Fallout and distribution of volcanic ash over Argentina following the May 2008 explosive eruption of Chaitén, Chile

Abstract

[1] The major explosive eruption of Chaitén volcano, Chile, in May 2008 provided a rare opportunity to track the long-range dispersal and deposition of fine volcanic ash. The eruption followed $10,000 years of quiescence, was the largest explosive eruption globally since Hudson, Chile, in 1991, and was the first explosive rhyolitic eruption since Novarupta, Alaska, in 1912. Field examination of distal ashfall indicates that $1.6 Â 10 11 kg of ash (dense rock equivalent volume of $0.07 km 3 ) was deposited over $2 Â 10 5 km 2 of Argentina during the first week of eruption. The minimum eruption magnitude, estimated from the mass of the tephra deposit, is 4.2. Several discrete ashfall units are identifiable from their distribution and grain size characteristics, with more energetic phases showing a bimodal size distribution and evidence of cloud aggregation processes. Ash chemistry was uniform throughout the early stages of eruption and is consistent with magma storage prior to eruption at depths of 3-6 km. Deposition of ash over a continental region allowed the tracking of eruption development and demonstrates the potential complexity of tephra dispersal from a single eruption, which in this case comprised several phases over a week-long period of intense activity.

An example of enhanced tephra deposition driven by topographically induced atmospheric turbulence

“…This spacing compares favourably with the sampling density of well-mapped historical deposits (Scasso et al 1994;Watt et al 2009). For example, the Mount St. Helens 1980 secondary maximum is mapped at a site spacing of ∼30 km across the plume axis and 60-120 km in a downwind direction, meaning that mapping of the secondary maximum is based on ∼10 sites ).…”

“…1) involved a single sustained Plinian explosive phase. This relative simplicity contrasts, for example, with the multiple overlapping units that form the 2008 Chaitén eruption deposit (Watt et al 2009;Alfano et al 2011), where deposition from multiple eruptive phases at a single site obscures depositional complexities arising from other processes. The latter event was far smaller than Cha1; for comparison, at a distance of 135 km, the thickest part of the 2008 deposit comprises 1 cm of medium-grained ash, compared with 15-18 cm of fine lapilli on the dispersal axis of the Cha1 deposit.…”

A review of volcanic ash aggregation

“…This is known as a distal mass accumulation maximum (or 'secondary thickening'), and results from aggregation in the drifting ash cloud (Carey and Sigurdsson, 1982;Sarna-Wojcicki et al, 1981;Sorem, 1982;. Distal mass accumulation maxima have been recognised in a number of recent ash deposits e.g., 1932 Quizapu eruption (Hildreth and Drake, 1992); May-Aug, 1980 eruptions of Mount St Helens, USA (Sarna-Wojcicki et al, 1981); 1991 eruptions of Unzen, Japan (Watanabe et al, 1999); 1991 eruption of Mt Hudson (Scasso et al, 1994); 1991 Pinatubo eruption (Wiesner et al, 2004); June-Sept1992 eruptions of Crater Peak, USA, , 2008 eruption of Chaiten, Chile, (Watt et al, 2009) as well as in ancient deposits (e.g., Lerbekmo, 2002).…”

“…As a result of aggregation, many distal ash fall deposits are poorly sorted and exhibit polymodal particle size distributions (e.g., Varekamp et al, 1984;Carey and Sigurdsson, 1982;Scott and McGimsey, 1994;Watt et al, 2009;). …”

Construction of volcanic records from marine sediment cores: A review and case study (Montserrat, West Indies)

“…Transitions or jumps in grading patterns can reflect changes in both the eruption parameters and/or wind conditions (e.g. Watt et al, 2009). …”