A re-examination of the three most prominent Holocene tephra deposits in western Canada: Bridge River, Mount St. Helens Yn and Mazama

Jensen, Britta J.L.; Beaudoin, Alwynne B.; Clynne, Michael A.; Harvey, Jordan; Vallance, James W.

doi:10.1016/j.quaint.2019.03.017

Cited by 28 publications

(15 citation statements)

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“…Glass geochemistry of the major tephra units of MSH and UA 3008 and UA 3295 (data for older sets from Mackay et al ., ; Jensen et al ., ; B. J. L. Jensen unpublished data). (A–D) FeO t , CaO, K 2 O and SiO 2 tend to most readily distinguish between the major tephra sets of the Spirit Lake stage.…”

Section: Resultsmentioning

confidence: 89%

“…In fact, it is one of the few rhyo-dacitic tephra from MSH outside of set B, with all other major eruptive units either with rhyolitic, basaltic andesitic or andesitic glass compositions (e.g. Hallett et al, 2001;Foit et al, 2004;Kuehn and Negrini, 2010;Pyne-O'Donnell et al, 2012Jensen et al, 2019;our unpublished data). This compositional diversity is not unexpected as the 'floating island' flow, emplaced just after layer T, is of whole rock andesitic composition (e.g.…”

Section: Layer Tmentioning

confidence: 99%

“…Results were compared to the internal University of Alberta tephra geochemical database, which contains data previously collected for layer T and other older MSH eruptions, some of which have been published (e.g. Pyne-O'Donnell et al, 2012;Mackay et al, 2016;Jensen et al, 2019;Tables 1,2). Major element analyses of glass are reported on a normalized basis.…”

Section: Study Areamentioning

confidence: 99%

“…Although some individual point glass geochemical data are available from more recent studies (e.g. Pyne‐O'Donnell et al ., ; Mackay et al ., ; Jensen et al ., ), most datasets, including those for the youngest MSH eruptive units produced during the Goat Rocks and Modern eruptive periods (ca. ad 1800–2008; Fig.…”

Section: Introductionmentioning

confidence: 99%

“…1B), such as Yn, Ye, We, Wn, member(s) of set P, and layer T, are important stratigraphic markers across western USA and Canada (e.g. Smith et al, 1975;Westgate, 1977;Foit et al, 2004;Jensen et al, 2019), and have been found as cryptotephra as far as Maine and Newfoundland (e.g. Pyne-O'Donnell et al, 2012;Mackay et al, 2016).…”

mentioning

confidence: 99%

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Glass geochemical compositions from widespread tephras erupted over the last 200 years from Mount St. Helens

Foo

Jensen

Bolton

2019

J Quaternary Science

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Building reliable chronologies from lake sediments, peat and other paleoenvironmental archives can be challenging, especially for historical times where radiocarbon is unreliable. Nineteenth-and 20th-century eruptions from Mount St. Helens (MSH) provide important chronostratigraphic markers for regional paleoenvironmental studies within this time frame, but are constrained by poorly geochemically characterized tephra and/or limited published data. Here, we present glass geochemistry from the most significant eruptions from this time. This includes proximal, medial and distal deposits of the 18 May 1980 MSH eruption, layer T (AD 1799/1800), a new tephra that we argue represents the AD 1842 eruption, and the 22 July 1980 eruption that had reported ashfall in Canada. Our results indicate that most tephras ejected during these eruptions, within a time frame of~200 years, have distinct glass geochemical characteristics that can be used to identify distal deposits for tephrochronological studies. Layer T is on trend with analyses of the 1980 eruption but has a distinct dacitic glass population. The 1980 and AD 1842 eruptions are similar, both having rhyolitic glass compositions, but the AD 1842 event can be differentiated by a more constrained SiO 2 range in the main geochemical population, and the presence of a unique SiO 2 sub-population.

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Section: Resultsmentioning

confidence: 89%

Section: Layer Tmentioning

confidence: 99%

Section: Study Areamentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

mentioning

confidence: 99%

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Glass geochemical compositions from widespread tephras erupted over the last 200 years from Mount St. Helens

Foo

Jensen

Bolton

2019

J Quaternary Science

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Sources of uncertainty in the Mazama isopachs and the implications for interpreting distal tephra deposits from large magnitude eruptions

et al. 2020

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Estimating the area of tephra fallout and volume of large magnitude eruptions is fundamental to interpretations of the hazards posed by eruptions of this scale. This study uses the tephra from the caldera forming eruption of Mount Mazama (Crater Lake, OR, USA) to demonstrate the challenges faced when working with large prehistoric tephra deposits and outlines the methodologies required to determine eruption volume and magnitude. We combine > 250 Mazama tephra occurrences, reported by a range of disciplines (including archaeology, paleoclimatology and volcanology), with new field studies to better understand the extent of the distal tephra. We find that the Mazama tephra has been remobilised to varying degrees over the past 7000 years, so each tephra locality was appraised for the likelihood that it records primary tephra fallout. We designated 45 of the distal (> 100 km from source) tephra sites as suitable for use in the production of isopachs using a spline fitting method. The new distal isopachs were then integrated with proximal fallout data and estimates of the ignimbrite volume from previous studies to revise the estimated bulk erupted volume from the climactic Mazama eruption to~176 km 3 (~61 km 3 dense-rock equivalent; DRE). This study demonstrates the importance of collating tephra localities from a range of disciplines and that even remobilised deposits provide valuable information about the extent of the deposit. Interpreting remobilised deposits can provide insight into post-eruptive processes that could potentially pose secondary hazards following large magnitude eruptions. We also show that in some circumstances, remobilised deposits preserve important physical properties such as grain size.

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Modelling the transport and deposition of ash following a magnitude 7 eruption: the distal Mazama tephra

et al. 2022

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Volcanic ash transport and dispersion models (VATDMs) are necessary for forecasting tephra dispersal during volcanic eruptions and are a useful tool for estimating the eruption source parameters (ESPs) of prehistoric eruptions. Here we use Ash3D, an Eulerian VATDM, to simulate the tephra deposition from the ~ 7.7 ka climactic eruption of Mount Mazama. We investigate how best to apply a VATDM using the ESPs characteristic of a large magnitude eruption (M ≥ 7). We simplify the approach to focus on the distal deposit as if it were formed by a single phase of Plinian activity. Our results demonstrate that it is possible to use modern wind profiles to simulate the tephra dispersal from a prehistoric eruption; however, this introduces an inherent uncertainty to the subsequent simulations where we explore different ESPs. We show, using the well-documented distal Mazama tephra, that lateral umbrella cloud spreading, rather than advection–diffusion alone, must be included in the VATDM to reproduce the width of the isopachs. In addition, the Ash3D particle size distribution must be modified to simulate the transport and deposition of distal fine-grained (< 125 µm) Mazama ash. With these modifications, the Ash3D simulations reproduce the thickness and grain size of the Mazama tephra deposit. Based on our simulations, however, we conclude that the exact relationship between mass eruption rate and the scale of umbrella cloud spreading remains unresolved. Furthermore, for ground-based grain size distributions to be input directly into Ash3D, further research is required into the atmospheric and particle processes that control the settling behaviour of fine volcanic ash.

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A re-examination of the three most prominent Holocene tephra deposits in western Canada: Bridge River, Mount St. Helens Yn and Mazama

Cited by 28 publications

References 42 publications

Glass geochemical compositions from widespread tephras erupted over the last 200 years from Mount St. Helens

Glass geochemical compositions from widespread tephras erupted over the last 200 years from Mount St. Helens

Sources of uncertainty in the Mazama isopachs and the implications for interpreting distal tephra deposits from large magnitude eruptions

Modelling the transport and deposition of ash following a magnitude 7 eruption: the distal Mazama tephra

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