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.
