I stand corrected, the portion of Etna most likely to collapse will be West, a steep slope 1000 metres above sea level.


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.

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Thank God for magnetism.