Character, mass, distribution, and origin of tephra-fall deposits from the 2009 eruption of Redoubt Volcano, Alaska: highlighting the significance of particle aggregation

The 2009 eruption of Redoubt Volcano included 20 tephra-producing explosions between March 15, 2009 and April 4, 2009 (UTC). Next-Generation radar (NEXRAD) data show that plumes reached heights between 4.6 km and 19 km asl and were distributed downwind along nearly all azimuths of the volcano. Explosions lasted between < 1 and 31 min based on the signal duration at a distal seismic station (86 km). From Moderate Resolution Imaging Spectroradiometer (MODIS) imagery and field data, we estimate that over 80,000 km² received at least minor ash fall (> 0.8 mm thick), including communities along the Kenai Peninsula (80–100 km) and the city of Anchorage (170 km). Trace ash (< 0.8 mm) was reported as far as Fairbanks, 550 km NNE of the volcano. We estimate the total mass of tephra-fall deposits at 54.6 × 10⁹ kg with a total DRE volume of 20.6 × 106 m³.

On March 15, a small (4.6 km asl) phreatic explosion containing minor, non-juvenile ash, erupted through the summit ice cap. The first five magmatic explosions (events 1–5) occurred within a 6-hour period on March 23. Plumes rose to heights between 5.5 km and 14.9 km asl during 2- to 20-minute-duration explosions, and were dispersed mainly along a NNE trajectory. Trace ash fall was reported as far as Fairbanks. Owing to a shift in wind direction and heavy snowfall during these events, field discrimination among many of these layers was possible. All deposits comprise a volumetrically significant amount of particle aggregates, yet only event 5 deposits contain coarse clasts including glacier ice. The most voluminous tephra fall was deposited on March 24 (event 6) from a 15 minute explosion that sent a plume to 18.3 km asl, and dispersed tephra to the WNW. Within 10 km of the vent, this deposit contains 1–11 cm pumice clasts in a matrix of 1–2 mm aggregate lapilli. A small dome was presumably emplaced between March 23 and March 26 and was subsequently destroyed during 1–14 minute magmatic explosions of events 7–8 (March 26) that sent plumes between 8.2 km and 19 km asl. Ash fell along a broad swath to the ESE, covering communities along the Kenai Peninsula with up to 1 mm of ash. Proximal deposits are largely composed of aggregate lapilli of 1–2 mm with very little coarse juvenile material. Events 9–18 (March 27) sent plumes between 5.2 km and 15.5 km asl during < 1–11-minute-long explosions. Ash clouds dispersed along trajectories to the NE, ENE and N and event 17 deposited up to 1 mm of ash on upper Kenai Peninsula and Anchorage. A moderate-size dome was emplaced between March 29 and April 4 and was subsequently destroyed during event 19 on April 4 which lasted 31 min and sent ash to 15.2 km asl. The proximal deposit is principally composed of dense dome rock, unlike earlier events, indicating that event 19 was likely caused by dome failure. The cloud dispersed to the SE along a narrow trajectory and up to 1–2 mm of ash fell on the lower Kenai Peninsula.

Particle size data showing a preponderance of fine ash, even in the most proximal locations, along with the abundance of aggregate lapilli documented in most samples, confirms that particle aggregation played a significant role in the 2009 eruption and induced premature fallout of fine ash.