The 1991-1993 Valle del Bove eruption

Etna produced its most voluminous and longest-lasting flank eruption since about 300 years between 14 December 1991 and 30 March 1993. Much of the southern part of Valle del Bove was filled with a thick compound lava field, with maximum thicknesses exceeding 100 m. About 205-250 m3 of lava were produced during the 473 days of the eruption at an average rate of 5-6 m3 per second. The eruption, though not particularly vigorous in terms of explosivity and eruptive rates, became the most publicized eruptive event at Etna so far, mostly due to the (at least partially) successful diversion of lava flows that threatened the town of Zafferana Etnea. The lessons from the 1991-1993 eruption should, however, not imply that man is now capable of controlling the volcano. Several favorable factors allowed the relatively successful lava diversion:
1) the low effusion rate. At a higher effusion rate, lava may have been delivered through more than one main lava tube or flow channel, which would have rendered any effort of diverting the lava more difficult. In other flank eruptions with high effusion rates, such as that of 1981 on the NNW flank, broad, voluminous lava flows advanced very rapidly towards towns and agricultural areas, without forming significant flow channels or tubes, and therefore no diversion would have been possible.
2) the high elevation of the eruptive vents. The eruptive fissure lay between 2350 and 2200 m altitude and therefore far away from inhabited areas. The lava flow had to travel about 8 km before it became a serious threat, and it took about 4 months to cover this distance. In the 1981 eruption, the lowest eruptive vents lay at 1150 m elevation, and the main lava flow covered a distance of almost 8 km in little more than 40 hours.
3) the drop in the effusion rate shortly after the final blasting operation. After that operation, carried out on 27 May 1992, lava did not travel much farther than 3 km from the diversion site. Had the effusion rate remained constant, then it would have been easy for the lava to build another lava tube system similar to that formed in the first months of the eruption, and this could have led to another advance of the lava fronts to the area of Zafferana.
4) the possibility to divert the lava far away from the threatened areas. The diversion was carried out at little below 2000 m elevation, and about 7 km from the nearest village. It did thus not result in a new threat to another area, because all the diverted lava could accumulate in an uninhabited area, far away from cultivated or inhabited land.

Map of the 1991-1993 lava flow-field

Map of the eastern flank of Etna, showing the lava flow-field of the 1991-1993 eruption in the southern part of the Valle del Bove, and key locations related to events during the eruption. The lavas erupted during the first phase of the eruption (14 December 1991) are shown in different color than the main lava flow. After the final diversion of the lava flow on 27 May 1992 (the site of this operation is indicated by a yellow asterisk), lava continued to accumulate only in the upper third of the lava flow-field, in the area where it is largest

	Left: Strombolian activity and lava emission from the main eruptive fissure onSW wall of Valle del Bove, seen in late-December 1991 from the Monte Zoccolaro area on the S rim of the Valle. Explosive activity at this fissure lasted only a few weeks, but lava continued to flow for one year and three and a half months, making this the longest eruption of Etna since the 1614-1624 eruption, and its most voluminous eruption since1669. However, the effusion rate was low: in 1669, about three times as much lava was produced in four months Right: Lava flows spilling down into the Valle del Bove at nightfall in late-December 1991, seen from the same viewpoint as photo at left. Both photos were taken by Carmelo Monaco
	Lava flows in the Valle del Bove on the evening of 20 March 1992, seen from Monte Pomiciaro on the S rim of the Valle. Lava issues from countless ephemeral vents (that is, short-lived vents where lava broke to the surface from lava tubes) as can be seen in both images. The photo at right shows the view down across the Val Calanna, an offspring of Valle del Bove near its SE end. Lava is gradually filling a basin behind an artificial earth barrier intended to halt the approach of the flow towards Zafferana
	Left: Lava in Val Calanna, above Zafferana which is out of the view to the right, on the evening of 20 March 1992. View is from Monte Pomiciaro. Lights in the background are of the villages of Milo and Fornazzo, not threatened by the lava flow. Right: One of the few buildings destroyed by the 1991-1993 lava flow near Zafferana. The owner wrote "Grazie governo" (Thank you Government) on its wall before it was surrounded by the lava flow, hinting to the "inactivity" of the authorities. Attempts to divert the lava were undertaken shortly after. Photo was taken almost one year after destruction, in February 1993
The following photos were taken by Giuseppe Scarpinati
	Photos taken on 1 January 1992, about two weeks after the onset of the eruption. Left photo shows lava spilling down the Salto della Giumenta, above Val Calanna. Right photo is a panoramic view of the southern part of the Valle del Bove with lava flows issuing from the eruptive fissure on the SW wall of the Valle, seen from Monte Zoccolaro on the S rim of the Valle del Bove
	Photos taken on 4 January 1992. Left: Black lava covers the Salto della Giumenta at left, and accumulates at its base in Val Calanna. The low hill in the center of the image is Monte Calanna Right: In an effort to halt the advance of the lava, an earth barrier is built across the outlet of Val Calanna, above the town of Zafferana
	Two close-up views of flowing lava in the upper Valle del Bove on 7 March 1992. Right photo also shows a peculiar natural sculpture at upper left, possibly a remainder of a drained lava tube

Active lava lobe extruding from an ephemeral vent (a vent located at the lower end of a lava tube) in Val Calanna, on 8 March 1992. The lava is gradually filling the basin behind the artificial earth barrier erected two months earlier; one month after this photo was taken, the lava spills over the crest of the barrier

Left: Giuseppe Scarpinati standing on what a few days before was the crest of the earth barrier, next to lava spilling over the barrier into a narrow valley leading directly towards the town of Zafferana. Three smaller barriers are hastily erected at ever lower elevations in that valley, only to be overrun after few hours by the advancing lava. Photo was taken on 11 April 1992 Center: A flow of bulbuous lava is burying a vineyard at painfully slow speed in a locality known as "Piano dell'Acqua" near Zafferana, as the lava flow is approaching alarmingly close to the eastern part of the town. This photo was taken on 25 April, when the first major surge of lava towards Zafferana occurred Right: A view towards W from the hills above Zafferana on 26 April 1992. Steaming lava is seen at left in the Val Calanna, from where the lava continues to flow towards Zafferana. The W wall of the Valle del Bove, covered with snow, is visible in the background

Left: An isolated building at Piano dell'Acqua above Zafferana, shortly after its destruction, on 9 May 1992, with an acknowledgement to the Italian government written on its wall by the owner. The lava has temporarily ceased advancing in the direction of Zafferana Right: A tongue of lava is burning trees above Zafferana, 9 May 1992

Lava flows burying fruit gardens above the Piano dell'Acqua, 9 May 1992. This spring is the last for many of the trees visible in these photos

	The photos above and at left were taken on 23 May 1992 in the area where efforts to divert the lava flow were carried out, in the upper part of the growing lava field in the Valle del Bove. The photos at upper left and right show the place where blasting of the main lava flow channel has resulted in a partial diversion into an artificial channel, with about three-thirds of the lava still flowing through the original channel (clearly visible in the left part of the upper right photo) into a tube that leads to the Piano dell'Acqua near Zafferana. Iron crosses visible in the photos were used to block the original flow channel, together with large concrete blocks. The final, and possibly successful, blasting of the flow channel was carried out on 27 May 1992

Three photos from the later stages of the 1991-1993 eruption, when lava continued to flow only in the upper third of the 1991-1993 lava flow-field. Left: Small lava flows gradually bury much of the suggestive landscape in the upper SW part of the Valle del Bove, including many dikes that were exposed in that area. Photo was taken on 18 July 1992 Center: Effusive vent, probably near the lower end of the 1991 eruptive fissure, at the base of the W wall of the Valle del Bove, on 8 September 1992 Right: A view of the SW wall of the Valle del Bove on 8 November 1992. A white gas plume is rising from the 1991 eruptive fissure, from which lava is still flowing in modest quantities

The winter of 1992-1993 has draped a thick white blanket over Etna, but the lava field forming since late 1991 remains black: lava is still flowing from the eruptive fissure, visible at the extreme left on the SW wall of the Valle del Bove. The photo was taken on 16 January 1993 from Monte Zoccolaro, which carries a large steel cross. Lava effusion ends in late-March 1993

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Copyright © Boris Behncke, "Italy's Volcanoes: The Cradle of Volcanology"