Report on Etna (Italy) — October 1996
Bulletin of the Global Volcanism Network, vol. 21, no. 10 (October 1996)
Managing Editor: Richard Wunderman.
Etna (Italy) Southeast Crater resumes activity after five-years
Please cite this report as:
Global Volcanism Program, 1996. Report on Etna (Italy) (Wunderman, R., ed.). Bulletin of the Global Volcanism Network, 21:10. Smithsonian Institution.
37.748°N, 14.999°E; summit elev. 3320 m
All times are local (unless otherwise noted)
The following summarizes the monthly reports of the Istituto Internazionale di Vulcanologia (IIV) for the period August-7 November 1996.
Activity during August. Strombolian activity continued from two vents in the cinder cone on the W border of the Northeast Crater (NEC) rim. There were almost continuous explosions; bombs and lapilli were thrown up to several tens of meters above the crater rim. Lava flowed from the NEC effusive vent on the S flank down into Voragine crater.
On 3 August an ash emission lasted three hours and three lava flows produced by ephemeral vents on the E rim of NEC moved toward the Valle del Bove. Activity increased on 4 August at 2045 and continued until the morning of 8 August without interruption. During these days explosions were heard in the villages Zafferana Etnea and Nicolosi (15 and 10 km from NEC, respectively). Three to four new vents emerged; they were aligned roughly N-S, produced Strombolian explosions, and modified the shape of the cinder cone from circular to elliptical.
Activity increased again on the morning of 14 August when several blasts from the middle flank of the volcano were heard. Two vents erupted together throwing lapilli and bombs up to 300 m above the NEC. An aa lava flow from the E slope of the cone moved into Valle del Bove, and sudden high lava jets were observed at the vent in front of the Voragine crater.
On 15 August some ash emissions occurred, and red glow was observed continuously during 15 and 16 August. During the night of 16-17 August the Strombolian activity suddenly decreased. On 17 August a few explosions ejected both incandescent and dark material. During the night of 17-18 August the lava flow in Valle del Bove stopped. On 19 August the lava flow in the Voragine also stopped and overall there was no eruptive activity. Poor weather conditions prevented clear observations.
On 21 August from 1040 to 1130 small Strombolian explosions occurred at NEC. On 28 August from 0700 to 1230 several ash explosions coincided with collapse of the cinder cone inside the NEC.
Activity at Bocca Nuova crater was characterized by mild and discontinuous Strombolian eruptions from vents in the N part of the crater floor. The other two summit craters, Southeast Crater and Voragine, produced only steam emissions. Although the degassing vent in the Voragine crater was filled almost continuously by lava from 21 July to 19 August, it remained open and degassing for the whole period.
Activity during September-October. Gas and steam were continuously emitted from the summit craters during September. On 17 September at 2100, weak Strombolian activity was observed at NEC for one hour.
On 1 October at 2100 Strombolian activity resumed at NEC with an explosive sequence. Initially the explosions occurred every 5 minutes; from 2230 the glow of the explosions became continuous and stronger, lasting all through the night. During the daylight of 2 October, puffs of black ash were observed until midday. Bad weather condition prevented observation for some days, and when the summit craters became visible they emitted only gas and steam.
Another short period of Strombolian activity occurred during 13-14 October, beginning around 1200 on 13 October with several small black ash puffs every 10-20 minutes. Weak red glows observed at night on NEC indicated low Strombolian activity. The following morning ash puffs were still emitted at NEC; eventually during the day the ash emissions became less frequent and then disappeared.
On 18 October a single ash explosion occurred and on 20 and 23 October several ash puffs were noted. These explosions opened a large pit crater on the E side of the intracrater cinder cone built during the strong eruptive activity of July and August.
On the N floor of Bocca Nuova (BN) during October, an incandescent hole produced sporadic weak Strombolian activity. The occurrence of ash in the plume after 24 October marked an increase in explosivity. Voragine had no eruptive activity in October; only quiet steam emission and some puffing were observed at the large hole on the lower part of the crater floor.
Activity during 1-7 November. On 2 November some intense red flashes were reported from BN and NEC at 0045; within the next 20 minutes larger explosions produced stronger glows at BN. Strombolian emissions at BN resumed during 0125-0130 and during 0205-0210, but ejecta reamined within the crater and minor emissions continued until 6 November when one ash puff reached 1 km above the summit. The emission came from an old degassing vent located on the inner wall, on the SE side of the crater. The associated explosion enlarged the vent and caused debris from the inner walls to cover a wide portion of the S crater bottom. A few small lava blocks fell on the S crater rim and a brown, totally non-juvenile ash deposit covered the E sector of the summit cone of the volcano down to a few km from the crater.
These events preceeded the resumption of activity at Southeast Crater (SEC), ending five years of complete inactivity (quiet since Autumn 1991, shortly before the beginning of 1991-93 eruption). On 6 November, after the large phreatic explosion at BN, very low Strombolian activity was reported from two vents on the floor of SEC. The close temporal occurrence of these phenomena and the alignment of the Strombolian vent of BN, the phreatic vent of BN, and the Strombolian vent of SEC, suggested that a very shallow magma intrusion moved from BN toward SEC and triggered its Strombolian activity. However, such interplay is normal among the Etna summit craters when the magma level reaches a very shallow depth. During the evening of 6 November some very low glows were observed above the SEC. In the following days SEC eruptive activity remained at very low levels of intensity.
Overflights of the summit craters on 5, 6, and 7 November confirmed that Strombolian activity occurred at BN almost continuously from one or more closely spaced vents on the top of the intracrater cone.
Geologic Background. Mount Etna, towering above Catania, Sicily's second largest city, has one of the world's longest documented records of historical 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 horseshoe-shaped 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: Mauro Coltelli and Paola Del Carlo, CNR Istituto Internazionale di Vulcanologia, Piazza Roma 2 Catania Italy (URL: http://www.ingv.it/en/).