Inside the Vent of the 2011-2012 Cordón Caulle Eruption, Chile: The Nature of a Rhyolitic Ash Plume Source

Abstract:

The 2011-2012 activity at Cordon Caulle has provided an unprecedented opportunity to observe a sustained explosive rhyolitic eruption. An initial 27 hour Plinian phase commenced on 4 June 2011, followed by ten months of hybrid explosive-effusive activity, which generated disruptive ≤6 km ash plumes. In January 2012 our close observations of the active vent[1] revealed how episodic release of gas and ash from several sub-vents on an incipient lava dome (Fig. 1b) merged to form a sustained ash plume. Sub-vents ranged from metric point sources to arcuate fractures (>10 m) in the dome carapace.

We visited the vent in January 2014, and found two ~50 m-wide, rubble-strewn vent areas adjacent to pancake-like obsidian domes, all within a breached, ~100 m-high tuff cone. Vent areas consist of fractured obsidian lava strewn by loose, rotated lava blocks ≤5 m across. Prominent red fracture surfaces (Fig. 1 d,e) occur in both the in-situ lava and the blocky veneer; these closely correspond to the type of sub-vents observed in 2012[1]. They range from smooth, curviplanar surfaces extending over several m to complex smaller-scale surfaces that follow pre-existing cooling joints in the lava carapace. In-situ fracture surfaces display prominent, predominantly vertical grooves and impact marks, but negligible displacement.

Surfaces are coated by µm-mm thick veneers of fine-grained ash, to which larger ash-coated clasts have adhered. Veneer thickness and sintering degree strongly decrease towards the upper carapace of the lava. SEM analysis of ash veneers reveals 1) a high proportion of sub-micron clasts, 2) strong clast sintering, 3) abundant ash aggregation textures spanning submicron-mm scales, and 4) local surface scouring and corrosion of glass and phenocrysts.

During ash venting the smallest particles are preferentially trapped on fracture surfaces and rapidly sintered, encouraging sub-vent blockage. Extensive ash aggregation may have been electrostatically aided, with strong charging induced by high venting velocities through narrow fracture networks. Fluctuating gas/ash ratios and velocities, as observed[1], led to periods of both deposition and erosion. Rafting of earlier ash nozzles down the advancing obsidian flow has created a record of evolving vent dynamics.

1. Schipper C.I. et al., 2013, JVGR 262, 25–37.