Initial stages of the Northeast Crater eruptions, October 1995

The 1995-2001 summit eruptions
Introduction

Etna reawoke after more than two years of slumber in late July 1995 with episodes of Strombolian and phreatomagmatic activity in the Bocca Nuova and NE Crater. For the following six years, all four summit craters were the site of highly variable, and often spectacular, activity which continued until the beginning of the complex summit/flank eruption of July-August 2001. The following report is a general summary intending to place the present activity in the framework of Etna's activity; the text is an excerpt from the original English version of a paper submitted to the Bulletin of the French "Association Volcanologique Européenne" (L.A.V.E.) in January 2000. It has been extended to cover the full period of the summit eruptions through mid-July 2001. To see many aspects of the most recent eruptive activity on Etna, visit the 1995-1999 photo gallery.

The summit craters of Etna in July 1997, showing locations of principal vents and other morphologic features. This map was included in a report by Behncke, Coltelli and Del Carlo, published in the August 1997 issue of the Bulletin of the Global Volcanism Network. Compare to October 1999 map below.

It is close to impossible to render a synthetic description of the activity initiated in 1995, since it consisted of tens of discrete paroxysmal eruptive episodes, and many periods of long-lived, mild activity with notable fluctuations in their intensity and eruptive styles; the activity was at times concentrated at one or two of the summit craters while in other periods it occurred from all four summit craters simultaneously. While each crater manifested a strongly individual behavior on its own, the whole summit crater system underwent periods of higher or lesser activity, reflecting intermittent magmatic "pulses" rising towards the surface. Many peculiar events and processes recorded during these years will be discussed in the paragraphs dealing with the four summit craters individually.

Magmatic activity first began in the Bocca Nuova in late July 1995 and was soon followed by a gradual reactivation of the NE Crater in the following month. For about two years, the activity of the Bocca Nuova remained at relatively low levels, while the NE Crater became quite energetic and produced 10 paroxysmal eruptive episodes between November 1995 and June 1996. After the eighth of these episodes (in February 1996), the crater remained in mild Strombolian eruption, and after the last episode, the activity became very similar to the classical persistent activity seen during the 1950's to 1970's, with spectacular but mild explosive activity from several cinder cones that grew within the crater, and slow effusion of lava onto the cone's flanks. The activity of the NE Crater declined markedly after mid-August 1996, but three months later, the Bocca Nuova began to erupt more vigorously, and the SE Crater ended five years of inactivity with the resumption of low-level Strombolian activity and the extrusion of small volumes of lava onto the crater floor.

Throughout the first half of 1997, the activity of the Bocca Nuova and the SE Crater continued at relatively low levels, but from early July onwards there was a distinct increase in the intensity of the activity, and sizeable pyroclastic cones began to build around the two main vents in the Bocca Nuova whereas lava completely filled the SE Crater and began to overflow onto its flanks. Between July and December 1997, the activity extended also to the Voragine and the NE Crater which produced mild Strombolian activity and (in the case of the Voragine) minor intracrater lava flows. After a period of vigorous activity in the Bocca Nuova in late-November and December 1997, the levels of activity at the summit craters decreased to relatively low levels again, and for several months the SE Crater remained the focus of activity at Etna's summit, interrupted only by an isolated, very brief but intense eruptive episode from the NE Crater in late-March 1998. This period of relatively low activity was initiated by a marked subsidence of the magmatic column within the Bocca Nuova in January 1998, and hundreds of small earthquakes on the W flank of Etna that aroused fears of an impending flank eruption.

The spring and early summer of 1998 saw a gradual reactivation of the summit crater system, with a particular increase of the eruptive vigor within the Voragine. From June until September this crater was the site of numerous episodes of spectacular lava fountaining, powerful explosions and intracrater lava flows. The most significant event of this period was a very strong paroxysm on 22 July that produced an eruption column 10 km high, and extensive falls of lapilli and ash onto the E and SE flanks. During this event a short lava flow advanced from the Voragine onto the NW flank of the main summit cone, the first lava flow from the area of the former Central Crater since 1964. The Voragine was largely filled during this period, and by the end of 1998 it was possible to walk across the crater floor to the "diaframma" to peer down into the nearby Bocca Nuova from a vantage point that had not existed a few months before, and that was to disappear within less than one year.

Also during the summer of 1998 the Bocca Nuova showed much higher levels of activity, with minor intracrater lava outflows and strong explosions, recovering the amount of crater filling lost during the subsidence earlier that year. On the other hand, the SE Crater, which had been in virtually continuous mild persistent activity since late-1996, stopped erupting a few days after the great paroxysm of the Voragine of 22 July, and remained totally quiet for about six weeks. In mid-September, just as the activity in the Voragine and the Bocca Nuova began to diminish, the SE Crater reawoke with very violent explosions, and from then until 4 February 1999 it was the source of 22 paroxysmal eruptive episodes that built up the summit of the SE Crater cone to more than 3250 m elevation (more than 70 m higher than its summit was in 1997). Apart from relatively mild explosive activity at the Bocca Nuova, the other summit craters remained essentially quiet during this period.

On 4 February 1999, the last of the paroxysmal episodes at the SE Crater resulted in the fracturing of its cone, and the opening of a new eruptive fissure at its SE base. Activity at this new fissure continued with mild explosive activity and vigorous lava emission, while all activity elsewhere in the summit area ceased. For the next four months, lava quietly issued from the 4 February fissure, forming an extensive and complex lava flow-field on the W wall of the Valle del Bove, with a maximum length of 2.5 km.

While the outflow of lava from the 4 February fissure continued with some fluctuations, eruptive activity resumed in the Voragine and the Bocca Nuova in June 1999, and during the following two months there was a gradual increase in the levels of activity mostly at the Voragine, while explosions occurred deep within the conduit of the NE Crater. This activity heralded what was probably the most violent eruptive event at the summit craters of the whole new eruptive cycle, on 4 September, at the Voragine. This event produced lava fountains 1500-2000 m high, and a heavy rain of scoriae and lapilli onto towns, roads and cultivated land on the E flank, causing significant damage. A brief revival of Strombolian activity occurred a few hours after this event at the SE Crater, followed by renewed lava emission from the fissure system at its SE base, active since 4 February.

During the following five weeks a very complicated sequence of eruptive events occurred in the summit area of Etna, with several violent eruptive episodes at the Voragine and the Bocca Nuova, intense Strombolian activity at the NE Crater, and fluctuating lava emission and sporadic mild explosive activity from the eruptive fissures near the SE Crater. On 5 October, near continuous, vigorous eruptive activity began in the Bocca Nuova and continued for about one month. It was during this period that the crater was completely filled with lava and overflowed, producing repeated spectacular surges of lava onto the W flank of the mountain, the first lava flows poured ever from the Bocca Nuova onto the external flanks of the volcano.

After less than three months of relatively mild activity (mostly within the Bocca Nuova), the SE Crater reawoke on 26 January 2000 with violent lava fountaining and production of lava flows. Its cone split open on its southern side, but differently from the 4 February 1999 eruption, the activity there lasted less than one day, and it became clear that this event was not a simple repetition of that of the previous year. It became clear three days later that the behavior of the SE Crater had once more changed, when another, brief but violent, eruptive episode took place at the same crater. During the following weeks similar events occurred in a frantic manner, with up to three eruptive episodes per day. All of them were characterized by a very brief duration (between 10 and 30 minutes) and production of lava fountains up to 1000 m high, tall tephra columns, and rapidly moving lava flows from fissures on the southern and northern flanks. In late February 2000 the intervals between eruptive episodes grew longer, but the episodes themselves also lasted longer, and in March they became distinctly stronger. In April and early May, the repose intervals were up to 10 days long, and each episode became a much observed spectacle to nearby residents and visitors. In mid-May and June the repose intervals were highly irregular, varying from 12 hours to 10 days. The last spectacular eruptive episode of this period, the sixty-fourth since 26 January, was also one of the most violent, with lava fountains up to 1200 m high, and a 3 km long lava flow that extended to the bottom of the Valle del Bove.

Two further eruptive episodes occurred at the SE Crater on 28 and 29 August 2000, during which the summit vent and the northern fracture of the cone were active, but the S flank fracture, which had opened during most of the January-June 2000 eruptive episodes remained inactive. These latest paroxysms were the closing act of the grand show displayed by the SE Crater to greet the new millennium; a period of minor activity was to follow.

From September to December 2000 relatively mild Strombolian activity occurred at two vents within the Bocca Nuova. In late November and early December, this was accompanied by what seemed an attempt of the SE Crater to return to its previous spectacular activity. In a manner identical to the initial stages of most of the January-August 2000 paroxysms, lava was extruded from a vent on the NNE flank of its cone, but after a few days this activity ceased without having evolved into a new paroxysm. At the end of 2000 and during the first weeks of 2001 Etna was quieter than it had ever been since the beginning of the summit eruptions in 1995. However, in late January the SE Crater slowly reawakened to build up for another grand show which, on 17 July 2001, culminated with the beginning of the impressive and complicated eruption which affected both the summit area and the southern and northeastern flanks of Etna - the first flank eruption since the 1991-1993 eruption. During May-July, sixteen eruptive episodes occurred at the SE Crater, which now had a new permanent satellite vent informally named "Levantino". Though less violent than the paroxysms of January-August 2000, these new events provided an awesome spectacle to who was fortunate enough to witness them. Then came the July-August eruption, and things at Etna changed dramatically.

The following pages describe the activity at the four summit craters in more detail and highlight some of the peculiar eruptive and depositional processes observed during the 1995-2001 summit eruptions.

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