Patterns of hydrothermal activity and groundwater flow at Mount Tongariro revealed through self-potential surveying

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Miller, C.A.; Mazot, A. 2013 Patterns of hydrothermal activity and groundwater flow at Mount Tongariro revealed through self-potential surveying. Lower Hutt, N.Z.: GNS Science. GNS Science report 2013/47 iii, 22 p.

Abstract: We undertook a large self-potential (SP), survey at Mount Tongariro together with soil CO2 flux and ground temperature measurements. The survey successfully delineated the Ketetahi and Red Crater hydrothermal surface features through an analysis of SP vs topographic gradient. The SP anomalies around hydrothermal areas were subdued, possibly due to low pH fluids, and/or hydrothermally altered and electrically conductive ground. We interpreted a large positive anomaly (+1100 mV) in Oturere Valley as the result of deepening of the ground water aquifer beneath a thick electrically resistive lava flow. Modelling of SP data to determine both water table depth, and the depth to a potential current source, agree with the thickness of the Oturere Valley lava flow as previously modelled from TOPSAR data, and imply that the lava itself is a poor aquifer, with the material underlying the lava hosting the aquifer. We interpreted a large negative (-600 mV) SP anomaly mapped on the NE flank of North Crater as a zone of down-flowing meteoric fluids, outside of the hydrothermal system. A mapped fault coincides with the boundary between the hydrothermal and groundwater flow at Ketetahi and may indicate some structural influence controlling the extent of the hydrothermal system. The implication of subdued SP anomalies in areas of surface hydrothermal activity as resulting from low pH fluids and hydrothermally altered ground, is that they represent areas of weakened ground which may be prone to future flank collapse, especially in areas of steep topography. Magmatic intrusion could cause flank collapse in a weakened part of the volcanic edifice by raising the pore pressure of surrounding rocks and promoting their ability to fail. Such a mechanism may have caused the landslide coincident with the 2012 Te Maari eruption. Therefore monitoring of SP changes over time may provide a method of determining changes in the hydrothermal system prior to an eruption or further flank collapse. (auth)