Morphologic features: The Valle del Bove
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, on clear days, 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.
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.
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.
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 intriguing (Deeming et al. 2001; K.R. Deeming, personal communication, 2003). 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 about 6800 years ago - still more recently than the date given by Calvari et al. (1998). To arrive at this age, they measured the abundance of a rare isotope of helium, 3He. This isotope appears in rocks exposed to cosmic rays that enter the Earth's atmosphere - that is, rocks that are not covered, for instance, by flowing water in rivers. The duration of exposition of a rock can be measured from the abundance of 3He isotopes. Such measurements were made in various fluvial features (valleys of rivers or torrents) that extend from the Valle del Bove rim down the outer slopes of the volcano. The existence of these incisions shows that water once flowed here and was fed by sources on a higher slope that disappeared when the Valle del Bove collapsed, causing the cessation of water flow. Prior to collapse, there were probably ice and snow fields in the summit area of the volcano, which fed a number of rivers and torrents. One of these palaeorivers flowed through a narrow valley on the southeast flank of Etna, now known as "Acqua della Rocca".
valley locally known as "Acqua della Rocca" (formally named
Vallone Acqua Rocca degli Zappini) forms a deep notch in the rim of the
Valle del Bove that is clearly visible above the Piano del Vescovo, a
popular picnicking area for local people on sundays and holidays. A hiking
trail leads up through the narrow valley, through lush forests, to the
rim of the Valle del Bove, from where there is a breathtaking view across
this enormous, lava-covered depression. About half way up in the "Acqua
della Rocca" valley, there is a steep scarp about 80 m high, which
appears strongly polished in its central portion. This effect is a result
of water that once cascaded down the scarp, forming a spectacular fall.
However, water has ceased to flow through the valley and down the scarp
long since, because the source of the water disappeared. The studies carried
out by the Manchester group of scientists revealed that the cessation
of water flow coincides with the collapse of the slope of the volcano,
which led to the formation of the Valle del Bove.
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).
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.
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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, and for submitting updated information. A publication on this subject is in preparation.
Copyright © Boris Behncke, "Italy's Volcanoes: The Cradle of Volcanology"
Page set up on 11 May 1999, last modified on 8 February 2004