Unravelling a complex volcanic history from fine-grained, intricate Holocene ash sequences at the Tongariro Volcanic Centre, New Zealand

Abstract

Holocene volcanism at the Tongariro Volcanic Centre (TgVC), at the south-western end of the Taupo Volcanic Zone (TVZ), New Zealand, involved eruptions from at least three centres and was dominated by numerous “small” eruptions of VEI 1–4. These produced very thin, grey to black, fine to coarse ash deposits of basaltic andesite to andesitic composition. Being unable to uniquely recognise and correlate these deposits presented a challenge in documenting the complete eruption record from the TgVC and to distinguish the periods of activity associated with each centre. Previous studies combined fine and poorly distinguishable tephras into mappable formations comprising many tens of units bracketed by rhyolitic marker tephras derived from the TVZ caldera centres to the north. Very fine-scaled mapping and analysis of glass of Holocene tephras in the TgVC were used to document the comprehensive explosive eruption record over the last ∼12 000 cal years. Glass compositional analyses distinguished the tephras from the three main centres of the TgVC: Mt. Ruapehu, Mt. Ngauruhoe and Red Crater (the latter two being part of the Mt. Tongariro complex). The three vents are sufficiently distinctive to be defined by ratios of major element oxides in the glasses, with the oxide pair K 2O/FeO being the most diagnostic. Ruapehu-sourced tephras are generally higher in K 2O (1.4–4 wt %) and lower in FeO (3.7–8.3 wt %) than Tongariro-derived units (Red Crater and Ngauruhoe). The diagnostic analyses provided the basis for correlating multiple tephra packages on the ring plains surrounding the TgVC and for establishing a revised eruption frequency for each of the volcanoes. The new data show that Ngauruhoe began erupting about 7000 cal years ago, up to 4500 cal years earlier than previously thought.