Report on Stromboli (Italy) — May 1996
Bulletin of the Global Volcanism Network, vol. 21, no. 5 (May 1996)
Managing Editor: Richard Wunderman.
Stromboli (Italy) Continued high levels of activity through mid-June; two larger explosions
Please cite this report as:
Global Volcanism Program, 1996. Report on Stromboli (Italy). In: Wunderman, R (ed.), Bulletin of the Global Volcanism Network, 21:5. Smithsonian Institution. https://dx.doi.org/10.5479/si.GVP.BGVN199605-211040.
38.789°N, 15.213°E; summit elev. 924 m
All times are local (unless otherwise noted)
Seismicity began slowly increasing in mid-March before a sudden jump in tremor intensity on 15-16 April (BGVN 21:04). Observations made by Marco Fulle confirmed that the elevated seismicity corresponded to increased eruptive activity. During the night of 15-16 April about 100 explosions occurred. Continuous fountains from the N part of vent 1/2 (see sketch in BGVN 21:04) rose 50 m and lasted 1-2 hours. The S part of vent 1/2 produced large explosions to heights of 150-200 m that deposited bombs on the terrace beyond vent 3/2. Activity from vent 3/1 consisted of continuous pulsing of incandescent gas and explosions every 2-3 hours. Vent 3/2 produced simultaneous explosions every 10-30 minutes from two vents. Similar activity and ~50 explosions were seen the night of 20-21 April. Additional observations included glowing ex-hornitos in vent 1/3 with regular steam pulses. Vent 3/2 explosions covered the terrace S of Crater 3 with bombs.
Observations of summit activity made during 21-28 April by Alean, Carniel, and Iacop revealed similar activity consisting of continuous spattering and intermittent explosions from Crater 1 (BGVN 21:04). Seismicity remained at high levels through mid-May (BGVN 21:04).
IIV report of 1 and 6 June explosions. At 2147 on 1 June, local seismic stations maintained by the Istituto Internazionale di Vulcanologia (IIV) recorded a powerful event lasting ~3 minutes. Eyewitnesses at Stromboli village reported a single strong blast followed by the fallout of red bombs on the upper N slope. Incandescent bombs fell on vegetation, causing a fire that was extinguished by Civil Protection aircraft in the late morning of 2 June. More than twenty tourists were visiting the summit at the time of the explosion. Some of them reported light burns caused by hot lapilli fallout and minor injuries made while escaping on the steep slope.
A field survey early on 2 June revealed that the explosion occurred at Crater 1. The chain of hornitos inside Crater 1 was blown out, leaving a large deep depression in the N side of the crater floor. The ejected material completely covered the summit, falling more than 500 m to the S and E, and reaching ~1,000 m on the N sector, where it fell on the vegetation. The deposit was made of black scoriaceous bombs, covered by Pele's hair, reddish blocks, and a small amount of fine material. On the Pizzo area, where people usually stay to observe the activity (250 m SE from the vent), the falling bombs ranged between 10 and 50 cm in size, and they covered the area with a density of 3-4/m2.
Strombolian activity after this event shortly returned to a medium intensity and a normal frequency (3-4 events/hour). In the days after there were several hours without any activity alternating to mild Strombolian activity and after 5 June spattering activity lasting several minutes was occasionally observed.
At 0452 on 6 June another strong seismic event from Crater 1 was smaller than the 1 June event and lasted ~1 minute. The eruption was recorded by the surveillance video camera on the Pizzo Sopra La Fossa, 120 m above the vent and 250 m away; the camera had been restored two days earlier. A few people observed the explosion and reported an ash column to a few hundred meters high and bomb fallout on the Sciara del Fuoco. The video showed a very fast gray-brown jet that ascended at ~30 m/second at the upper limit of the camera view; most of the bomb and block fallout was behind the camera. The ash emission lasted ~2 minutes, but at the end only overpressured steam was emitted.
After the explosion, Strombolian activity continued at Crater 1. During fieldwork that afternoon, activity was characterized by low-intensity explosions with emission of bombs and brown ash, interrupted by sporadic strong explosions that produced a larger amount of bombs followed by an almost continuous spattering for 5-15 minutes. All pyroclastic materials fell close to the craters but during the larger explosion some bombs were thrown a few hundred meters from the vents. The Strombolian activity continued through at least 10 June, showing periods of mild explosions interrupted by strong explosions and short periods of continuous spattering.
Observations on 8-9 and 11-12 June. Marco Fulle made observations from Pizzo sopra la Fossa for six hours on the night of 8-9 June. Vent 1/2 exhibited continuous fountaining 50 m high with larger pulses every 5-10 minutes and ejection of meter-sized lava clots. The vent also produced 35 explosions 100-200 m high, with bombs over the Sciara del Fuoco and the terrace up to Crater 2, and meter-sized lava clots inside Crater 1. Vent 3/1 was inactive, but vent 3/2 produced 20 explosions 50 m high with a lot of ash and bombs ejected inside the crater.
Observations from Pizzo sopra la Fossa were again made for six hours on the night of 11-12 June. Vent 1/2 again produced continuous fountaining and 46 explosions. Vent 3/1 remained inactive. Vent 3/2 generated 37 explosions 100-250 m high with minor ash. Fountaining occurred during the explosions and near-vertical jets of bombs fell S of the crater rim and over vent 3/1.
Geologic Background. Spectacular incandescent nighttime explosions at this volcano have long attracted visitors to the "Lighthouse of the Mediterranean." Stromboli, the NE-most of the Aeolian Islands, has lent its name to the frequent mild explosive activity that has characterized its eruptions throughout much of historical time. The small island is the emergent summit of a volcano that grew in two main eruptive cycles, the last of which formed the western portion of the island. The Neostromboli eruptive period from about 13,000 to 5000 years ago was followed by formation of the modern edifice. The active summit vents are located at the head of the Sciara del Fuoco, a prominent horseshoe-shaped scarp formed about 5000 years ago as a result of the most recent of a series of slope failures that extend to below sea level. The modern volcano has been constructed within this scarp, which funnels pyroclastic ejecta and lava flows to the NW. Essentially continuous mild strombolian explosions, sometimes accompanied by lava flows, have been recorded for more than a millennium.
Information Contacts: Mauro Coltelli, CNR Istituto Internazionale di Vulcanologia (IIV), Piazza Roma 2, Catania, Italy (URL: http://www.ingv.it/en/); Marco Fulle, Osservatorio Astronomico, Via Tiepolo 11, I-34131 Trieste, Italy.