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Long-lasting, small-to-moderate eruptions at composite volcanoes: reconstructing the largest eruption of Mt. Ruapehu (New Zealand) of the last two millennia

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Abstract

Small-to-moderate explosive eruptions involve VEIs ≤ 3, tephra volumes ≤ 0.1 km3 and often eject a significant amount of ash-sized pyroclastic material. This reduces the preservation potential of associated deposits and leads to an underrepresentation of these low- to mid-intensity explosive eruptions in long-term, frequency-magnitude datasets. Mt. Ruapehu is one of New Zealand’s most active volcanoes, having produced at least 32 small-to-moderate eruptions over the past 1800 years. The largest of these eruptions deposited the widespread T13-sequence and lasted several months to years. The cumulative deposit volume is estimated at 0.15 km3, thus being an order of magnitude larger than the average deposit volumes of the last 1800 years at Ruapehu. The sequence of pyroclastic fall deposits can be subdivided into six depositional sub-units representing at least five eruption phases of variable intensity and magnitude. The ash-lapilli sequence displays variable dispersal, deposit textures and pyroclast characteristics. While the initial phase is characterised by dispersal limited to the proximal 11 km and a tephra volume of 8.5 × 105 m3 (± 3%), the following high-intensity “peak” phase is estimated at 8.8 × 107 m3 (± 37.8%), representing about ⁓60% of the cumulative tephra volume. The combination of deposit characteristics with textural analysis of different types of juvenile clasts suggests that changes in eruption style and intensity were mainly controlled by shallow processes in the conduit, such as degassing and crystallisation and changes in conduit geometry. Multilobate, irregular dispersal patterns and laterally variable pyroclast assemblage indicate unsteady eruption conditions characterised by weak eruption plumes controlled by prevailing winds. This study testifies the complexity of tephra sequences associated with long-lasting, small-to-moderate eruptions, and describes the key eruption parameters that can be obtained through a detailed characterisation and identifies the main limitations related to the classification of these eruptive styles.

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Acknowledgements

MV thanks the Tongariro Natural History Society for receiving the Memorial Award. The authors are grateful for the support of H. Gabrielsen (Department of Conservation), K. Woods (Ernslaw Karioi Forest) and staff of the Waiouru Military Training Area for field access facilitation. R. Brahm Scott, E. Brosch, K. Kreutz, L. Roither and A. Todde are thanked for field assistance. The work of C. Durham with the help of C. Lormand with regards to image analysis are greatly acknowledged. Finally, the authors thank T. Esposti Ongaro for editorial handling and helpful comments on the manuscript as well as A. Aravena and an anonymous reviewer for their detailed and constructive suggestions.

Funding

This research is part of the research requirements for MV’s PhD thesis that was funded through the research program “Quantifying exposure to specific and multiple volcanic hazards” from the Natural Hazards Research Platform (2015-MAU-PC-01) and through the Resilience to Nature’s Challenges Volcano Program (GNS-RC047). Open access funding provided by Università di Pisa within the CRUI-CARE Agreement.

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Correspondence to Marija Voloschina.

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Editorial responsibility: T. Esposti Ongaro

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Voloschina, M., Lube, G., Moebis, A. et al. Long-lasting, small-to-moderate eruptions at composite volcanoes: reconstructing the largest eruption of Mt. Ruapehu (New Zealand) of the last two millennia. Bull Volcanol 86, 34 (2024). https://doi.org/10.1007/s00445-024-01723-x

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  • DOI: https://doi.org/10.1007/s00445-024-01723-x

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