrusion take place in the area (this has happened five times since 1980), the area to the E of the intruding dike is displaced eastward, and this area is the steep W face of the Valle del Bove. In other words, the upper part of the Valle del Bove headwall is pushed away from the rest of the mountain. This is a steep slope about 1000 m high, and if a large part of it were to transform into a major landslide, it would fall from a height of may hundreds of meters toward the valley floor, which would give the falling mass an incredible acceleration and momentum. The resulting debris avalanche would speed eastward across the floor of the Valle del Bove and almost surely extend far beyond its lower end into the densely populated area below, possibly down to the Ionian Sea. All this would occur within a few minutes. Devastation would be beyond imagination, and, worst of all, the present state-of-the-art of volcanology has few means to predict events of this kind in time. It can simply be hoped that those who believe that Etna is prone to further collapse in the near future are wrong, and that the volcano will rather behave in the same manner it has done during the past 2000 years.
But even if a sector collapse of catastrophic dimensions is probably a remote possibility, the eyes of many scientists are fixed on the W rim of the Valle del Bove as this area is moved eastward by one intrusion after the other, at intervals of a few years, and on the SE Crater, whose cone sits immediately on that mobile rim and which has grown at an unbelievable speed since the late 1990s. Can the rapidly increasing weight of that cone, plus the weight of several voluminous lava flow fields emplaced since 1999 on the W rim of the Valle del Bove, destabilize that rim and trigger its collapse? What if such a collapse entrains the SE Crater cone, exposing its conduit and all the magma it contains to the fresh air and instantaneous decompression? Wouldn't this be a perfect scenario uniting the hypotheses of sector collapses and major explosive eruptions in the past? The truth is, no one knows. Most scientists studying Etna prefer to assume that the volcano will do the same things they have seen personally during their many years of observations of the volcano and its activity, and it is very likely that things will exactly go that way. But at times, like a nightmare, the vision of that area plunging into the Valle del Bove and beyond, leaving behind it a mass of decompressing magma that transforms into a huge explosion, haunts the minds of some of us. This is where volcanologists can only wish that the more catastrophism oriented colleagues possess a vivid phantasy. Most of the hazards described on this page can be mitigated if the involved people and institutions are willing to reason and to collaborate. A sector collapse and all that it might unleash simply must not take place, not here and not now.


Copyright © Boris Behncke, "Italy's Volcanoes: The Cradle of Volcanology"

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