Report on Etna (Italy) — October 1986
Scientific Event Alert Network Bulletin, vol. 11, no. 10 (October 1986)
Managing Editor: Lindsay McClelland.
Etna (Italy) Lava flows and Strombolian activity from SE fissures and crater
Please cite this report as:
Global Volcanism Program, 1986. Report on Etna (Italy) (McClelland, L., ed.). Scientific Event Alert Network Bulletin, 11:10. Smithsonian Institution.
37.748°N, 14.999°E; summit elev. 3320 m
All times are local (unless otherwise noted)
A new system of eruptive fissures opened at the base of the central crater on 30 October. The new eruption was marked by vigorous Strombolian activity and intense seismicity. Several new lava flows emerged from the fissures.
The seismic activity observed during the first several days of October continued during the following days. Both shallow and deep earthquakes occurred, mainly on the W flank of the volcano. The deepest earthquakes were at 20 km and maximum magnitude was 3.0.
An intense but brief seismic swarm that included > 40 shocks in one hour began at 2345 on 29 October. The strongest event (M 3.7), centered on the NE flank (at Piano Provenzana), occurred ~1/2 hour later at 0018. Only a few events were recorded during the following days.
Harmonic tremor remained at late September-early October levels until 30 October, when the seismic swarm was joined by a brief, ten-fold rise in tremor amplitudes, between 0000 and 0400, that accompanied the opening of a system of eruptive fissures. The 2-km-long fissure system, oriented ENE, stretched from the base of the central crater to the Valle del Leone (from 2,900 to 2,500 m above sea level (asl), on the W flank of the Valle del Bove). Several explosive vents that formed along the upper part of the fissure system were the source of gas and vapor emission, strong ash expulsions, and occasional phreatic explosions. Weak Strombolian activity occurred from the lower part of the fissure system (~ 2,500 m altitude) and several lobes of lava flowed away from the fissures. Eight hours after the start of the eruption, two main flows were moving SE. The southern flow had advanced ~ 2.5 km and reached the Valle del Bove at 1,750 m altitude.
During the early afternoon the fissure system continued to propagate downslope, creating a new eruptive fissure NW of Mt. Simone, around 2,300-2,200 m altitude, just behind and to the side of the N "wall" of the Valle del Bove. Violent explosive Strombolian activity started immediately, as four eruptive vents formed. Lava flows from the two lower vents coalesced into a single lobe that moved SE. In the late evening, strong lava fountaining began along a few hundred meters of the central part of the fissure system (around 2,600-2,500 m altitude). The ash and lapilli from this activity were transported by the wind 30 km to the SW.
Similar explosive activity was continuing the next morning. Lava formed three main flows (around 2,700 m altitude), that moved SE and ESE. Tremor amplitude decreased significantly, but increases were later recorded during various periods of increased eruptive vigor. Strong Strombolian activity from Southeast Crater began in the early afternoon. Isolated but violent explosions continued until the next day (1 November) when lava overflowed from Southeast Crater and moved SE for a few hundred meters. Southeast Crater lava effusion ended during the morning of 2 November and ash expulsions, sometimes violent, began.
During the first few days of November, violent explosions audible on the entire SE flank marked periods of intense ash expulsion from the new eruptive vents (particularly those around 2,300-2,200 m altitude). The numerous lava flows remained above 1,500 m altitude, forming a large lava field.
Strombolian activity from the bottom of the 2 central crater vents had continued through October. From the beginning of the 30 October eruption, vapor emission of fluctuating intensity alternated with expulsions of significant amounts of dark ash from these vents. Fumarolic activity occurred along the edges of Northeast Crater.
On 3 November a M 3.5 earthquake, the strongest since 30 October, occurred at 0833 in the lower part of the Valle del Bove. As of the morning of 10 November, weak Strombolian activity was occurring from six explosive vents, two around 2,500 m altitude, and four around 2,300-2,200 m. Lava from the two eruptive systems merged and formed a single flow. It advanced 200 m past Rocca Musarra (on the N side) and penetrated farther inside the Valle del Bove than any other flow, to 1,450 m. At least two large scoria cones were forming over the vents.
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: R. Romano, G. Budetta, T. Caltabiano, D. Condarelli, and O. Consoli, IIV; G. Luongo, IIV and OV; S. Gresta, Univ di Catania.