The origin of a mixed-avalanche deposit generated during the 2008 eruption of Llaima volcano, Chile
Breard, Eric Christophe Pascal
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Llaima volcano is one of the most active volcanoes of the Southern volcanic Zone of the Andes. In January 2008, it entered into eruption and displayed Strombolian to Violent-Strombolian eruption styles (Chile). Two bomb-rich basaltic-andesite pyroclastic flow deposits were found on the northeastern flank, their emplacement was not witnessed during eruption. Generation of pyroclastic flows is not common for the typical eruptive activity at Llaima volcano, thus it needs to be deduced from field relating shape, granulometry, componentry and petrographic studies the processes of formation of the deposits. Studies undertaken were mainly focused on the coarser flow deposit that has a lobate front with an H/L ratio of 0.4. The deposit is poorly sorted and consists of subangular clasts and spherical cauliflower and breadcrust bombs set in a non-cohesive matrix. Juvenile clasts are blocky and slightly vesicular. The cauliflower bombs suggest that the Llaima pyroclastic flow was generated by phreatomagmatic explosions at the source conduit which was filled with a mixture of solidified. Water involved in the explosion likely resulted of the melting of the glacier that covered the volcano. The model proposed is the following. Lava fragmented by phreatomagmatic explosions created a coarse pyroclastic density current that incorporated clasts in the edge of the main crater. Then the low fluidized flow reached the glacier. Erosion processes dominated in the part where the slope is higher than 30°, Young non-juvenile materials deposited on snow in the previous few days/weeks were added to the juvenile material. Deposition began where the slope reached 30° (below the angle of repose). Deposition occurred in unconfined settings, the granular PDC is interpreted to have been quasi-steady, forming lobes. Spreading over snow/ice and eventually incorporating it, enhanced the runout (low friction) of the pyroclastic flow that eventually became a mixed-avalanche and stopped on slope angles between 0-10°. The second PF is smaller than the first one, and flowed over the first coarse deposit. The Crystal Size Distribution (CSD) study undertaken of the juvenile cauliflower clasts proves that different magmas evolved in different reservoirs in the plumbing system at Llaima volcano and the magmas were mixed and had a somewhat similar evolution in the same reservoir or conduit before eruption at the main crater. It proves that the two deposits studied have an origin at the main cone. Hence, the snow interaction can create new hazards at Llaima volcano, in fact, numerous lateral explosion events occurred in January 2008 on the east flank, suspected also to be linked to fragmentation of lava by phreatomagmatism.