Report on Etna (Italy) — 27 February-5 March 2013
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 27 February-5 March 2013
Managing Editor: Sally Kuhn Sennert
Please cite this report as:
Global Volcanism Program, 2013. Report on Etna (Italy). In: Sennert, S K (ed.), Weekly Volcanic Activity Report, 27 February-5 March 2013. 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 seismic activity at Etna's Bocca Nuova Crater (BN) gradually increased at night during 26-27 February. Copious, puffing emission of dense vapor from BN observed at sunrise slowly grew more energetic over the next few hours. Between 1030 and 1045, the volcanic tremor amplitude rapidly rose; at the same time an eruption column largely composed of vapor formed and hot material was ejected. The plume contained moderate amounts of reddish-brown volcanic ash, mostly during 1215-1220 and 1314-1316, which led to the fall of small quantities of very fine ash on the SE flank between Zafferana and Santa Venerina. These ash emissions likely resulted from collapse or sliding of unstable material on the steep inner crater wall; the eruptive vent, which lies in the SE part of the crater, is leaning against the wall and the rapid accumulation of pyroclastic deposits in that area might have facilitated collapses.
During the phase of most intense eruptive activity, INGV staff carrying out fieldwork in the summit area, noted that volcanic bombs fell outside the crater rim onto the SW flank of the central cone. Intense explosive activity was also observed within the Voragine (VOR), which since early October 1999 had not exhibited magmatic activity. The activity began to diminish around 1320, evident from a reduction in the volcanic tremor amplitude; at 1430, the episode was essentially over, even though strong degassing continued at both BN and VOR. During the evening of 27 February vigorous Strombolian activity in VOR was reported by observers on the W and E flanks. Minor and discontinuous Strombolian activity also occurred at BN which often launched incandescent volcanic bombs up to 150 m above the crater rim. The activity continued through the night, into at least early 28 February, when NSEC also started showing signs of renewed activity after more than four days of repose.
Weak explosive activity and sporadic weak ash emissions at New SE Crater (NSEC) were observed during the early morning hours of 28 February. Weak Strombolian activity in the W part of the NSEC appeared at 0917. Thirteen minutes later Strombolian activity was occurring at the main vent in the center of NSEC. Contemporaneously, the volcanic tremor amplitude started to rise. During the following hour, eruptive activity gradually increased, while the volcanic tremor amplitude increased rapidly; at 1030, at least three vents were erupting including the former "pittino" to the W, in the saddle between the old SEC cone and NSEC cone. This activity generated a dense gas plume containing modest quantities of ash. At 1040 lava flowed through the through the deep breach cutting the SE crater rim; 25 minutes later, explosive activity increased dramatically and lava fountains rose about 100 m above the crater rim. From 1115 on, frequent powerful explosions generated visible shock waves and heavily showered the NSEC cone with large volcanic bombs. The activity intensified further between 1117 and 1122, accompanied by a conspicuous cloud of vapor and brown ash issued from the saddle between the two SEC cones. This cloud marked the progressive collapse of a large portion of the saddle, which destroyed nearly all of the SW flank of the NSEC cone, and parts of the E flank of the old SEC cone. Voluminous amounts of lava flowed from the deep notch left by the collapse, expanding first S and then SE, in the direction of the Belvedere monitoring station. Lava was also emitted from the eruptive vents at the base of the NSEC cone; this lava mixed with the flow emitted directly from the NSEC toward SE.
Sustained lava fountaining, with intense pyroclastic fallout and the generation of a huge cloud of gas and ash, continued at maximum intensity for about 20 minutes. The ash cloud drifted E, leading to abundant ash and scoria fall in the Milo-Fornazzo and Giarre-Riposto areas. At 1142 the activity started to diminish, although dense clouds of vapor and grayish-brown ash were emitted from the collapsed saddle area. At about 1150, the activity at the "pittino" became phreatomagmatic; vapor and ash were emitted and hot, wet blocks that formed spectacular vapor trails were ejected. Shortly after 1200, explosive activity at the NSEC ceased, whereas lava emission continued from the collapsed saddle area as well as from the SE flank of the NSEC cone, at a slowly diminishing rate. The lava flows continued to advance during the night of 28 February-1 March, and ceased sometime during 1 March. Strombolian activity within the VOR continued without significant variations, and was observed by INGV staff during a field visit on the morning of 1 March, at least through 1500 on 3 March.
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.