Activity of the Voragine, 1995-2001

Italy's Volcanoes: The Cradle of Volcanology

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Voragine eruption, 22 July 1998

Powerful explosive eruption of the Voragine on 22 July 1998, seen from near Bronte, a town on the W flank of Etna. This eruption caused heavy tephra falls on the SE side of the volcano. Photos were taken by a photograph of Bronte, courtesy of "Photo Mary" Bronte

The Voragine from 1995 to 2001

 

Although the Voragine was the last of the four summit craters to join in the 1995-1998 activity and its activity initially was very weak, it was the crater to produce two of the most significant eruptive events of the current eruptive cycle, on 22 July 1998, and 4 September 1999. About one year before magmatic activity resumed on the crater floor, the open pit that had been degassing quietly for years was invaded by lava flows from the NE Crater, but degassing continued from the pit for months after the cessation of lava inflow. During the spring of 1997, however, a landslide blocked the pit, and no activity was observed there until mid-July 1997.

A very small cone displaying Strombolian activity began to grow at the site of the central pit in late July 1997; its activity was weak and sporadic, and it ceased erupting in late December 1997. In September-October minor volumes of lava flowed from this cone onto the surrounding crater floor. Activity had in the meantime started at another vent in the southwestern part of the crater floor, immediately below the "diaframma". The initiation of this activity coincided with the activity of vent C in Bocca Nuova, situated on the back side of the "diaframma", suggesting that both vents were connected at shallow depth. During low levels of gas emission the conduit of the vent could be observed to dip steeply southwestwards, passing under the "diaframma" to below the crater floor of the Bocca Nuova. It appears that this vent was actually related to that crater and "accidentally" found its way into the Voragine. During the first months of 1998, however, this vent was the main focus of activity within the Voragine, and during May-June the conduit appeared to be no longer inclined towards the Bocca Nuova.

A notable increase of the activity occurred in May, and in June the central vent reopened to produce spectacular Hawaiian-style activity while the "diaframma" vent varied from episodes of sustained fire-fountaining to copious ash emission. Rapidly moving lava flows formed at the base of sustained fountains at the central vent on a few occasions in mid-June. The two Voragine vents were often seen to erupt alternatingly although both erupted simultaneously when the activity increased in mid-July. In contrast to the slow growth of the initial conelet at the central vent in July-December 1997, growth of the two cones during June-July 1998 was quite rapid. In mid-July the pyroclastic edifice around the southwestern vent stood as high as the crest of the "diaframma" and the mean depth of the Voragine had decreased from about 90 m (in June 1997) to about 30 m below the northwestern crater rim.

Spectacular lava blisters that grew and exploded in the course of a few seconds were observed at the "diaframma" vent in early June and again at the central vent in mid-July. Those of the earlier vent were tens of meters in diameter while those in the central vent were at best 10-15 m large; many exploded even before attaining that size. The bursting of these lava bubbles produced terrific cannon-shot like detonations audible more than 25 km away, and witnesses watching the activity from the southern rim of the NE Crater felt strong air concussions and slight ground shaking.

Episodes of strongly increased activity occurred in early July, producing scoria falls that extended SE to the rim of Valle del Bove. A much more powerful paroxysmal episode of fire-fountaining and tephra emission on 22 July was accompanied by the first lava flow from the area of the former Central Crater since 1964. This paroxysm ranks among the most significant eruptive events at the summit craters during the 20th century.

Eyewitnesses reported that during the hours prior to the paroxysm the activity was very intense, similar to that of the preceding weeks, but nothing indicated the imminent culmination. At about 1830 h that day, there occurred a powerful explosion followed by the uprush of a tall lava fountain, and an eruption column rapidly rose to an elevation of about 10 km, being impressively visible from locations up to 150 km away. Heavy falls of pumiceous scoriae began soon in the summit area, and sand-sized tephra fell over a wide sector to the northeast, east and southeast, causing serious traffic disruptions and forcing the closure of Catania international airport, almost 30 km from the summit. Sustained lava fountaining lasted little more than one hour and was followed by ash emissions of decreasing intensity that ceased altogether the next day.

Field inspections later revealed a number of unique eruptive and depositional processes during the 22 July 1998 event. The paroxysmal activity appears to have been entirely restricted to the Voragine although the Bocca Nuova and the SE Crater continued to erupt simultaneously. Both the central and SW vents of the Voragine participated in the eruption. Fluid spatter fell back into the crater, accumulating rapidly, and lava overflowed the crater rim both in the SSW (over the crest of the "diaframma") and northwest. At the NW overflow, the lava moving along the S base of the NE Cone, demonstrated a striking erosive force. It gradually undercut the flank of the cone causing its partial collapse and carried pieces of the collapsed flank with it. This process was exacerbated by heavy showering of the NE Cone by huge bombs and spatter from the central vent, leading to increasing instability of its southern flank. The pyroclastic deposit on the SW rim of NE Crater was 10-15 m thick and completely obliterated any trace of the cones that had grown there in July-August 1996. There even appears to have been some flowage of the still-fluid deposit on the floor of the NE Crater into its central pit, which, however, remained open.

The lava overflowing the crest of the "diaframma" cascaded tens of meters down into the Bocca Nuova, forming a flow that passed between the cones at the NW and SE vents in that crater, and ponded in its SW portion. Some erosion by this flow occurred on the W side of the cone at the SE vents.

The proximal pyroclastic deposit of the eruption consists of dense bombs up to several meters in diameter, which is thickest on the western, northern and eastern sides of the Voragine and thins rapidly to the southwest and south but extends for hundreds of meters across NE Crater where it is thickest. The medial deposit consists of pumiceous black scoriae and forms prominent fans to the NE, E and SE whereas little fallout occurred to the southwest and west. The axis of maximum thickness of the southeastern fan passed across the SE Crater where the deposit was up to 1 m thick, and at the Torre del Filosofo mountain hut, the thickness was up to 40 cm when freshly fallen. Towns on the SE and SSE flank, including Catania, were blanketed with several millimeters of sand-sized tephra. The volume of the deposit was in the range of 1-5 million m3.

Vigorous activity at the Voragine resumed in early August, punctuated by another paroxysmal eruptive episode on the 6th (this was of minor proportions compared to the 22 July paroxysm), and continued through early September, producing almost incessant loud detonations and occasional minor effusive activity, followed by a drop of the magma in the vents. From November 1998 onwards the crater was very quiet and it was possible to walk across the crater floor, look into the vents that had been so vigorously active a few months before, and peer into the Bocca Nuova from the crest of the "diaframma"!

After about 7 months of quiet, the Voragine became active again in June 1999, and from then on the activity gradually increased, indicating a new rise of the magma column, and the arrival of fresh, gas-rich magma at the surface. The sequence of events in 1998 made many of the geologists working on Etna suspect that this activity would culminate in another paroxysm similar to that of 22 July 1998 - and these suspicions proved right.

On the afternoon of 4 September 1999 the Voragine exploded in a paroxysm that may have been even more violent than that of the preceding year. While the volcano was veiled in dense weather clouds on most sides, people had a tremendous view of the eruption from Piano Provenzana on the N flank, thanks to a gap in the clouds. It was possible to see from here a gigantic lava fountain that roared from the Voragine to a height of at least 1500, and possibly 2000 m, and a black ash plume that rose even higher before being blown to the E by a strong wind.

A group of people who were on the SE side of the summit area to observe the effusive activity near the SE Crater did not see much of the activity but heard its beginning and intensification between about 1745 h and 1815 h, and understood that a major event was in progress as the audible impacts of large objects appeared to come closer and closer, and escaped to safety in time. The culmination occurred between 1900 and 1930, and was followed by many isolated but extremely violent explosions. The whole summit area lay under the bombardment of blocks and bombs with diameters of up to 5 m or more (the largest single bomb seen after the eruption had a diameter of 15 m!). After the eruption, people who visited the summit area of Etna found what appeared to be a battlefield. Thousands of bombs and blocks littered the surface up to 1 km from the Voragine, many had created spectacular impact craters, piercing even the summit dirt road in tens of places. The great amount of dense blocks of older lava indicated that a portion of the crater floor had been shattered during the eruption, possibly by phreatic or phreatomagmatic activity at its beginning.

On the lower E flank, villages, towns and cultivated land received a heavy shower of lapilli-sized scoriae which left a deposit up to 10 cm thick, and many scoria clasts were 5-10 cm large. Fortunately there were only few people outside because of the bad weather, otherwise there would have been many cases of injury due to the fall of so large projectiles. The damage to agricultural areas was great, especially in vineyards. Car traffic suffered serious disruptions due to the pyroclastic deposit on roads and on the Catania-Messina highway, and it took weeks to remove the lapilli from roads and houses. However, the sector affected by the pyroclastic fall was relatively limited in comparison to the fallout area of the 22 July 1998 eruption, due to the very strong wind blowing at the time of the eruption. The volume of the pyroclastic deposit of 4 September 1999 lies between 1 and 5 million m3, similar to that of 22 July 1998.

4 September 1999 map

Sketch map showing the distribution of pyroclastics of the 4 September 1999 eruption of the Voragine, based on field work during the week following the eruption. Click on the map for an enlarged version.

The morphology of the Voragine was again overwhelmed. A large new pit at least 100 m in diameter was carved out in its SW part, uniting the former central and "diaframma" vents. The crater floor to the E of the pit was raised further to only about 30 m below the E rim, and the "diaframma" was breached in correspondence to the S part of the new pit. Lava flows, formed by the accumulation of fluid bombs from the lava fountain on the crater rims, moved downslope for a few hundred meters on the E and W side of the Voragine. The W flow was significantly shorter than the very similar flow of 22 July 1998, but the E flow, consisting of two lobes, was somewhat longer.

Like its predecessor, the 4 September 1999 paroxysm strongly affected the adjacent NE Crater. While the collapse scar produced on the SSW flank of the NE Crater cone during the 22 July 1998 event was largely healed by burial under a thick pyroclastic deposit, a new collapse scar formed on the SSE flank. Few observations of the effects on the internal part of the crater were possible due to the continuing activity at the Voragine, the increased activity at the NE Crater and the violent activity at the Bocca Nuova during the following weeks, but the morphological changes there appeared to be relatively unimportant.

The Voragine experienced several further episodes of vigorous activity during the weeks following the 4 September eruption, but none of these was of similar magnitude. The last reported activity at the Voragine occurred on 12 October 1999; after this the crater was subjected to heavy fallout from the activity in the Bocca Nuova, and in late October most of the large pit formed on 4 September had been filled with ejecta coming from that crater.

No further eruptive activity occurred at the Voragine after October 1999. The only significant event since then was the opening of a small degassing vent on the SW side of the 4 September 1999 pit in May 2000. This vent, which initially had a diameter of a few meters, gradually enlarged through mid-2001 to a diameter of about 50 m and generally released more gas than the neighboring main pit. The position of the new vent roughly coincides with the location of the 1998 "diaframma" vent.


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

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