New Insights into Eruption Styles in the Auckland Volcanic Field based on Textures, Volumes and Dispersal of Proximal to Medial Pyroclastic Fall Deposits

Abstract

It is well known that tephra fall originating from both distal and local (Auckland Volcanic Field; AVF) sources poses a significant hazard to the Auckland Region, and has been a major focus of several past studies. However, many of these studies focus on fine-grained ash fall, and do not consider the hazard and potential effects of more coarse-grained material. This study aims to shed light on the potential eruption styles, dispersal, volumes and impacts of proximal to medial (i.e. lapilli-sized) tephra generated from magmatic eruptions from six case study volcanoes within the AVF. Dispersal and volumes are estimated by combining field data with borehole data supplied by the New Zealand Geodatabase (NZGD), Petlab and DEVORA. Proximal tephra volume estimates for two of the case study volcanoes were able to be calculated. A volume estimate for proximal to medial tephra of 3.6 x 106m3 was calculated for Mt Wellington, allowing the total eruptive volume to be refined from 82.3 x 106m3 to 86 x 106 m3. For Three Kings a volume estimate for proximal to medial tephra fall of 3.5 x 106m3 was calculated, allowing the total eruptive volume to be refined from 69.3 x 106m3 to 72.9 x 106m3. These significant volumes indicate that proximal to medial tephra could pose a much greater hazard than originally thought in the event of a future volcanic eruption in the AVF, and should in future be considered separately to fine-grained tephra. Insight into eruption styles and fragmentation mechanisms of several volcanoes of the AVF through detailed characterisation of the textural properties and density of lapilli clasts are also explored. This study quantified bulk density and a range of porosity features, as well as examined qualitative aspects of texture and crystal content. Within lapilli clasts. This showed contrasting lapilli types that could be associated with specific eruption mechanisms. The main contrast was between isolated-bubble rich, low density lapilli (associated with Strombolian and higher energy styles) and those rich in connected bubbles with higher densities (Hawaiian fountains). Of the volcanoes studied, Mt Wellington and Three Kings clearly showed Strombolian eruption styles, while Rangitoto lapilli were produced by Hawaiian fountains. The other volcanoes, Pupuke, Motukorea and Purchas Hill, were more complex, with the fragmentation styles influenced by phreatomagmatism. Finally North Head showed evidence of eruption of very gas-rich magma, but rapidly chilled by surface water in a Surtseyan eruption. New insights into the subtleties of eruption style will help improve our planning for more appropriate eruption scenarios in future from the AVF and similar volcanoes.