Magmatic priming of a phreatic eruption sequence: the 2012 Te Maari eruptions at Mt Tongariro (New Zealand) imaged by magnetotellurics and seismicity

Abstract

SUMMARY Magnetotelluric data from Mount Tongariro have been analysed using an unstructured tetrahedral finite-element inversion code that incorporates topography, which was not included in previous analysis of these data. Incorporating topography adds information, which stabilizes the resistivity inversion modelling, and for the first time allows details of the shallow hydrothermal system and its relationship with the underlying magmatic system to be resolved. Specifically, an electrically conductive zone between 4 and 12.5 km depth marks the underlying magmatic system, which is shown to directly connect via conductive pathways to the area where the most recent phreatic eruptions at Tongariro occurred in 2012. The resultant phreatic eruptions in 2012 August and November showed no new magmatic component to the eruption deposits. Nevertheless, by combining the magnetotelluric resistivity image with relocated seismicity, we can see that seismicity (a proxy for magma ascent) migrated from the top of the magmatic system into the hydrothermal system in the months preceding these eruptions. Magmatic interaction with the extant hydrothermal system likely caused the over pressurization for the phreatic eruption. This work highlights the utility of combining geophysical methods and petrological data to constrain phreatic eruption processes.