Bimodal grain size distribution and secondary thickening in air-fall ash layers

Brazier, S.; Sparks, R. S. J.; Carey, Steven; Sigurdsson, Haraldur; Westgate, John A.

doi:10.1038/301115a0

Cited by 105 publications

(44 citation statements)

References 16 publications

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“…Larsson (1937) tried to rationalize the secondary thickness maximum by appealing to atmospheric turbulence induced by the Sierra de C6rdoba > 250 km farther north, but (if real) the downwind thickening is more likely to have been caused by premature fallout of fine ash as larger aggregates, locally concentrated by atmospheric moisture or electrostatic attraction. Such a secondary thickness maximum was recorded ~ 325 km downwind by Sarna-Wojcicki et al (1981) for the 18 May 1980 ashfall from Mount St. Helens, and it has generally been attributed to such particle aggregation (Sorem 1982;Carey and Sigurdsson 1982;Brazier et al 1983). Aggregation of fallout particles at La Plata (1160 km E of Quizapu) was indeed described by both Dartayet (1932) and Lunkenheimer (1932), who measured clusters (copos) as large as 1 mm even though > 98 wt% of the ash particles were smaller than 0.1 mm (Kreutz and Jurek 1932).…”

Section: Thicknessmentioning

confidence: 99%

Volc�n Quizapu, Chilean Andes

1992

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Abstract. Quizapu is a flank vent of the basalt-to-rhyodacite Holocene stratocone, Cerro Azul, and lies at the focus of a complex Quaternary volcanic field on the Andean volcanic front. The Quizapu vent originated in 1846 when 5 km 3 of hornblende-dacite magma erupted effusively with little accompanying tephra. Between 1907 and 1932, phreatic and strombolian activity reamed out a deep crater, from which 4 km 3 of dacite magma identical to that of 1846 fed the great plinian event of 10-11 April 1932. Although a total of > 9 km 3 of magma was thus released in 86 years, there is no discernible subsidence. As the pre-plinian crater was lined by massive lavas, 1932 enlargement was limited and the total plinian deposi t contains only ~ 0.4 wt % lithics. Areas of 5-cm and 1-cm isopachs for compacted 1932 fallout are about half of those estimated in the 1930's, yielding a revised ejecta volume of -9.5 km 3. A strong inflection near the 10-cm isopach (downwind -1 1 0 km) on a plot of log Thickness vs Area 1/2 reflects slow settling of fine plinian ash -not of coignimbrite ash, as the volume of pyroclastic flows was trivial (<0.01 km3). About 17 vol.% of the fallout lies beyond the 1-cm isopach, and ~ 82 wt% of the ejecta are finer than 1 mm. At least 18 hours of steady plinian activity produced an exceptionally uniform fall deposit. Observed column height (27-30 km) and average mass eruption rate (1.5 x 108 kg/s) compare well with values for height and peak intensity calculated from published eruption models. The progressive "aeolian fractionation" of downwind ash (for which Quizapu is widely cited) is complicated by the large compositional range of 1932 juvenile pumice (52-70% SiO2). The eruption began with andesitic scoria and ended with basaltic scoria, but > 95% of the ejecta are dacitic pumice (67-68% SiO2); minor andesitic scoria and frothier rhyodacite pumice (70% SiO2) accompanied the dominant dacite. Phenocrysts (pl > hb -opx > mt > ilm ~ cpx) are similar in both abundance and composition in the 1846 (effusive) and 1932 (plinian) dacites. Despite the contrast in mode of eruption, bulk compositions are also indistinguishable. The only difference so far identified is a lower range of ~D values for 1846 hornblende, consistent with pre-emptive degassing of the effusive batch.

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Section: Thicknessmentioning

confidence: 99%

Volc�n Quizapu, Chilean Andes

1992

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“…If this is the case, more proximal thickening is likely to occur in the unmapped region toward the volcano. Such a feature may arise from fine-particle aggregation [Carey and Sigurdsson, 1982;Brazier et al, 1983], resulting in a larger effective grain size, owing to ash interaction with frozen or liquid hydrometeors [Textor et al, 2006;Durant et al, 2008]. The less explosive 3 May eruptive phase produced 0.05 km 3 of tephra, dispersed by strong winds (Figure 1), and depositing ash in a sharp-edged linear lobe.…”

Section: Eruption Volumesmentioning

confidence: 99%

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

et al. 2009

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[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.

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“…2) shows the distinct secondary increase of ash thickness in the north of the Kamchatka River valley, in the region of the Tolbachik plateau and in the region of Shiveluch volcano. Such a phenomenon was reported by Brazier et al (1983) for the 1980 Mount St. Helens and 1979 Soufriere eruptions, and is obviously the result of the accretionary lapilli formation.…”

Section: Oa Braitseua Et Al /Journalmentioning

confidence: 96%

The caldera-forming eruption of Ksudach volcano about cal. A.D. 240: the greatest explosive event of our era in Kamchatka, Russia

Braitseva

Мелекесцев

Пономарева

et al. 1996

Journal of Volcanology and Geothermal Research

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The largest Plinian eruption of our era and the latest caldera-forming eruption in the Kuril-Kamchatka region occurred about cal. A.D. 240 from the Ksudach volcano. This catastrophic explosive emption was similar in type and characteristics to the 1883 Krakatau event. The volume of material ejected was 18-19 km3 (8 km3 DRE), including 15 km3 of tephra fall and 3-4 km3 of pyroclastic flows. The estimated height of eruptive column is 22-30 km. A collapse caldera resulting from this eruption was 4 X 6.5 km in size with a cavity volume of 6.5-7 km3. Tephra fall was deposited to the north of the volcano and reached more than 1000 km. Pyroclastic flows accompanied by ash-cloud pyroclastic surges extended out to 20 km. The eruption was initially phreatomagmatic and then became rhythmic, with each pulse evolving from pumice falls to pyroclastic flows. Erupted products were dominantly rhyodacite throughout the eruption. During the post-caldera stage, when the Shtyubel cone started to form within the caldera, basaltic-andesite and andesite magma began to effuse. The trigger for the eruption may have been an intrusion of mafic magma into the rhyodacite reservoir. The eruption had substantial environmental impact and may have produced a large acidity peak in the Greenland ice sheet.

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Bimodal grain size distribution and secondary thickening in air-fall ash layers

Cited by 105 publications

References 16 publications

Volc�n Quizapu, Chilean Andes

Volc�n Quizapu, Chilean Andes

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

The caldera-forming eruption of Ksudach volcano about cal. A.D. 240: the greatest explosive event of our era in Kamchatka, Russia

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