Postglacial effusive activity at Mt Ruapehu, Aotearoa New Zealand: Lava flow volumes and eruptive rate

Hazards posed by volcanic eruptions are directly related to the frequency and magnitude of eruptions, and understanding these parameters for past events helps to assess future hazards associated with individual volcanoes. Ruapehu (Aotearoa New Zealand) is an andesitic stratovolcano that produced several dozens of lava flows and tephra units during the last 20 kyr, and has been affected by two flank collapses during the Early and Middle Holocene. Recent studies have significantly improved the chronology of young (<20 ka) lava flow forming eruptions, but the magnitude of these events remain poorly constrained. In this study, we estimate the volume of 52 young lava flows at Ruapehu, and combine this data with a compilation of all available chronological constraints and published volumetric and chronological data of pyroclastic eruptions, to produce a detailed cumulative erupted volume curve for the last 20 kyr at Ruapehu. After the emplacement of large lava flows during the end of the Last Glacial Maximum (LGM), the eruptive rate remained relatively constant at around 0.27 km³/kyr (dense rock equivalent or DRE) until ca. 7.4 ka. Then, activity ceased for c. 2 kyr, after which it resumed less intensely than before 7.4 ka and from the southern vent only. Our data suggests that the two Holocene flank collapses that affected Ruapehu’s summit area have had different effects on the plumbing and magmatic system; the larger Early Holocene collapse forced a change in eruptive style towards effusive eruptions, while the smaller Mid-Holocene collapse did not have a significant impact on the volcanic activity. This study shows the importance of robust volumetric and chronological analyses in providing insights on the evolution of stratovolcanoes, and particularly when analysing the causes of the temporal variability of their activity.