Volcano instability on the Earth and other planets

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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 years.

Photo taken in by Boris Behncke. 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 two deaths , nine deaths , and two deaths ; in all cases the victims were visitors to the summit crater who were surprised by sudden steam-blast phreatic explosions.

Volcano Instability on the Earth and Other Planets

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 , 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. 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. In most cases the losses have been cultivated land, but on a number of occasions buildings have been destroyed.

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More rarely have population centers been impacted and partly or completely destroyed — during the past years this has happened only three times, in , , and 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.


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In a recent study, Behncke et al. 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.


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  • Top image shows the rough subdivision by Behncke et al. 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. 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 one of these episodes, on 26 April , a passenger airplane starting from Catania airport with more than 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 and flank eruptions, and again during the 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. 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 years.

    Obviously, tephra falls will be locally devastating if a flank eruption occurs close to the populated areas, as in 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.


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    • 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.

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      Building codes are applied for new constructions since the s, but an amazing quantity of residential buildings as well as hospitals and school buildings were constructed during the s to s without applying any codes, so that a tremendous number of such buildings are vulnerable. Volcanic sector collapse is known to have occurred at least once during the history of Etna, about 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.

      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 and , 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 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 Photo courtesy of Jean-Claude Tanguy, published in Behncke 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 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.

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      Similar events occurred in at the Northeast Crater, repeatedly during the numerous lava fountaining episodes from the Southeast Crater in and possibly also during similar events in early , and more recently, on 10 May and on 8 April A very different scenario was the one we encountered on 25 October , during the one-month-long eruption that filled the Bocca Nuova to overflowing Behncke et al.

      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 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 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 , 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 , 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.

      Different interpretations of the causes of the larger flows were proposed by Norini et al. 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.

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      Dramatic sequence of photos showing development of a large pyroclastic flow from the southeastern base of the Southeast Crater cone on 16 November , photographed from about 1. 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 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 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, years ago, and during the B.

      Plinian eruption. Magellan radar image data of Sapas Mons, a km diameter volcano located on the flanks of the Arla Rise, permit the distinction of widespread volcanic units on the basis of radar properties, morphology, and spatial and inferred temporal relations, each representing a stage or phase in the evolution of the volcano. Six flow units were identified and are arranged asymmetrically about the volcano. Although there is some evidence for overlapping of units, the stratigraphy clearly indicates a younging upwards sequence. The estimated volume of this 2. Although it is difficult to clearly identify a single lava flow, estimates of apparent single flow volumes range from 4 km 3 for an average unit 5 flow of 3.