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Long-lasting, small-to-moderate eruptions at composite volcanoes: the largest eruption of Mt. Ruapehu (Aotearoa New Zealand) in the last 1800 years

Marija Voloschina1 , Gert Lube2, Anja Moebis2, Costanza Bonadonna3, Marco Pistolesi1, Jonathan Procter2

  • Affiliations:  1Department of Earth Sciences, University of Pisa, Pisa, Italy; 2Volcanic Risk Solutions, Massey University, Palmerston North, New Zealand; 3Department of Earth Sciences, University of Geneva, Geneva, Switzerland 

  • Presentation type: Poster

  • Presentation time: Friday 16:30 - 18:00, Room Poster Hall

  • Poster Board Number: 168

  • Programme No: 3.4.41

  • Theme 3 > Session 4

Abstract

Small-to-moderate explosive eruptions involve tephra volumes ≤0.1 km3 and VEIs≤3. The ejected material often comprises significant amounts of ash-sized pyroclastic material, reducing the preservation potential of associated deposits and leading to the underrepresentation of these eruptions in long-term, frequency-magnitude datasets. Mt. Ruapehu has produced at least 32 small-scale eruptions over the past 1800 years, with the largest of these eruptions lasting several months to years and depositing the widespread T13-sequence. The cumulative deposit volume of 0.15 km3 is an order of magnitude larger than average deposit volumes of the past 1800 years. The T13-ash-lapilli-sequence comprises six depositional units with variable tephra dispersal, deposit texture and pyroclast characteristics. These represent at least five explosive phases of variable intensity and magnitude: the initial phase shows a dispersal limited to 11 km and a tephra volume of 8.5 × 105 m3, while the following peak phase comprises a widely dispersed unit with a volume estimated at 8.8 × 107 m3. Combination of deposit characteristics with the textural analysis of different types of juvenile clasts suggests that shallow processes in the conduit, such as degassing and crystallisation are the main factors controlling eruption style and intensity. The multilobate and irregular tephra dispersal, together with laterally variable pyroclast assemblage, indicate unsteady and wind-controlled eruption plumes. This study allows to discuss the complexity of tephra sequences associated with long-lasting, small-to-moderate eruptions, as well as their key eruption parameters that can be obtained through detailed deposit characterisation and the main limitations associated with existing classification schemes.