Etna Week (Part 3) - Etna's Volcanic Hazards

The final part of Etna Week, brought to us by guest blogger Dr. Boris Behncke. Check out Part 1 and Part 2 as well!

Etna Volcanic hazards
By guest blogger Dr. Boris Behncke.

Etna is one of the most active volcanoes on Earth, and a population of nearly one million people dwell on its flanks, many in areas that have been repeatedly invaded by lava flows during the historical period. A few villages have been constructed very close to the vents of eruptions only a few hundred years old.

Top: Residential areas surrounding numerous pyroclastic cones on the lower southeast flank of Etna, seen from Monte Arso, a cone that erupted in the late Middle Ages looking toward the metropolitan areas of Acireale and Catania. The cones seen in this image are all prehistoric but just outside the field of view are a few cones that erupted during the past 2000 years. Photo taken in 2000 by Boris Behncke. Bottom: A volcanologist's dream and nightmare - one day, a new crater will open and grow into a new cone in our backyard, like shown in this apocalyptic photomontage (the big cone formed during the 2001 south flank eruption projected onto a photograph of my hometown Trecastagni on the southeast flank of Etna). Both original photos by Boris Behncke

As mentioned before, few people are known to have lost their lives due to eruptions of Etna. During the last century, three deadly incidents are known, in 1929 (two deaths), 1979 (nine deaths), and 1987 (two deaths); in all cases the victims were visitors to the summit crater who were surprised by sudden steam-blast (phreatic) explosions. Amazingly, many people have escaped unscathed during a number of much more violent explosive magmatic eruptions, which, however, always showed a conspicuous buildup for some time before culminating. In contrast, phreatic explosions occur virtually without warning, as has been tragically demonstrated at Galeras volcano (Colombia) in 1993, when nine people, including six volcanologists, were surprised and killed by a relatively small explosion - they happened to be near the very crater (some were even within the crater taking gas samples). [For further detail on the Galeras incident, there are two rather gripping and contrasting stories, "Surviving Galeras" written by one of the survivors, Stan Williams (in collaboration with Fen Montaigne), and "No Apparent Danger" by former-geologist-turned-into-reporter Victoria Bruce'. I recommend to read Williams first and then Bruce, after which you may be having some sort of a balanced view of things.]

Volcanic hazards at Etna are: (1) lava flows, (2) tephra falls (and volcanic ash plumes endangering air traffic), (3) earthquakes related to eruptive activity and magma movement, (4) volcanic sector collapse, (5) tsunami, (6) pyroclastic flows.

Lava flows are by far the most common hazard at Etna. About half of Etna's historically recorded eruptions have caused damage to human property due to lava flow invasion. In most cases the losses have been cultivated land, but on a number of occasions buildings have been destroyed. More rarely have population centers been impacted and partly or completely destroyed - during the past 400 years this has happened only three times, in 1651-1653, 1669, and 1928.

Etna eating towns and villages - luckily this happens quite rarely. Top image is a reproduction of a fresco exposed in the sacristy of the cathedral of Catania, which neatly shows the erupting vent (Monti Rossi) low on the south flank of Etna and the lava flow being diverted around the city of Catania by its city walls; people can furthermore be seen fleeing on boats, others holding processions, and a few housewives hanging their laundry next to the hot lava to make it dry faster.

In a recent study, Behncke et al. (2005) for the first time tried to quantify the risk posed by lava flows, diving the Etnean area into six different zones of increasing hazard, from the coastal areas to Etna's summit. This work revealed a moderately high risk of lava flow invasion in a densely populated area on the southeast flank of Etna, including Trecastagni where I and my family are living. However, such hazard zonation is of relatively limited use for land use planners and civil defense, since the boundaries of different hazard zones are relatively vague and do not reflect the morphological variations of the terrain on a scale of a few tens to a few hundreds of meters.

Mapping the lava flow hazard at Etna. Top image shows the rough subdivision by Behncke et al. (2005) into six hazard zones, bottom image renders a much refined impression of the vulnerability to lava flow invasion based on the SCIARA lava flow simulation model (Crisci et al., 2010).

A much more sophisticated effort was thus launched in recent years, which involved several groups of scientists from various universities in Italy and abroad, and the Istituto Nazionale di Geofisica e Vulcanologia. I participated in some of this work, and the results are encouraging. The main means of defining the hazard from lava flow invasion, and fine-tuning the hazard zonation and the vulnerability of the Etnean area at very high resolution, is computer simulation of lava flows. Different models have been applied at Etna for the simulation of lava flows, which are described in much detail in publications by Crisci et al. (2010), Favalli et al. (2009), and Herault et al. (2009).

It is now possible to recognize those areas that would be impacted first - obviously those lying in morphologically low areas - and where to concentrate rescue efforts and salvage operations once the location of an imminent or starting eruption is known. The tens of thousands of computer simulations carried out during the project have not only served to produce very detailed hazard maps for Etna, they also produced virtually all possible eruption (lava flow) scenarios for any location on this volcano. These scenarios can be extracted with a few mouse clicks on demand, so that simulating new scenarios causing loss of precious time will not be necessary.

The risk of damage and disruptions caused by tephra fall has been seriously underestimated at Etna until recently, mainly due to the (false) notion of Etna being a non-explosive volcano. During the about 150 episodes of lava fountaining during the 1995-2001 "Millennium Fireworks" at the summit craters, heavy showers of ash and scoria (very porous, black, centimeter-sized fragments of lava) occurred frequently on the flanks of Etna, causing damage to crops, and sometimes breaking car windshields and disrupting road traffic. During one of these episodes, on 26 April 2000, a passenger airplane starting from Catania airport with more than 100 passengers on board encountered the tephra plume and falling scoria cracked its windshield, whereupon the airplane had to return for an emergency landing in Catania. Since that incident, air traffic is severely restricted during explosive eruptions at Etna.

This became a particularly biting issue during the prolonged ash falls from the 2001 and 2002-2003 flank eruptions, and again during the 2006 summit eruption. For periods of days to weeks, the airport of Catania remained closed, sometimes even the airport of Reggio Calabria, on the Italian mainland about 70 km northeast of Etna, had to be closed as well. For this reason, people in Sicily were not particularly shocked when Iceland's Eyjafjalljökull brought all air traffic in Europe to a grinding halt in the spring of 2010 for a few days, including northern Italy.

The revelation that ash-producing flank eruptions are far more common at Etna than previously thought indicates that the population around Etna and people travelling from and to Sicily in airplanes will experience further disruptions due to ash falls about once every 10-20 years. Obviously, tephra falls will be locally devastating if a flank eruption occurs close to the populated areas, as in 1669. A similar event would bury the villages to the east and southeast - including Pedara, my home town Trecastagni, Mascalucia, Tremestieri, and a few more - under up to several meters of tephra. In this moment, I know of no preparations for such a case, and educating the public (administrators and inhabitants) to create an awareness of this and other hazards is overdue.

Earthquakes accompanying the movement of magma or caused by magma-induced flank displacement are frequent on the eastern, southeastern, and southern flanks of Etna, and often cause significant material damage and occasionally kill people. Such events cannot be predicted, and prevention such as earthquake-resistant construction is essential. Building codes are applied for new constructions since the 1980s, but an amazing quantity of residential buildings as well as hospitals and school buildings were constructed during the 1960s to 1980s without applying any codes, so that a tremendous number of such buildings are vulnerable. This is an issue of unimaginable proportions, and extends far beyond Etna's magma-related seismic activity, because all of eastern Sicily is a high-risk seismic zone due to the presence of several major regional tectonic fault systems, and a number of large population centers such as Catania and Messina lie in this area, having a building stock of which maybe 20 per cent would resist (not collapse) during a major earthquake.

Volcanic sector collapse is known to have occurred at least once during the history of Etna, about 9000 years ago, forming the Valle del Bove. This event is believed by some researchers to have caused a massive tsunami, which ravaged the coasts around the eastern Mediterranean (Pareschi et al., 2006). Although the flanks of Etna continue to be severely affected by instability, the risk of a major sector collapse and related tsunami is currently considered low.

Pyroclastic flows are a fairly new discovery at Etna, although this volcano has proved more inventive in different mechanisms to generate such flows than any other volcano. I have had the doubtful privilege to witness small pyroclastic flows in the summit area of Etna on two occasions, in 1999 and 2006, and at very close range (less than 1 km), and a few colleagues have made similar experiences. These flows were showing nearly all the characteristics of pyroclastic flows on other, generally more explosive volcanoes, but were - luckily - very small and some apparently were much cooler than their more common counterparts.

Pyroclastic flow caused by the collapse of an oblique eruption column from the Southeast Crater, on 16 April 2000. Note the numerous people near the building in the left foreground, immediately after this photo was taken they fled downslope, and no one was touched by the flow. Note the large pyroclastic fragments in the air above the pyroclastic flow. Building is Torre del Filosofo, which was buried under tephra during the flank eruption of 2002-2003. Photo courtesy of Jean-Claude Tanguy, published in Behncke (2009).

During the past 25 years, small pyroclastic flows have occurred on at least 10 occasions in the summit area and on the upper flanks of Etna. A few were caused by collapse of eruption columns, which is one of the most common mechanisms of pyroclastic flows worldwide. A fine example of this type occurred on 16 April 2000 at the Southeast Crater, when a heavily charged pyroclastic jet shot out obliquely from an opening flank vent, the heavy downpour of gas-charged fragments developing into a pyroclastic flow that passed a few hundred meters from dozens of spectators, luckily without reaching any of them. Similar events occurred in 1986 at the Northeast Crater, repeatedly during the numerous lava fountaining episodes from the Southeast Crater in 2000 (and possibly also during similar events in early 1999), and more recently, on 10 May 2008 and on 8 April 2010.

A very different scenario was the one we encountered on 25 October 1999, during the one-month-long eruption that filled the Bocca Nuova to overflowing (Behncke et al., 2003). On that day, magma pushing through hot, though largely solid material filling the crater, uplifted a portion of that material, raising it like a lava dome and thrust it over the crater rim onto the steep outer flank of the central summit cone. The flank of this dome-like mass steepened, becoming unstable and collapsing like the flanks of a growing silicic lava dome, much the same way as the lava domes of Soufrière Hills on Montserrat or Merapi in Indonesia. The collapsing masses of hot, gas-charged rock transformed into small pyroclastic flows that traveled at a speed of about 70 km per hour, and some of us were just a few hundred meters away from these flows.

Ground and aerial views of the small pyroclastic flows that formed during the Bocca Nuova eruption on 25 October 1999. These flows were generated by the collapse of a mass of hot lava, which was pushed from inside the crater over its western rim. Note the vigorous Hawaiian-style lava fountains in both images. Photos by Marco Fulle (top) and Marco Neri (bottom)

The most impressive pyroclastic flows seen in recent years at Etna were those of November 2006, and they were also the most enigmatic in terms of the mechanisms which generated them. On 16 November, during one of many eruptive episodes at the Southeast Crater between July and December 2006, lava issuing from the summit of the cone interacted explosively with wet, hydrothermally altered rocks into which it was eroding, causing numerous small and two larger (up to 1.5 km long) pyroclastic flows (Behncke et al., 2008). Different interpretations of the causes of the larger flows were proposed by Norini et al. (2009) and Ferlito et al. (2010), the earlier proposing a purely gravitationally induced collapse of the cone (the pyroclastic flow was in fact described as a debris avalanche by Norini et al., 2009), the latter envisaging a sudden decompression of shallow magma when the cone's flank collapsed. These scenarios fail to take into account that removal of a significant portion of the cone was not an instantaneous event but occurred over more than 6 hours at a rather slow speed, a process that I and numerous colleagues had observed since early on that day. But whatever the details of the causes, the pyroclastic flows were large enough to engulf people had they travelled southward rather than southeastward. The temperature was probably low, because plastic-coated wooden signs placed along a tourist path had not suffered any heat effects, but the mechanical impact might have been deadly for any living being in the path of the pyroclastic flows.

Dramatic sequence of photos showing development of a large pyroclastic flow from the southeastern base of the Southeast Crater cone on 16 November 2006, photographed from about 1.5 km to the south. The first photo (upper left) shows the initial explosive jets that generated the flow, consisting entirely of ash, blocks, and water vapor, but little incandescent material. The abundance of water vapor indicates involvement of a large volume of wet, hydrothermal-fluid-soaked rock, which mixed and interacted explosively with hot lava flows. Building visible in the first three frames is what remains of Torre del Filosofo, largely buried under 2002-2003 tephra. Photos courtesy of M. La Rosa

Similar pyroclastic flows occurred on 24 November and were again observed at close range by a geologist (Robin Campion from Belgium), but no study of the deposits and on the triggering mechanisms was carried out in this case.

Finally, a fourth mechanism producing pyroclastic flows was discovered during an episode of lava fountaining and emission of voluminous, fast-moving lava flows from the Southeast Crater on 29 March 2007. In this case, a large lava flow encountered deep snow on a steep slope, and apparently disintegrated as snow melted and failed under the moving lava; this caused powerful explosions which in turn produced pyroclastic flows and mudflows that advanced for about 1 km downslope into uninhabited areas.

As far as can be understood from current knowledge, pyroclastic flows as those observed in the past few decades are a severe hazard for visitors to the summit area, but do not threaten the lives and property of the people living on the slopes of Etna. However, larger pyroclastic flows were generated during the cataclysmic eruptions at the end of the Ellittico stage about 15,000 years ago, and during the 122 B.C. Plinian eruption. Chances of an Ellittico-style event are extremely remote, because the magma composition is different today, but an event like in 122 B.C. cannot be fully excluded to occur even in the short term.

Much, much more could be told about Etna - such as the relationship of the volcano with the people who live next to it, and the various monitoring techniques now being applied by the Istituto Nazionale di Geofisica e Vulcanologia of Catania, as well as a number of groundbreaking research efforts. It would be well worth to indicate how Etna can be discovered by volcano and nature lovers without beating the common tourist paths. All this may come in a future guest blog - and, obviously, one day there will be a new eruption to inform you of. We are all waiting for it - hoping that we will still have a peaceful summer and maybe a peaceful Christmas, and a peaceful next year. One day time will run out, and be certain that you'll receive the news firsthand here on this blog.

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