Report on Etna (Italy) — 17 August-23 August 2011
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 17 August-23 August 2011
Managing Editor: Sally Kuhn Sennert
Please cite this report as:
Global Volcanism Program, 2011. Report on Etna (Italy). In: Sennert, S K (ed.), Weekly Volcanic Activity Report, 17 August-23 August 2011. Smithsonian Institution and US Geological Survey.
37.748°N, 14.999°E; summit elev. 3295 m
All times are local (unless otherwise noted)
Sezione di Catania - Osservatorio Etneo reported that eight days after the preceding episode, Etna's New SE Crater was the site of another paroxysmal eruptive episode on the morning of 20 August, the eleventh event of this type since the beginning of 2011. The event began on 18 August with increased gas emissions from New SE Crater. On 19 August a powerful explosion ejected incandescent bombs and produced a small ash plume. The event was followed by a few more minor explosions, all accompanied by an increase in volcanic tremor amplitude and focal shift from the NE Crater toward the New SE Crater. Throughout the day small dilute ash was emitted. During the evening weak Strombolian activity commenced, with small explosions occurring about every 30 minutes.
On 20 August weak but continuous incandescence due to lava emissions appeared in the crater. Strombolian activity intensified, and lava overflowed the rim through a breach in the E crater rim traveling towards the Valle del Bove. Almost five hours later lava fountaining generated heavy fallout of large pyroclastics onto the flanks of the cone. Dense plumes of gas and tephra rose 5-6 km from the crater and drifted SW, causing ash- and lapilli-fall in areas such as Paternò (22 km SSW), Ragalna (13 km SSW), and Biancavilla (16 km SW). Closer to the crater, in the Torre del Filosofo area to the S, clasts up to several tens of centimeters in diameter landed on the ground.
Light brown dust clouds appeared in an area on the lower E flank of the cone, where a small depression had formed a few hours after the 12 August event. Shortly thereafter, the continuous, intense ejection of pyroclastics onto the flanks of the cone generated avalanches resembling pyroclastic flows, which descended a few hundred meters beyond the base of the cone, mainly towards the S. In the meantime, the lower portion of the E flank of the cone began to slide and collapse under the push of lava from within the channel. A new lava flow issued from the collapsed area, taking a more southerly path than the lava emitted until then, and divided into numerous branches. Lava fountaining slowed later in the evening and eventually ceased, followed by ash emissions from the crater for a few minutes. A series of ash explosions lasted for five minutes in the early morning on 21 August. The morphological changes affecting the pyroclastic cone surrounding the New SE Crater were significant. Besides the collapse on the lower E flank of the cone, the S and NE rims of the cone had grown in height.
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.