SIMULATIONS OF TEPHRA DISPERSAL FROM THE 1991 EXPLOSIVE ERUPTIONS OF HUDSON VOLCANO, CHILE

The 1991 explosive eruptions of Hudson volcano in southern Chile produced 2.7 km³ (DRE) of basaltic and trachyandesite tephra during the period August 8-15. The initial basaltic phase (Aug 8-9) produced a maximum column height of 12 km and tephra fallout was directed to the north and northeast by the prevailing winds. The paroxysmal phase (Aug 12-15) involved at least three separate events with a maximum ~18-km-high eruption column inferred from satellite temperature data. Tephra fallout from this phase was initially directed towards the south, before swinging towards the east as the wind changed direction. The plume was ultimately directed to the southeast by another shift in wind direction. The plume associated with the paroxysmal phase produced a multi-layered deposit composed of alternating layers of fine ash and pumice lapilli. Simulations of the 1991 explosive eruptions were conducted using the ash tracking model PUFF, which successfully reproduced the aerial distribution and temporal evolution of the plumes. The optimal agreement between the observed and simulated plumes occurs when the highest concentration of ash particles coincides with the tropopause, a height that is typically lower than the maximum observed column height. The highly stratified nature of the August 12-15 fall deposit is likely the result of multiple eruptive events coupled with a time varying wind field. A strongly changing wind direction that occurred during the earlier stages of the paroxysmal eruption could have produced variations in the dominant grain size being deposited between fine ash and pumice lapilli during individual eruptive sequences. Another consequence of a shifting plume direction is the cross-axis widening of the overall deposit. Such a widening could result in overestimated eruption column heights based on models that rely on field-based measurements of lithic and pumice dispersal.