Identifying Pyroclastic Density Currents From Partial Outcrop Exposure on Mt. Ruapehu, New Zealand

Abstract

Pyroclastic density current (PDC) deposits, especially small to medium volume events, have low preservation potential at many volcanoes, particularly when unconsolidated or deposited on steep, glaciated slopes. This may lead to an underrepresentation of these events in the eruptive record, and consequently, in hazard management planning; leaving populations on and around the volcanoes unprepared for the threat of these smaller eruptions. Therefore, it is important to investigate and recognize these smaller events in the volcanic record to create more comprehensive plans for future eruptions. Mt. Ruapehu is one of New Zealand’s most active volcanoes, last erupting in 2007. Few studies have investigated the PDC occurrence on this volcano, despite PDCs being one of the most hazardous volcanic processes. Poor preservation of PDC deposits, due to small volume, past glaciations, erosion, burial, and poor consolidation has left a significant gap in Mt. Ruapehu’s eruptive record. By identifying and characterizing PDCs on Mt. Ruapehu this paper provides an updated account of PDC occurrence on this volcano, especially for smaller scale PDCs. Comprehensive field-mapping forms the basis for this study by identifying PDC deposits from partial outcrop exposures. We use field observations of these deposits to describe the lithofacies and infer PDC behavior. Relative stratigraphy and whole-rock geochemistry are used to correlate deposits with dated units from literature and provide approximate age ranges. This study describes 12 PDC deposits representing at least 10 previously unidentified flows. Combined with PDCs identified in previous studies there is a total of 23 PDC deposits found on Mt. Ruapehu, including the PDC observed during the 1945 eruption. These PDCs have been emplaced throughout Mt. Ruapehu’s 250 ka eruptive history. The PDCs were concentrated and dominated by granular flow or granular fluid-based flow transport regimes. The lithofacies show PDCs forming from column collapse and dome collapse or explosion events. This demonstrates that Mt. Ruapehu is capable of producing a spectrum of PDC styles and sizes, something that must be considered during future hazard planning on the volcano.