Tanguy and Clocchiatti reported that in late May and early June explosions, usually 10-30/minute, ejected lava fragments and scoria to 50-200 m above the inner cone that had formed inside the Southeast Crater. Explosive activity sometimes declined to weak puffs of gas without much tephra, but at other times the ejecta were rich in large fragments of magmatic material. During periods of more vigorous activity, occasional bursts hurled the smallest tephra to 500-600 m height. In contrast, lava effusion occurred at a steady rate of a few (perhaps 2-5) cubic meters per second, significantly lower than in paroxysmal eruptions (>10 m³/s) but probably higher than in typical subterminal persistent activity (1 m³/s). The maximum temperature measured at small effusive vents on 31 May and 5 June was 1,075-1,076°C (at 60 cm depth).

Romolo Romano reported that explosive activity at the Southeast Crater in June and early July was limited to ejection of incandescent tephra at varying intervals, occasionally accompanied by ejection of dark ash. Effusive activity continued, resulting in further enlargement of the lava field, especially to the N. Lava flows did not extend much below the 2,800 m level. During the last 10 days of June, lava overflows occurred from the SE side of the Southeast Crater, but did not flow over the edge of the Valle del Bove. On 4 July, an overflow of very thin (0.5 m thick) fast-moving lava occurred from the still-active SE side of the Southeast Crater. Tanguy and Clocchiatti reported Bocca Nuova was filled with lava to within ~100 m of its rim in April, but by early June the lava column was again very deep (> 300 m) and activity was limited to quiet emission of large amounts of SO₂. Romano reported that violent ejection of reddish ash from Bocca Nuova was observed beginning in the second half of June. Recently, the ash has been gray (indicating presence of new material). On 3 and 9 July, violent explosions ejected lava fragments that fell outside the crater rim, especially on the SW flank. During this period, strong emission of gas under pressure was noted at the Chasm.

Tanguy reported that in late April seismicity recorded by Christian Archambault from a geophone about 1 km SSE of the Southeast Crater increased from less than 500 to more than 2,200 events/day (figure 14). A seismic crisis was also recorded January-March, accompanied by lava filling and strombolian activity at Bocca Nuova. The temperature at 3 cm depth at the station about 400 m SSE of the Southeast Crater decreased before the 1983 flank eruption but increased before the current eruption began in April 1984.

Figure 14. Number of seismic events recorded per day (averaged over a 10-day period) at the TDF geophone about 1 km SSE of the Southeast Crater (top) and plot of the difference in subsurface temperature (measured at 3 cm depth) between the CC1 (~400 km SSE of the Southeast Crater) and TDF stations (bottom). Arrows indicated periods of eruptive activity. Courtesy of J.C. Tanguy.

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: R. Romano, IIV; J. Tanguy, Univ. de Paris VI; R. Clocchiatti, CEN, Saclay, France; C. Archambault, CNET, France.