A frame recorded by the visible-light monitoring camera of the INGV-Catania on the Schiena dell'Asino, about 5 km southeast of the summit craters of Etna, in the instant of the first explosion from the New Southesat Crater at 03.36 GMT on 19 August 2011

Early stage of the pase of sustained lava fountaining and ejection of abundant large pyroclasts, causing heavy fallout into the flanks of the New Southeast Crater cone, shortly after 07.00 h GMT on 20 August 2011. Note the presence, on the eastern (center right) side of the cone, of light brown dust clouds, rising from the portion of the cone that is about to collapse due to the push of lava from within the main flow channel (at right). Photo taken by Stefano Branca, INGV-Catania

The "prelude" to this episode started on the morning of 18 August with an increase in the gas emissions from the New Southeast Crater. At 03.36 GMT (= local time -2) on the 19th, the visual monitoring systems of the INGV-Osservatorio Etneo (Catania section) recorded a powerful explosion that launched incandescent bombs and a small plume of ash; this esplosions was followed by a few minor ones. These events were accompanied by an increase in the volcanic tremor amplitude and a gradual shift of its source from its previous position below the Northeast Crater toward the New Southeast Crater. During the day of 19 August, small emissions of dilute ash occurred; meanwhile the volcanic tremor amplitude showed fluctuations on a level that remained generally lower than the first increase on the morning. On the evening of the same day, a weak Strombolian activity was under way, with small explosions about every 30 minutes or so.

Shortly before 02.00 GMT on 20 August, a weak but continuous glow appeared within the active crater, which gradually increased during the following 30 minutes; this glow was produced byh the emission of lava within the crater. At about 02.30 the Strombolian activity started to intensify, and around 02.55 lava started to overflow through the deep breach in the eastern crater rim. For the next four hours, the Strombolian activity continued with fluctuations in the intensity and frequency of the explosions, while the lava flow slowly advanced toward the western slope of the Valle del Bove. Finally, shortly before 07.00 GMT the activity showed a notable intensification, and in a few minutes passed into lava fountaining, which generated heavy fallout of large pyroclastics onto the flanks of the cone. A few minutes later, light brown dust clouds appeared in an area on the lower east flank of the cone, where a small depression had formed a few hours after the 12 August paroxysmal episode, due to the sliding of a part of the still-hot deposit of that episode. Shortly thereafter, the continuous, intense projection 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 toward south.

Twin lava fountains are seen in this image captured from video recorded by the thermal monitoring camera of the INGV-Catania on the Montagnola, located about 3 km south of the summit craters of Etna, at 07.24 GMT on 20 August 2011

Lava flows descending the western slope of the Valle del Bove, at the end of the paroxysmal eruptive episode of 20 August 2011. Photo taken from the Schiena dell'Asino by Boris Behncke, INGV-Catania

In the meantime, the lower portion of the east flank of the cone, on the southern side of the large breach and lava overflow channel in the eastern crater rim, began to slide and collapse under the push of lava from within the channel. Eventually a new lava flow issued from the collapsed area, taking a more southerly path than the lava emitted until then. The lava divided into numerous branches, most of which took the same parcourse as the lavas emitted during the preceding paroxysmal episodes.

Lava fountaining began to show signs of diminishing shortly after 07.30 and totally ceased at about 07.50. For a few more minutes, ash was emitted from the crater, before all returned calm. At 11.59 GMT, a series of explosive ash emission started, which lasted until 12.04.

This latest paroxysmal eruptive episode, one of the shortest and most violent in the series of such episodes initiated in January 2011, occurred 8 days after its predecessor, a slightly longer interval than the previous ones. The early buildup phase was characterized by rather low levels of activity until 02.00 GMT on 20 August, while the passage from relatively modest Strombolian activity to sustained lava fountaining and the rise of an eruption column took place in only a few minutes.

The morphological changes affecting the pyroclastic cone surrounding the New Southeast Crater were significant: besides the collaspe on the lower east flank of the cone, the southern and northeastern rims of the cone have grown in height.