asked questions about Etna
The answer is: under certain favorable circumstances (high elevation of the eruptive vents, low mass eruption rate, enough time to carry out the necessary preparations) a diversion of lava flows is possible. There are various limits to this possibility, which will be evident in the following discussion of the historically documented attempts to divert lava flows at Etna.
Interventions to change the course of lava flows coming from flank eruptions on Etna have a certain tradition. It was here that the first historically documented attempt of a lava flow diversion away from threatened areas was undertaken, during the 1669 eruption whose lava flow was menacingly advancing towards Catania. A group of men from Catania ruptured the walls of a channel along which the supplied lava to its front near the town in order to force it out of its bed which served as a protection from heat loss. From descriptions of this manoeuvre in historical sources it is apparent that it was crowned by some success - at least from a technical point of view, but the effect of the successful diversion of the lava flow was that now it threatened another town (Paternò), whose inhabitants prevented the Catania team from maintaining the breach in the lava channel wall open. As a result, the breach closed, and lava continued to advance towards Catania where it caused significant damage. Less known measures to prevent the spread of lava flows within the city of Catania during the same eruption include the construction of barriers across the main roads leading to the center of the city which actually halted the advance of the lava and saved the center of the town from destruction.
The principal attempt of lava diversion in 1669 led to the creation of a particular legislation which prevented the diversion of lava flows at Etna, a law that was valid until 1983. In May of that year, a lava flow from a south flank fissure at about 2300 m elevation advanced slowly in the direction of the towns of Belpasso and Nicolosi (although it was still several km from both), having already consumed parts of the ski lifts, several restaurants and other tourist facilities, isolated houses (many of them constructed illegally), arable land, forests and various sections of the road from Nicolosi to the Rifugio Sapienza area (at about 1900 m elevation). It has been a matter of controversy whether there was any real threat to the towns, but in any case it was decided that the lava flow be diverted by means of blasting the walls of its main feeding channel with explosives. This operation was carried out on 14 May 1983 and was described as a partial success; technical problems during the preparation of the intervention prevented the complete destruction of the lava channel wall, and only a small fraction of the flow was diverted for short time into an artificial bed.
It was during the same (1983) eruption that another kind of measure was used to protect several major buildings which were immediately threatened by the lava flow (Rifugio Sapienza, the Astronomical Observatory and the Grand'Albergo dell'Etna), this consisted in the construction of earthen barriers parallel to the flow margins. It was thus aimed to prevent the lateral spreading of the lava field towards those structures, and in fact none of them were destroyed.
Nine years later,
a lava flow from a fissure high on the southwestern wall of the Valle
del Bove gradually advanced towards the town of Zafferana on the southeastern
flank, and a new series of protective measures were introduced to halt
the lava flow or at least slow its speed. The first such measure was the
erection of a large earthen barrier at the end of Val Calanna, a southern
outlier of the Valle del Bove, perpendicular to the flow direction. As
expressed explicitly by Barberi et al. (1993), this barrier was not intended
to stop the advance of the lava flow but to temporary slow it, in order
to let some time pass during which other protective measures could be
taken or the eruption could end. The basin behind the barrier was gradually
filled by overlapping lava flows, and then the lava eventually began to
spill over the crest of the barrier and down into the narrow valley leading
down to Zafferana. A series of hastily erected minor barriers at lower
elevations in the valley (again perpendicular to the flow direction) were
rapidly overwhelmed by the advancing lava which destroyed orchards and
a few small buildings.
Although the 1992 operation appears to have been a success from a technical point of view, it has been a matter of debate whether the threat for Zafferana was as great as indicated by supporters of the intervention, and whether the lava tube would have been re-established had not the mass eruption rate diminished right at the time of the successful blasting operation. In any case, the final 1992 blasting operation is regarded a success, with some consequences to the attitude of the people living near the volcano. In interviews made during the late 1990s with people from the Zafferana and Nicolosi area, when asked about their fear of a future eruption, many of them expressed that they had no fear because "when there will be a lava flow it will be diverted anyway". However, the prospects were not all that simple at that time, and the July-August 2001 eruption was to teach a few interesting lessons.
During the July-August 2001 eruption on the southern flank, three (out of a total of seven) eruptive fissures delivered lava flows that caused damage and threatened to cause much more extensive destruction, especially in the area of the Rifugio Sapienza and the Etna cable car, where there are numerous tourist facilities (hotels, restaurants, souvenir shops). In order to limit destruction to a minimum, earth barriers were erected and channels were excavated, which were hoped to prevent the menacing lava flows from extending laterally. Two major lava surges, on 26 and 30-31 July, rapidly spilled down the steep slope above the threatened structures and were largely confined by the barriers and artificial channels, with minor overspills occurring in a few locations. One of these came to within less than 50 m of the departure station of the cable car, another nearly reached a building of the Provincial Tourism Agency, but both were relatively small and short-lived, and the latter was cooled by spraying water onto its front.
In eruptions like those of 1983 and 1991-1993 which were characterized by low effusion rates and whose eruptive vents lay at higher elevations than 2000 m, there is a good chance that the lava will take quite a while before reaching inhabited areas - if at all, since at low effusion rates the lava tends to form overlapping and adjacent flows of approximately the same length instead of forming a single long flow which simply extends further and further downslope. A diversion is possible where there are pathways for an artificial flow not occupied by areas and structures of value. If, on the other hand, an eruption were to occur in the densely populated area on the lower southeastern flank of Etna, even at low effusion rates the problem would be one of lacking space where a lava flow could be diverted. The problem would be much more evident in the case of a high-effusion rate eruption, even when the eruptive vents are located relatively high on the mountain. The March 1981 eruption serves as a dramatic example: in this event, a system of eruptive fissures rapidly propagated down the north-northwestern flank, feeding voluminous lava flows that advanced frighteningly swiftly towards the area of Randazzo and the small village of Montelaguardia lying to its east. Fortunately, the main phase of the eruption lasted less than two days, and the main lava flow narrowly missed Randazzo and Montelaguardua, passing right midway between these towns. However, during its advance the flow overwhelmed numerous isolated buildings and interrupted two main roads, two railway lines, and power supply to Randazzo. In the case of the 1981 eruption, there would have been no chance of a lava diversion.
The necessary conclusion
is that although some possibilites to control and influence the advance
of a lava flow do exist, they are limited to eruptions from vents at high
elevations which are characterized by low effusion rates, in sectors of
the volcano where there is space to accomodate an artificial lava flow
produced by diversion without the production of significant damage.
Copyright © Boris Behncke, "Italy's Volcanoes: The Cradle of Volcanology"
Page set up on 24 February 1998, last modified on 18 February 2002