Report on Etna (Italy) — March 1990
Bulletin of the Global Volcanism Network, vol. 15, no. 3 (March 1990)
Managing Editor: Lindsay McClelland.
Etna (Italy) Lava fountains and flow then strong block ejection from Southeast Crater
Please cite this report as:
Global Volcanism Program, 1990. Report on Etna (Italy). In: McClelland, L. (ed.), Bulletin of the Global Volcanism Network, 15:3. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199003-211060.
37.748°N, 14.999°E; summit elev. 3295 m
All times are local (unless otherwise noted)
Summit activity. (S. Calvari, M. Coltelli, O. Consoli, M. Pompilio, and V. Scribano.) February activity was characterized by a single strong eruptive episode at Southeast Crater. Summit-area craters generally remained quiet through the rest of February and March. The 1-2 February eruptive episode was similar to several in January. A gradual increase in Strombolian explosions was followed by lava fountaining, and lava flowed over the crater's E rim for 5 hours beginning at 2200 on 1 February. The flow turned toward the Valle del Bove, advancing to ~ 2,000 m altitude, near the terminus of the mid-January flow. During the morning of 2 February, discontinuous Strombolian activity was followed by ejection of scoria that seldom reached a few tens of meters from the rim. Activity changed at about 1330 to energetic, discontinuous explosions that generated rumbling heard at a considerable distance. Blocks more than a meter across fell within a few hundred meters of the crater; much of the slightly vesicular ash was non-juvenile. Similar activity continued until about midnight. After the eruptive episode, the crater was completely obstructed, without any gas emission, until 27 February, when sporadic ejection of dark tephra began from two vents on the crater floor. February activity at other summit-area craters was limited to vapor emission from floors and walls. Emissions were particularly strong from Northeast Crater, where the active vent's walls were strongly incandescent.
Degassing was continuous at the summit craters in March but was not accompanied by Strombolian activity. Degassing occurred from an elliptical vent on the W floor of La Voragine accompanied by sporadic rumbling. Gas was also emitted from two sites on the SE and NW floor of Bocca Nuova. Weak fumarolic activity, from collapse steps that have formed along concentric fractures in Southeast Crater, was strongest from the center of the crater. Degassing also continued in Northeast Crater. On 29 and 30 March, sporadic tephra ejection and incandescence were observed, apparently from a sudden rise in the magma column.
Seismic activity. (E. Privitera, C. Cardaci, O. Cocina, V. Longo, A. Montaldo, M. Patanè, A. Pellegrino, and S. Spampinato.) Volcanic tremor amplitude began a progressive increase on 1 February at 1239, probably associated with increased Strombolian activity at Southeast Crater. Amplitudes peaked at 1940 that day, and at 0048 the next morning as activity was changing from Strombolian to lava fountaining. Other substantial increases in tremor amplitude occurred at 0600-1100, 1855, and 1935. The first of two sequences of discrete earthquakes on 2 February began at 0352. Eight of the events, centered at ~15 km depth on the volcano's N sector, were larger than M 1, the strongest at M 2.6 between 0424 and 0619. The second series of shocks started at 1321, with the two largest events (M 2.8) at 1322 and 1337. Hypocenters were on the Valle del Bove at <1 km depth. From 3 February until the end of the month, seismic activity was at very low levels, with little variation in tremor amplitude or the number of low-frequency shocks. Nine fracturing events exceeded M 1, with a maximum magnitude of 2.5.
Seismic activity in March was characterized by a significant increase in the number of fracturing events. Swarms on 16 and 18 March totaled 124 shocks (M>=1) and brought the month's recorded earthquakes to 153, ~ 3 times as many as in January and February. The 16 March swarm began at 0530 and continued until 0050 the next day. Of the 107 shocks stronger than M 1, 28 were of M>=2 and three of M>=3. The bulk of the most energetic events originated from the central to W part of the edifice at 10-20 km depth, although one (at 1052) was located just NNW of the central crater at ~5 km depth. The strongest shock of the 18 March sequence, which included 17 events, occurred on the SW flank (a few kilometers S of Monte Nero) at ~10-15 km depth. An M 3.3 earthquake on 22 March at 1159 was ~15 km deep, roughly 6 km SSW of the summit (just S of Monte Vetore). The March seismicity was not accompanied by changes in volcanic tremor amplitude, which remained low throughout the month. The number and amplitude of low-frequency events showed little change after 3 February. A new seismic station (PZF) was installed on the lower NW flank (near Maletto), replacing station RCC, stolen in August 1989. With the new site, IIV's Etna network numbers 8 stations.
Ground deformation. (A. Bonaccorso, O. Campisi, G. Falzone, B. Puglisi, and R. Velardita.) Two tilt stations (SPC and CDV) operated during February, both on the S side of the volcano. Data from station SPC generally remained within resolution limits through February and March. A weak anomaly was recorded on the tangential component 18-20 February, then tangential data returned to the normal range. Radial values from recently installed station CDV remained within resolution limits through February, while tangential data began a (negative) excursion on 18 February that totalled 5 µrad by the end of the month. All instruments from this station were stolen on 1 March. Reoccupation of sites that form a triangle along the fracture zone between 1,800 and 1,500 m altitude on the S-SE flank (between benchmarks Bocche 1792, Serra Pizzuta Calvarina, and Mt. Stempato) did not show significant deformation since the previous measurements on 19 January.
Summit SO2 flux. (T. Caltabiano and R. Romano.) Rates of SO2 emission during Southeast Crater's eruptive episode on 2 February were three times mean values. Measurements 7, 14, and 21 February showed considerable variation. The five March measurements yielded SO2 flux of 2,500-14,000 t/d, increasing at the end of the month.
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. Santacroce, IIV.