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A few thousand years ago, Etna's eastern flank suffered a catastrophic sector collapse, similar to that of Mount St. Helens on 18 May 1980. This means that a part of the volcanic edifice slid sidewards, forming a voluminous avalanche of rock, which rushed towards east, devastating and burying all that lay in its path, and possibly slamming into the sea. A huge depression was thus formed in the side of the volcano, now known as Valle del Bove (Valley of the oxen). Its north-south width is roughly 5.5 km while its extension from west to east is 7 km. In plan view it is roughly horse-shoe shaped, resembling a key hole. At its western end, the depression is bordered by an imposing wall up to 1000 m high (Serra Giannicola area) while it is open to the east; to the south and north there are spectacular crests hundreds of meters high from where a visitor enjoys commanding views of the Valle del Bove and its surroundings. The northern crest, named Serra delle Concazze, culminates in the Pizzi Deneri, where it actually delimits a subsidiary depression in the northeastern part of the Valle del Bove that is known as Valle del Leone (Valley of the lion). The southern crest is named Serra del Solfizio in its lower (eastern) part and Schiena dell'Asino in its upper part, which terminates with the prominent pyroclastic cone of the Montagnola, formed by a flank eruption in June-September 1763.
The origin of the name "Valle del Bove" remains somewhat mysterious. Actually the might suggest that it was once possible to use parts of it as pasture ground for cattle. Presently some 80-90 per cent of the floor of the Valle del Bove are covered by lavas erupted during the 20th century, making it one of the most hostile and least accessible areas on Etna. But this needs not always have been the state of the valley. There have been prolonged periods when little lava was deposited on the valley floor, like the 100 years following the 1669 eruption, and probably also a large part of the 17th century itself. It can be assumed that there have been similar periods before, when there was enough time for vegetation growth in the lower part of the Valle del Bove, especially when earlier lavas had covered by subsequent tephra falls, which facilitates plant colonization. During the frequent tephra falls from the Southeast Crater in 2000 a thick layer of scoriae was deposited on the 20th century lavas in the valley, and in the following spring grass was seen growing in numerous spots. If no new lavas bury significant portions of the area in the near future, the Valle del Bove might assume a green color within a few years, so that grazing cattle would become possible.

Distant views of Valle del Bove
	Photo 1 (left): View from the Giarre area of Etna with snow cover (in April 1997). The vast depression of Valle del Bove is visible below the main summit cone, centered slighly to the left of it. The two major "shoulders" on both sides of the main summit cone are the southern (left) and northern (right) rims of the Valle del Bove. Photo 2 (right): A similar view, but during the summer (August 1997), and from a close location.

The Valle del Bove is a key area for studies of the geological evolution of Etna, for in its walls the traces of numerous older edifices pre-dating the presently active volcano are exposed. It has been here that the first evidence for the eventful history of the volcano has been found in the mid-19th century, and most of what is known now about the sequence of constructive and destructive events in the course of the past tens and hundreds of millennia has been derived from geological research within the Valle del Bove.
The circumstances and age of the collapse event - or series of events - that generated the Valle del Bove have remained mysterious for a long time, and a number of hypotheses regarding the origin of the collapse depression have been forwarded in the past 150 years. Various authors proposed ages varying between 5000 and 80.000 years of the Valle del Bove. Lyell (1830, 1849, 1858) discussed various hypotheses regarding the origin of the Valle del Bove: caldera collapse, marine erosion, and fluvial erosion. In 1947, the Catania based scientist Vagliasindi published two extensive papers in which he postulated that not collapse but glacial erosion had scoured out the Valle del Bove, but this hypothesis was rejected by most subsequent authors (whereas recent research indicates that glaciers were indeed present on Etna during the Pleistocene, and possibly into the Holocene). Klerkx (1970) attributed the Valle del Bove to caldera collapse of the Trifoglietto edifice (see the chapter on the geological evolution of Etna), but it is now known that it is much younger than Trifoglietto, as stated by Guest et al. (1984) who believed that it post-dates the Ellittico edifice.

Southern part of the Valle del Bove
	Photo 3 (left): A view from the southern rim of Valle del Bove (Schiena dell'Asino, see also photo 4) onto the southwestern part of the valley floor covered with black lava of the 1991-1993 eruption. Note the numerous dikes outcropping in the flanks above the valley floor - This and the following three photos were taken on 24 May 1998. Photo 4 (right): Detail of the preceding, with dikes cutting through more erodible pyroclastic deposits in the valley walls.
	Photo 5 (left): Zoom on the lower part of the 1991-1993 eruptive fissure on the southwestern wall of Valle del Bove. Large upper vent was the site of Strombolian activity during the first weeks of the eruption while lava flowed mainly from the vent rimmed by white sublimates further downslope. Photo 6 (right): Looking up the Schiena dell'Asino (south rim of Valle del Bove) towards west. Dark cone at the skyline is Montagnola, a cone formed in the summer of 1763.

Without having any clear evidence (in the form of characteristic debris avalanche deposits such as those known from other volcanoes), various authors speculated that collapse of the Valle del Bove might have been related to a catastrophic gravitational collapse of the eastern flank of the modern Mongibello edifice, similar to the collapse of Mount St. Helens (Washington, USA) on 18 May 1980. Guest et al. (1984) suggest that actually there was a series of such collapse events. These speculations were stimulated by the results of detailed geophysical monitoring of the upper southern flank of Etna during eruptions in the 1980's, in particular the 1983 eruption (Murray and Pullen, 1984) which showed that each time when magma intruded under the flank of the volcano, in a direction more or less parallel to the southwestern rim of Valle del Bove, the area lying east of the intruding dike was forecefully displaced eastwards, that is, in the direction of the Valle del Bove. Indeed between 1983 and 1992, the southwestern rim of Valle del Bove was displaced between 5 and 6 m eastwards during four major events, increasing the instability of the steep western face of the Valle, and causing concern about future catastrophic collapse of that area. Until very recently, however, no deposits that could be clearly attributed to sector collapse and a resulting debris avalanche were found. Such deposits were believed to lie buried below a thick succession of fluvial debris and conglomerates, known as the Chiancone, on lower eastern flank of Etna below the eastern mouth of Valle del Bove and thus hidden from exposure. Calvari et al. (1998) finally succeeded in finding debris avalanche deposits in a few outcrops of the Chiancone area and thus found firm evidence for a debris avalanche responsible for at least the initial stage of formation of the present Valle del Bove. They reported a minimum age of 8400 years for the debris avalanche deposit and infer that the Chiancone deposits derive from remobilization and later fluvial reworking of the deposit.

Northern part of the Valle del Bove
	Photo 7 (left): Northern part of the Valle del Bove as seen from its northern rim (Serra delle Concazze) on 23 June 1998. Dark lava flows visible on valley floor are of the 1986-1987 and 1989 eruptions. Note that like on the southern side, the valley walls are traversed by numerous dikes. Photo 8 (right): Northeastern part of Valle del Bove and the summit cone complex seen from Serra delle Concazze, and 1986-1987 lavas in the foreground. The summit cones are NE cone (right), SE Cone (left), and, partly hidden by these two cone, the main summit cone.

But the fascinating story of the study of the Valle del Bove does not end here. During the past few years a group of scientists from Manchester University (U.K.) and the Benfield Greig Hazard Research Centre (London, U.K.) has done research on features related to what they believe was the pre-collapse volcanic edifice, and the preliminary results of these studies are haunting (Deeming et al. 2001; K.R. Deeming, personal communication, 2002). Using abundances of a rare isotope of helium, ³He, for dating of rocks in an area known as "Acqua della Rocca" on the outer wall of the Valle del Bove on the southeastern flank of Etna, they revealed that the catastrophic collapse of the Valle most likely occurred much less than 5000 years ago and correlate it with a major eruption about 1500 BC, which is decribed by Diodore of Sicily (Diodorus Siculus). While the studies so far have not revealed clear geological evidence of deposits of eruptive activity related to the collapse, it is possible - if not likely - that like at Mount St. Helens the sudden unloading of a part of the shallow plumbing system triggered a major explosive eruption. Coltelli et al. (2000) report an age of 3210-3090 years before present for a tephra layer consisting of fall and pyroclastic flow deposits on the upper northeastern flank of Etna, which they interpret as a product of phreatomagmatic activity. This might match with the dates (2600-4800 years before present) attributed by Deeming and co-workers to the Valle del Bove collapse.
Diodore's writings indicate that the Sicans, an indigenous people living on the eastern flank of the volcano, were forced by the eruption (and/or the landslide) to migrate to the western part of Sicily. An event of this type can be expected to have caused total destruction and heavy loss of life in the affected areas. Since the "1500 BC" event is the earliest historically recorded eruption of Etna, the volcano entered into the historical record in quite a cataclysmic manner. In the light of earlier studies by Calvari et al. (1998) this event seems to be but the latest episode in a series of sector collapses, as suggested by Guest et al. (1984).

REFERENCES

Calvari S, Tanner LH and Groppelli G (1998) Debris-avalanche deposits of the Milo Lahar sequence and the opening of the Valle del Bove on Etna volcano (Italy). Journal of Volcanology and Geothermal Research 87: 193-209.

Coltelli M, Del Carlo P and Vezzoli L (2000) Stratigraphic constraints for explosive activity in the past 100 ka at Etna volcano, Italy. International Journal of Earth Sciences (formerly Geologische Rundschau) 89: 665-677.

Deeming KR, Harrop PJ, Turner G and McGuire WJ (2001) Catastrophic lateral collapse at Mount Etna in historical times (abstract). European Geophysical Society, 26th General Assembly Nice (France). Download file in PDF format

Guest JE, Chester DK and Duncan AM (1984) The Valle del Bove, Mount Etna: its origin and relation to the stratigraphy and structure of the volcano. Journal of Volcanology and Geothermal Research 21: 1-23.

Lyell C (1830) Principles of Geology (1st edition). J Murray London.

Lyell C (1849) On craters of denudation, with observations on the structure and growth of volcanic cones. Quarterly Journal of the Geological Society of London 6: 209-234.

Lyell C (1858) On the structures of lavas which have consolidated on steep slopes; with remarks on the mode of origin of Mt. Etna, and on the theory of "Centres of Elevation". Philosophical Transactions of the Royal Society of London 148: 703-786.

Kim Deeming (Isotope Geochemistry Group, Manchester University, U.K.) is acknowledged for permitting the presentation of preliminary information about research on the Valle del Bove collapse on this site. A publication on this subject is in preparation.