Report on Etna (Italy) — 15 August-21 August 2012
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 15 August-21 August 2012
Managing Editor: Sally Kuhn Sennert
Please cite this report as:
Global Volcanism Program, 2012. Report on Etna (Italy). In: Sennert, S K (ed.), Weekly Volcanic Activity Report, 15 August-21 August 2012. 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 a new phase of activity at Etna's Bocca Nuova summit crater started on 2 July, was virtually continuous for three weeks until 24 July, then rapidly decreased. During the first 10 days explosions were separated by quiet intervals lasting several minutes, then later became more continuous. During periods of the strongest activity, some lava bombs were ejected beyond the crater rim, landing on a gently sloping area E of the crater. Lava was emitted during 4-24 July from vents on the flanks of the pyroclastic cone that had begun to grow around the explosive vent. Rarely, lava was emitted from the main explosive vent. The lava progressively covered the crater floor next to the pyroclastic cone, which had grown 30-40 m, before the activity ceased on 24 July. Frequent ash emissions observed the next day were mostly due to explosions, and possibly from minor collapses of the summit area of the new cone. Concurrent with the decrease in eruptive activity, volcanic tremor amplitude dropped to background levels.
Episodic activity occurred during 26 July-11 August. At daybreak on 26 July, strong incandescence marked the onset of the first episode of activity within Bocca Nuova, which had the same characteristics as the previous activity; frequent Strombolian explosions were accompanied by lava flow emissions from a vent located on the W flank of the cone. The next day, the lava had almost completely filled the central subsidence pit on the floor of Bocca Nuova, and the cone had grown in height. The activity then decreased, and essentially ceased by the evening. There were intermittent ash emissions on 28 July, and volcanic tremor amplitude decreased to background levels.
The second episode, during 29 July-1 August, was characterized by small Strombolian explosions and lava emission from a vent located on the S flank of the pyroclastic cone. Volcanic tremor amplitude showed strong oscillations, but never reached the same peak levels associated with the 26-27 July episode. Volcanic tremor amplitude dropped sharply on 1 August, marking the cessation of the activity; a few ash emissions were observed the next day.
The third episode began on 3 August with a rapid increase in the volcanic tremor amplitude and strong incandescence from Bocca Nuova, signaling the beginning of Strombolian activity and lava emissions onto the crater floor. Activity ceased the next day, and was followed by weak residual activity within Bocca Nuova, which generated small ash clouds. The fourth episode began on 6 August and ended the next day, and showed essentially the same characteristics as the previous episode.
The fifth episode began on 10 August and was again marked by a rapid increase in volcanic tremor amplitude; at nighl, bright glow illuminated the gas plume rising from the crater, which was visible up to tens of kilometers away. Some of the explosions ejected incandescent pyroclastic material well above the crater rim. The activity started to decrease just after midnight and then ended on 11 August. Subsequently, ash emissions resumed, which were particularly frequent on 13 August, and which produced small, brownish-gray clouds of fine-grained tephra. Some of the ash emissions later that evening were accompanied by ejected incandescent material; volcanic tremor amplitude, however, remained at background levels.
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.