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

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.