Report on Etna (Italy) — 7 September-13 September 2011
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 7 September-13 September 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, 7 September-13 September 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 the thirteenth paroxysmal eruptive episode of 2011 took place at the New SE Crater of Etna on the morning of 8 September. Prior to the episode, a few emissions of ash from the New SE Crater occurred on 6 September. Sporadic, very weak Strombolian explosions from the crater started during the late evening on 7 September, and then continued in a subdued manner through the night.
On 8 September a series of ash emissions were followed by a rapid increase both in the intensity and frequency of Strombolian explosions. Loud detonations were audible across a vast sector of Etna's densely populated SE to E flanks. Simultaneously the volcanic tremor amplitude sharply increased and shifted from below the NE Crater toward the SE Crater. The Strombolian activity turned into a pulsating lava fountain, accompanied by increasing amounts of volcanic ash. Lava fountaining and ash emissions became more vigorous. Lava flowed through a deep breach in the E crater rim and along the fracture that had opened on the SE side of the cone during 29 August. The lava overflow was accompanied by repeated collapse and rockfalls from unstable portions of the cone in that area. Later brief periods of repeated emissions of brown ash mixed with white water vapor occurred from two or three vents on the N flank of the New SE Crater cone, in an area of the lava overflows from the N rim of the crater that had started shortly after the onset of the activity. The paroxysmal activity ceased in the evening and was followed by a series of progressively more passive ash emissions. Lava flows descended on the W slope of the Valle del Bove; expansion of the most advanced lava fronts continued for some time after feeding of the lava had ceased, mostly due to gravitational flow. Small active lava flows were observed for many hours after the cessation of the paroxysmal activity, remaining confined to the immediate vicinity of the crater.
The pyroclastic cone that grew around the New SE Crater during the recent series of eruptive episodes had undergone significant morphological changes. The S and N crater rims had further increased in height, whereas degradation and mass wasting on the SE flank had become more conspicuous. A large chunk of rock on the lower SE flank was rotated and uplifted, forming a steep-sided "spine" about 20-30 m tall, with locally vertical and sub-vertical flanks.
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.