Etna update, 17 July 2012

Strombolian activity within the Bocca Nuova on the evening of 3 July 2012, seen from the northwestern crater rim. The full moon illuminates the sky at upper left. Photo taken by Boris Behncke (INGV-Osservatorio Etneo)

Strombolian activity within the Bocca Nuova

On the late evening of 2 July 2012, a weak and intermittent glow began to illuminate the gas plume issuing from the Bocca Nuova, the largest of the summit craters of Mount Etna, marking the resumption of eruptive activity in this crater nearly one year after a short period of Strombolian and effusive activity. A field visit made to the crater by INGV-Osservatorio Etneo (INGV-OE) staff on the next evening revealed that mild Strombolian activity was occurring from a single vent a few meters in diameter, which lay at the base of the southeastern crater wall. At intervals of a few minutes, this vent produced Strombolian explosions; usually a first, strong explosion was followed by a series of progressively weaker bursts over the next few tens of seconds. Incandescent bombs and scoriae launched by these explosions did not reach the crater rim and fell around the active vent.

During the following days, the activity within the Bocca Nuova continued with minor fluctuations; on 4 July, a few bombs fell outside the eastern crater rim, and on 8 July, a small lava flow was observed on the flank of the pyroclastic cone growing around the active vent. Similar activity - Strombolian explosions and intermittent lava effusion from shifting sources on the flanks of the small pyroclastic cone - was observed during further visits by INGV-OE staff on 11 and 13 July. On 16 July, the amplitude of the volcanic tremor recorded by the summit seismic network of the INGV-OE showed a consistent increase, which was accompanied by an intensification of the activity within the Bocca Nuova; during the night of 16-17 July, continous, bright glow was visible at the crater with the naked eye from the populated areas around the volcano.

This sequence of video frames shows how the upper portion of the small pyroclastic cone within the Bocca Nuova swells before one of the stronger explosions on the evening of 11 July 2012; note also the sliding of incandescent material from the left flank of the cone, just a few instants before the onset of the explosion. Images were extracted from video recorded by Boris Behncke (INGV-Osservatorio Etneo) from the southern crater rim

This new eruptive activity within the Bocca Nuova is very similar to that obseved almost exactly one year ago, between 11 and 17 July 2011, at the same crater; also on that occasion, nearly continuous Strombolian activity was accompanied by small intracrater lava flows. However, the current activity started more than two months after the latest episode of lava fountaining at the New Southeast Crater, a very different dynamic framework compared to July 2011. In the past, the Bocca Nuova has produced long periods of Strombolian activity often accompanied by lava emission onto the crater floor. The most recent such period - apart from the brief July 2011 activity - lasted from July 1995 until early 2001. During that period, the crater was progressively filled, both by the growth of pyroclastic cones and intracrater lava emission, and overflowed repeatedly in October-November 1999.