Sedimentology and event timing of a catastrophic volcaniclastic mass flow, Volcan Hudson, Southern Chile

Eruption of Volcan Hudson, Aisen region, Southern Chile on 12 August 1971 melted up to 80% of ice within the 9-km-wide Hudson Caldera. Rapid mobilisation of abundant fluvio-glacial sediment within the adjacent Huemules valley led to the generation of a catastrophic mass flow which swept westwards along the valley, eventually entering the fjord some 40 km downstream. The mass flow, developing from one source and being unaffected by tributary input, was characterised by three events within the flood hydrograph: (1) a precessional flood possibly linked to breakage of an early formed dammed lake; (2) a main mass flow which varied in its rheological behaviour from plug-flow to hyperconcentrated flow; and (3) a recessional flood at the tail of the main flow which diluted towards a concentrated streamflow. The main debris flow displayed characteristics of pulsing, internal segregation/density stratification, significant downstream sediment assimilation, cohesive/frictional freezing with ensuing fluidisation and development of an advancing runout phase. Consideration of these sedimentary characteristics and spatial/temporal flow transformations are used to propose a schematic model of event timing within this catastrophic volcaniclastic flow.