The following report, jointly submitted by scientists from the Università di Catania and the Istituto Internazionale di Vulcanologia, expands on information from the first half of July (BGVN 22:07) and adds observations of activity at each crater through the end of the month. A review of the eruption since July 1995 places the current activity into context. Between 19 and 31 July, moderate eruptive activity continued from the summit craters, including occasional small overflows from Southeast Crater (SEC). Magmatic activity began at the Voragine in mid-July, followed by continuous Strombolian activity from a central pit in Northeast Crater (NEC) at the end of July (after about eight months of relative quiet). A visit on 5 August revealed magmatic activity from all four summit craters (figure 68).

Figure 68. Sketch map of Etna's summit craters as of late July 1997, showing vents active in 1996-97. For Bocca Nuova, locations A and B indicate eruptive centers. Numbered features in Northeast Crater identify: 1) the cluster of cones and craters active in July-August 1996 (the two westernmost craterlets are collapse structures formed after that activity); 2) tilted slabs of lava and agglutinate; 3) the outer platform; 4) the inner platform covered by lava flows from the cones; and 5) a collapse pit over the central conduit. Courtesy of Boris Behncke and IIV.

Review of activity since July 1995. Magmatic activity resumed in Etna's summit craters in July 1995, more than two years after the end of the voluminous 1991-93 E-flank eruption in Valle del Bove. Initially this activity was limited to Bocca Nuova (BN), the westernmost of Etna's four summit craters, but it soon extended to nearby NEC. During November 1995-August 1996, NEC was the site of spectacular explosive and effusive activity, including ten episodes of high lava fountaining (BGVN 20:11/12, 21:02, 21:03, 21:06, 21:07, and 21:10), while BN continued to emit pyroclastics and small intracrateral lava flows on a moderate scale. In early November 1996, magmatic activity resumed at SEC, which had been inactive since late 1991 (BGVN 21:10). SEC activity was initially very mild, gradually building a small intracrateral cone. In late January 1997 it began to emit minor lava flows that were confined to the ~200-m-wide crater. Activity at SEC and BN continued through late June 1997 without major modifications. Etna's fourth summit crater, Voragine (also known as The Chasm), has been actively degassing from a pit on its floor without the emission of any juvenile material in past years.

Voragine. The central degassing pit was obstructed by talus on 14 June, with only minor wisps of steam escaping from the pit's rims. Apparently, a lava flow erupted into the Voragine from NEC in July- August 1996 had partially collapsed, covering the pit. On the evening of 13 July 1997 dense gas clouds possibly containing some ash were seen rising from the Voragine for about 15 minutes. On 18 July, the Voragine was found in a state similar to previous years, but the day after, fluid lava had risen to the lip of the central pit. A small cone (~5 m high) began to grow over the former pit on 24 July; this cone was the site of mild but continuous Strombolian activity, throwing incandescent bombs to several tens of meters above the vent. Similar activity continued through 28 July, but a visit to the crater on the following day revealed that eruptive activity had stopped and only weak steaming occurred from the new conelet.

Northeast Crater (NEC). The intense activity of 1995-96 and subsequent collapse completely altered the morphology of NEC, which was a single deep pit until early November 1995. The WSW part of the crater was occupied in July by a cluster of partially overlapping craters and small cones, some being minor collapse features (figure 68, number 1). The northern part of NEC contained an outer platform ~30-50 m wide (figure 68, number 3) of agglutinate that filled the crater in June-July 1996. The W edge of the platform was marked by large craggy slabs of lava (figure 68, number 2), which were tilted upwards as the lava fill subsided. A lower inner platform in the central northern part of NEC (figure 68, number 4) was covered by late lava flows erupted from the cone cluster. In the southern central part of the crater there was a collapse pit (figure 68, number 5) ~100 m across that showed vigorous degassing and possibly deep-seated Strombolian activity. During a 20-minute visit to NEC on 14 June loud roaring noises were followed by pit emissions of dense yellowish gas plumes containing some ash at intervals of 1-2 minutes. On 14 July ash emissions occurred from the crater, a day after similar emissions from the Voragine. During the last week of July, there was a gradual resumption of Strombolian activity, which had become continuous by the 31st. Most of this activity was confined to the pit, but occasional larger explosions ejected incandescent bombs high above the summit of NEC.

Bocca Nuova (BN). Since the resumption of magmatic activity in 1995, two principal eruptive centers have been active in this crater, one at the base of its vertical SE wall (figure 68, location A) and another in the NNW sector of BN (figure 68, location B). Throughout July, Center A consisted of two closely spaced vents that built a large half-cone against the crater wall, ~30 m above the crater floor. While one of the vents at times displayed quasi-continuous small Strombolian bursts, the other produced very violent explosions at intervals ranging from 30 seconds to 5 minutes, often ejecting bombs >150 m above the vent. These explosions were accompanied by detonations that could be heard several kilometers away. During some of the largest explosions bombs fell on the SE rim or outside BN to the S in the area of the former summit crater platform.

Center B had a highly variable number of explosive and effusive vents that formed a complex cone surrounded by a lava field covering almost the entire crater floor. Between early June and about 15 July, Strombolian activity occurred from several closely spaced vents with diameters not exceeding 10 m each while lava issued from vents at some distance from the explosive ones. By 16 July there was one large vent ~30 m across at the top of a broad cone. The vent was filled with vigorously boiling and fountaining lava, with some fountains ejecting spatter as high as the crater rim (~100 m above the vent). By late July, a second cone was growing on the E side of the first one; both exhibited lava splashing and fountaining while lava issued from vents at the base of the cones. The floor of BN underwent repeated and rapid resurfacing during this time and the maximum depth of the crater was 114 m (determined with laser rangefinder) at the end of July.

A 5 August visit revealed four erupting vents, two in the northern eruptive area and two on the half-cone growing in the SE corner of the crater. Bursts of bombs and spatter often rose significantly higher than the crater rim, reaching heights of up to 200 m above the SE vents. Large explosions from these vents at times ejected bombs onto the SE rim of BN and over the S flank of the central cone, making this area very dangerous.

Southeast Crater (SEC). This crater is gradually being filled by the growing intracrateral cone and lava flows, and lava has spilled onto its outer slopes. Explosive and effusive activity occurred at variable intensity from vents varying continuously in size, number, and location. The main focus of activity was at the summit of the cone (~20 m above its N base and ~40 m above its S base) in the central northern part of SEC (figure 69), but effusive activity often took place on the flanks of the cone, either from small hornito-shaped cones or from wedge-shaped purely effusive vents. More rarely, lava flows were directly emitted from the explosive summit vents.

Figure 69. Development of the intracrateral cone and lava flows at Southeast Crater of Etna during July 1997. Active flows are indicated as white areas; recent but inactive flows by a dark pattern. "Fortino" is a small stone shelter built on the highest point of the W crater rim. Courtesy of Boris Behncke and IIV.

Activity increased in early July, and explosions became visible from Catania (~30 km S) at night. During the following days Strombolian activity was discontinuous, occurring in cycles that lasted 10-15 minutes separated by quieter periods of similar duration. Visits to the crater once or twice per week, and helicopter overflights, documented shifts in eruptive vents and development of lava flows (figure 69). By 11 July the level of the intracrateral lava field had risen to within a few meters of the lowest part of the crater rim (on the SE side of SEC; figure 69a); late on 19 July a small lava tongue spilled over the SE crater rim, advancing ~30 m downslope before stopping. On that evening, the intracrateral cone had three explosive vents; the most vigorous one was the lowest and easternmost (figure 69b). Lava issued in surges from that vent, forming a high lava ridge with several steep-sided pinnacles on the SE flank of the cone. A second minor overflow onto the SE flank occurred on 22 July, forming a second lava lobe on top of the first one, but without reaching farther downslope. During the following days, Strombolian activity became continuous.

On 25 July the intracrateral cone had grown by ~10 m since 19 July, and two explosive vents on its summit ejected bombs ~100 m high. Lava effusion from a hollow-shaped vent fed a flow that advanced towards a low point on the NE crater rim (figure 69c). Effusive activity occurred in pulses coinciding with periods of increased explosive activity at the cone's summit. Larger volumes of lava began to spill onto the outer SE flank of SEC on 27 July, forming a black tongue ~150 m long and 50 m wide. On the 29th, lava effusion occurred both on the NE and SE sides of the cone (figure 69d), but the lava field showed little growth because the flows on the SE flank apparently overlapped rather than extended downslope. Continuous explosive activity from a single summit vent ejected bombs ~200 m above the vent, and pyroclastics fell onto the high W rim of SEC, in the area of a small stone shelter ("Fortino" in figure 69d). During the night of 31 July-1 August, another overflow formed a new lava lobe on the N side of the earlier flows, and advanced some 30 m farther downslope.

Activity at SEC on 5 August continued in the same manner as in previous weeks with vigorous explosions at an intracrater cone and lava effusion from its eastern flank. Several small overflows from the SE rim had spilled onto the cone's flank almost every day in early August, and a small overflow had occurred onto the NE flank.

Geological Summary. Mount Etna, towering above Catania on the island of Sicily, has one of the world's longest documented records of volcanism, dating back to 1500 BCE. Historical lava flows of basaltic composition cover much of the surface of this massive volcano, whose edifice is the highest and most voluminous in Italy. The Mongibello stratovolcano, truncated by several small calderas, was constructed during the late Pleistocene and Holocene over an older shield volcano. The most prominent morphological feature of Etna is the Valle del Bove, a 5 x 10 km caldera open to the east. Two styles of eruptive activity typically occur, sometimes simultaneously. Persistent explosive eruptions, sometimes with minor lava emissions, take place from one or more summit craters. Flank vents, typically with higher effusion rates, are less frequently active and originate from fissures that open progressively downward from near the summit (usually accompanied by Strombolian eruptions at the upper end). Cinder cones are commonly constructed over the vents of lower-flank lava flows. Lava flows extend to the foot of the volcano on all sides and have reached the sea over a broad area on the SE flank.

Information Contacts: Boris Behncke, Istituto di Geologia e Geofisica, Palazzo delle Scienze, Università di Catania, Corso Italia 55, 95129 Catania; Mauro Coltelli and Paola del Carlo, Istituto Internazionale di Vulcanologia, Piazza Roma 2, 95123 Catania, Italy (URL: http://www.ingv.it/en/).