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MARS - Article - ,NewZealandahigh Mount Ruapehu in the...

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The Hydrothermal Vent System of Mount Ruapehu, New Zealand – a high  frequency MT survey of the summit plateau Mount Ruapehu, in the Tongariro Volcanic Centre (TgVC) of the central North Island of New Zealand ( Fig. 1 a), is the most southerly andesite volcano of the Kermadec volcanic arc. The summit plateau of the mountain ( Fig. 1 b) occupies an area of approximately 1 km 2 and Crater Lake, 500 m to the south and ~ 100 m lower, are surrounded by three main peaks. In its last series of major eruptions in 1995–96 Ruapehu erupted approximately 0.1 km 3 of magma, emptying Crater Lake. Present knowledge of the hydrothermal/vent system on Mount Ruapehu was derived initially from analysis of the mass and energy budget of Crater Lake ( Hurst et al., 1991 ) and from geochemical analyses ( [Christenson and Wood, 1993] and [Christenson, 1994] ). More recent analyses of the geochemistry ( Christenson, 2000 ) and seismicity ( [Sherburn et al., 1999] and [Bryan and Sherburn, 1999] ), associated with the eruptions of 1995–96, have supported the earlier inferences of an essentially open vent system allowing significant heat transfer to Crater Lake through a heat pipe. In the upper region of the vent, a hydrothermal system which has both single-phase liquid and vapour regions separated by a two-phase liquid–vapour region is inferred. However no geophysical measurements have been made which can verify the extent of the inferred system. Ingham et al. (submitted for publication) have recently reported a 3-D inversion of a regional magnetotelluric (MT) survey of Ruapehu which indicated a narrow central zone of low resistivity beneath the mountain and extending to some 5–6 km in depth. This was interpreted as the conduit by which hot volcanic gases and fluids pass from depth to the volcanic hydrothermal system. In this paper we present a 3-D resistivity image of the hydrothermal and volcanic vent system on Mount Ruapehu derived from new high frequency MT measurements on and around the summit plateau. The resulting resistivity structure not only suggests that the heat pipe system beneath Ruapehu is more extensive than has previously been proposed, but also allows us to make some inferences about the volcanic history of the system. In the MT technique simultaneous measurements of variations in the naturally occurring magnetic and electric fields at the surface of the earth are used to derive the electrical resistivity structure beneath a measurement site. The background electrical resistivity of the earth is generally of the order of 100s–1000s of Ω m, but within volcanic and
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geothermal systems factors such as high temperatures, the presence of high salinity fluids, hydrothermal alteration products and, potentially, magma, all lead to significantly lower resistivity (~ 1–30 Ω m). These systems therefore provide ideal targets for study using geophysical techniques that are sensitive to electrical resistivity. Over the last 10 years MT has become the most widely used technique in the study of volcanic and geothermal systems. Not only have many such systems been studied using MT (e.g.
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