IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

1P2_2H-O16

Room A1

Date/Time: July 21 17:15-17:30

Ductile and brittle processes of permeability development and outgassing during hybrid
explosive-effusive activity at Cordon Caulle, Chile (2011-12)
C Ian Schipper1 , Jonathan M Castro2 , Hugh Tuffen3 , Michael James3 , Penny How31SGEES, Victoria University of Wellington, New Zealand, 2 Institute of Geosciences, Johannes
Gutenberg University Mainz, Germany, 3 Lancaster Environment Centre, United Kingdom
E-mail: schipper.ian@gmail.com

After initial Plinian to sub-Plinian phases of activity, the 2011-12 eruption of Cordon Caulle, Southern Chile,transitioned into a months-long period of ash jetting and Vulcanian blasts, with effusion of a voluminous obsidianlava flow. Ejected pyroclasts ranged from fine tube-like pumiceous ash to large (<3 m) bombs of dense obsidian.
Field campaigns before, during, and after the eruption allow us to describe the shallow conduit that fed the eruption
using direct observations of activity and features of the pyroclasts.
The vent complex and site of lava effusion was represented by two loci of Vulcanian blasts within a single tephra
cone. These loci each consisted of clusters of long-lived sub-vents that expressed correlated shifts in eruption
intensity, indicating the presence of partially connected and/or branching zones of high permeability within theupper conduit. Sub-vents ejected the tube-like pumiceous ash, with porosity (39-67 %) and anisotropic Darcian
permeability (3.1x10-15 m2 perpendicular to fabric to 3.8x10-11 m2 parallel) indicating that the permeable networksconsisted of highly sheared, tube-like bubbly magma. These contrasted with the low porosity (17%) and nul
permeability of bombs ejected to hundreds of metres from the vent in Vulcanian blasts. Residual H2 O content ofash (0.14 wt%) and bombs (0.2-0.25 wt%) indicate pyroclast degassing to near-atmospheric pressures. Together,
the pyroclasts demonstrate that both ductile and brittle processes of permeability development and gas transfer
were important.
Ash textures and modeling indicate the onset of permeability by ductile processes of shear-enhanced bubble
coalescence in the upper 1 to 1.5 km of the Cordon Caulle conduit. Textures in pumices collected early in theeruption indicate a significant role for shear localization in the ductile development of permeability, which weextrapolate to the hybrid activity. Bombs of tube pumice collected after the eruption indicate that the ash represents
fragmented parts of larger bodies. Ongoing work aims to reconcile the very low H2 O equilibration pressures of theash (< 1 MPa) with the considerably higher overpressures (5.7-12.5 MPa) theoretically required to fragment meltwith the measured porosity and permeability.
Ash jetting and Vulcanian blasts show that ductile processes were not sufficient to accommodate degassing, andadditional brittle (e.g., fragmentation) events assisted in gas transfer to the atmosphere. Brittle processes of gas
transfer are examined through the frequency of Vulcanian blasts recorded during the eruption, as well as in texturesof dense-to-breadcrusted, tuffisite vein-bearing obsidian bombs collected afterwards. Ductile and brittle processes
combined to permit the open-system degassing required for hybrid explosive-effusive activity.

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