Pyroclastic density currents resulting from the interaction of basaltic magma with hydrothermally altered rock: an example from the 2006 summit eruptions of Mount Etna, Italy

Abstract

After 16 months of quiescence, Mount Etna began to erupt again in mid-July 2006. The activity was concentrated at and around the Southeast Crater (SEC), one of the four craters on the summit of Etna, and eruptive activity continued intermittently for 5 months. During this period, numerous vents displayed a wide range of eruptive styles at different times. Virtually all explosive activities took place at vents at the summit of the SEC and on its flanks. Eruptive episodes, which lasted from 1 day to 2 weeks, became shorter and more violent with time. Volcanic activity at these vents was often accompanied by dramatic mass-wasting processes such as collapse of parts of the cone, highly unusual flowage processes involving both old rocks and fresh magmatic material, and magmawater interaction. The most dramatic events took place on 16 November, when numerous rockfalls and pyroclastic density currents (PDCs) were generated during the opening of a large fracture on the SE flank of the SEC cone. The largest PDCs were clearly triggered explosively, and there is evidence that much of the energy was generated during the interaction of intruding magma with wet rocks on the cone's flanks. The most mobile PDCs traveled up to 1 km from their source. This previously unknown process on Etna may not be unique on this volcano and is likely to have taken place on other volcanoes. It represents a newly recognized hazard to those who visit and work in the vicinity of the summit of Etna.

Generation of pyroclastic flows by explosive interaction of lava flows with ice/water-saturated substrate

“…Three paroxysmal eruptive episodes during this eruptive period, on 16 and 24 November 2006, and 29 March 2007, were accompanied by vigorous secondary phreatomagmatic explosions that generated PFs (Behncke et al, 2008;Norini et al, 2009). …”

“…These secondary explosions were somewhat different from the others described in this paper, because they occurred on slopes that were free of snow or frozen rocks. In contrast, in these cases lava flows interacted with substrate saturated with hydrothermal fluids (Behncke et al, 2008;Behncke, 2009). Both, the 16 and 24 November eruptive paroxysms in the primary volcanic vents lasted approximately 10-12 hours each.…”

“…The recognition that small PFs are a rather frequent product of Etna's summit eruptions follows eight events documented for the past~20 years (Behncke et al, 2008;Behncke, 2009). …”

“…Volcanoclastic flows of various scales and origins were observed during both paroxysmal eruptive episodes on 16 and 24 November 2006 (Behncke et al, 2008;Behncke, 2009;Norini et al, 2009). We will focus here on the formation of the two largest PFs that were formed as a result of secondary explosions on lava flows.…”

“…Fisher and Schmincke, 1984;Morrissey et al, 2000). They were described for lava flows on Hawaii (Mattox and Mangan, 1997), Kamchatka (Vinogradov and Muravyev, 1985;Belousov, 1990Belousov, , 1996, Iceland (Morrissey and Thordarson, 1991;Hamilton et al, 2010), and Italy (Behncke et al, 2008). Secondary explosions typically represent rather mild vertical ejections of steam containing some amount of fragmented lava.…”