Activity at Etna since the end of the last flank eruption on 28 January 2003 (BGVN 28:01) was characterized by intense degassing at the Northeast Crater (NEC). In April, ash emission was observed from Bocca Nuova crater (BN), and ash fell for about 1 hour on E-flank villages. On 17 April a helicopter survey, aided by use of a thermal camera, revealed a cinder cone within the S pit of BN with a hot vent at its top. However, no degassing was taking place from this vent, and the pit appeared mostly obstructed by debris from the crater walls. Rare explosions from the vent caused little emission of juvenile material on the crater floor. Another helicopter-borne thermal survey in May showed that the summit craters were mostly obstructed.

Only a hot crack within the S pit of BN was observed during a June field survey. A new vent on the N rim of the Voragine (VOR), detected during a June field survey, was ~0.5 m wide, and the temperature measured through a thermal camera was ~500°C, much higher than the two vents within the crater. Given the presence of hot features within the summit craters and the obstructions observed inside BN, Southeast Crater (SEC), and VOR, it is possible that renewal of explosive activity at these summit craters could be accompanied by sudden, unpredictable gas explosions.

On the afternoon of 11 August an increase in volcanic tremor at the summit seismic stations lasted about 15 minutes and was followed by about 30 minutes of strong explosion earthquakes recorded at all summit stations of the INGV-CT seismic network. This was the first such event recorded at Etna since the end of the flank eruption. The INGV-CT web camera at Milo (~11 km from the summit) showed a puff of red ash from the summit of NEC. Red glows from the same crater were reported that night. A field survey on 14 August did not reveal any explosive activity or sounds of explosions from the crater. There were no explosion earthquakes or increased volcanic tremor between 11 and 16 August.

Periodic measurements of the gas plume from the summit using both COSPEC (SO₂ flux) and FTIR (SO₂/HCl and HCl/HF ratios) showed decreases in all three values since the end of the flank eruption. This suggests a general decreasing trend in gas output from Etna's summit craters.

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: Sonia Calvari, Istituto Nazionale di Geofisica e Vulcanologia, Piazza Roma 2, 95123 Catania, Italy (URL: http://www.ct.ingv.it/).