Merging eruption datasets: building an integrated Holocene eruptive record for Mt Taranaki, New Zealand

Abstract

Acquiring detailed eruption frequency datasets for a volcano system is essential for realistic eruption forecasts. However, accurate datasets are inherently difficult to compile, even if one or more well-dated eruption records are available. A single record typically underrepresents the eruption frequency, while combining two or more records may result in an overrepresentation. Although glass compositions have proven to be successful in tephrochronological studies of dominantly rhyolitic tephras; microlitic growth and thin glass shards inhibit their application to andesitic tephras. A method consisting of a combination of two techniques for correlating syn-eruptive deposits is demonstrated on data from the typical andesitic stratovolcano of Mt. Taranaki, New Zealand. Firstly, tentative matches are identified using the radiocarbon age and associated error of each event. Secondly, the compositions of titanomagnetite micro-phenocrysts are used as an independent check, and shown to be a useful correlation tool where age data is available. Using two lake-core records containing tephra layers in an overlapping timeframe, the radiocarbon age-correlation procedure suggested 31 tephra matches. Geochemistry data were available for 15 of these pairs. In three of these cases, the titanomagnetite compositions did not match. Hence, these "paired" tephras were from compositionally distinct magmas and therefore likely represent separate events. An additional three matches were reassigned within the temporal uncertainty limits of the dating procedure, based on better geochemical pairing. The final combined dataset suggests that there have been at least 138 separate ash fall-producing eruptions between 96 and 10 150 years B.P. from Taranaki. Using the combined dataset the mixture of Weibulls renewal model forecasts a probability of 0.52 for an eruption occurring in the next 50 years at this volcano. The present annual eruption probability is estimated at 1.6%. This likelihood is almost double that obtained when relying on a single stratigraphic record.

Tephra from andesitic Shiveluch volcano, Kamchatka, NW Pacific: chronology of explosive eruptions and geochemical fingerprinting of volcanic glass

“…There is also 682 a strong effect of mixing on concentration of refractory trace elements in hybrid magmas. Geochemical studies of the detailed tephra record for individual volcanoes are few (e.g., 712 Donoghue et al 2007;Oladottir et al 2008;Turner et al 2009) though they permit to study 713 evolution of volcanoes with great details and sometimes show certain regular temporal patterns 714 in the eruptive records (Oladottir et al 2008). Our work at Shiveluch and Kliuchevskoi 715 volcanoes also shows that both volcanoes exhibit wave-like changes of SiO 2 contents in glass 716 from rapidly quenched tephras during Holocene roughly correlating in time between the 717 volcanoes (Portnyagin et al 2009(Portnyagin et al , 2011.…”

Tephrochronology and its application: A review

“…15). These biotite differences provided the key to identifying one of the distal tephras in the Kaipo bog sequence in eastern North Island that previously had been misidentified because its major-element glass chemistry matched that of two eruptives identical in age but derived from different volcanic sources (Shane et al, 2003a(Shane et al, , 2003b Analyses of Fe-Ti oxides, titanomagnetite and ilmenite, by EPMA have been useful for tephra fingerprinting for many years (e.g., Westgate and Evans, 1978;Hogg and McCraw, 1983;Beaudoin and King, 1986;Julian et al, 1988;Cronin et al, 1996;Smith et al, 2002;McHenry, 2005;Turner et al, 2009). The validity of such analysis as a correlation tool was supported by crystal field theory (Hodder, 1981).…”

“…She demonstrated that a single core or site is not sufficient to develop a regional stratigraphic record because of erosion and reworking, and that composite overlapping records, rather than a principal reference section or core, may be required to develop a comprehensive record (see also Turner et al, 2009;Schiff et al, 2010). A similar finding was reported for northern New Zealand at Lake Maratoto where Green and Lowe (1985) and Lowe (1988b) used 33 lake cores in total, and multiple peat cores, to reconstruct the lake"s tephrostratigraphic record and its developmental history since ca.…”

“…Also, andesitic glass is more susceptible to weathering than rhyolitic glass, meaning it can become unsuitable for analysis for correlation purposes. Consequently, wide compositional ranges may occur (Nakagawa et al, 1999;Shane et al, , 2008bDonoghue et al, 2007;Turner et al, 2009). …”

“…As well as providing isochrons for palaeoenvironmental research, tephras provide a comprehensive record or dossier of explosive volcanism and recurrence rates in the Quaternary or earlier and can therefore be used to establish time-space relationships of volcanism and insight into petrogenesis (e.g., Kohn and Topping, 1978;Wilson and Hildreth, 1997;Nakagawa et al, 1999;Smith et al, 2002Smith et al, , 2005Smith et al, , 2006Óladóttir et al, 2008;Turner et al, 2008b), and for volcanic hazard prediction and risk management (e.g., Shane and Hoverd, 2002;Smith, 2004, 2010;Jenkins et al, 2007;Turner et al, 2008aTurner et al, , 2009Lindsay et al, 2009). Tephra sequences preserved in peat or lake or marine sediments, or in ice, at medial or distal sites paradoxically may provide records potentially more comprehensive, or more assessable, than those proximal to volcanoes where the stratigraphy can be compromised by deep burial or erosion or other factors Alloway et al, 2005;Narcisi et al, 2005Narcisi et al, , 2010Lowe et al, 2007;Allan et al, 2008;Paterne et al, 2008;Turner et al, 2008b;Davies et al, 2010b).…”