Physical characterization of volcanic particle resuspension: the case of the 2011-2012 Cordón Caulle eruption (Chile)

LUCÍA DOMÍNGUEZ; COSTANZA BONADONNA; PABLO FORTE; LEONARDO MINGARI; RAFAELLO CIONI; DONALD BRAN; JUAN ESTEBAN PANEBIANCO

Viena

Congreso; European Geosciences General Assembly; 2018

Primary tephra fallout represents one of the most widespread volcanic hazard in terms of spatial and temporalscales. Several studies during the last couple of decades have provided new insights into our understanding oftephra fallout deposits. Less attention has been dedicated to the characterization of wind resuspension of ash asa secondary but often syn-eruptive volcanic hazard. Resuspension of volcanic particles is a global concern sinceit is not only confined to the source area (i.e. primary tephra deposit) but it can impact large areas up to severalhundred kilometres far from the source. In addition, ash resuspension can occur years, decades and even centuriesafter the eruption driving significant impact on large areas over extended periods of time and/or exacerbatingthe consequences of primary hazards and pre-existing conditions (e.g. droughts). Better understanding of thisphenomenon, frequency of events as well as the type, size and shape of particles remobilized by the wind istherefore necessary in order to i) improve our preparedness and mitigation measures for volcanic ash impacts inthe long-term; and ii) constrain accurate inputs for numerical models and consequently improve the forecasting ofresuspension events. We have conducted a detailed physical characterization of ash resuspension associated withthe 2011 eruption of Cordón Caulle volcano (Chile) based both on post-deposition field observations and on asystematic syn-deposition collection of airborne material through dedicated samplers at different heights abovethe soil surface. The 2011 Cordón Caulle eruption produced plumes of 3-14 km generating about 1 km3 of tephradeposited during several months (up to December 2012), but mostly during the first few days. Tephra was depositedtowards the Argentinian Andes and Patagonia due to the prevailing westerly winds. The finest and upper layer ofthis deposit has been remobilized since 2011 and it continues even up to today. Grain size analysis shows thatwind is capable to remobilize particles of a very specific range from 1 to 500 µm, with a median of 63 to 90µm, associated with particle saltation. However, dust devils charged with finer particles ( 500 µm) have been also observed in the area of Ingeniero Jacobacci in the Patagonian steppe.Our analysis also shows how grainsize sorting is correlated with collection height, with the highest samples (1.50 mfrom surface) being better sorted than the lowest samples (0.15 m from surface). Frequency of resuspension eventsand mass flux are mostly controlled by the amount of available ash on the surface with syn-eruptive tephra fallout.However, specific meteorological conditions, such as winds and soil moisture, become dominant with time. Thechronological evolution of mass flux shows that resuspension events were more frequent during the austral springand summer when prevailing winds are stronger (10 ? 18 km h−1), gusts reach 90 km h−1and precipitations arelow. Even though resuspension events still occur today, a turning point was represented by the strong precipitationsof April 2014, which was correlated with a strong decrease in mass flux.