Characteristics of hydrothermal eruptions, with examples from New Zealand and elsewhere

Hydrothermal eruptions have occurred in many hot water geothermal fields. T his paper concentrates on examples from New Zealand but also mentions other s elsewhere, which demonstrate points of particular interest. Numerous smal l eruptions (maximum focal depths of about 90 m) have occurred in historic times (past 150 years) at Wairakei/Tauhara. Rotorua, Tikitere, Ngatamariki, Mokai and Waimangu, The presence of breccia deposits shows that much large r (with estimated maximum focal depths of about 450 m), prehistoric hydroth ermal eruptions have also occurred at Kawerau, Wairakei, Tikitere, Orakeiko rako, Te Kopia, Rotokawa and Waiotapu. One of the largest known hydrotherma l eruptions in New Zealand took place at Rotokawa 6060 +/- 60 years ago; th is produced a deposit that extended over an area with a diameter of 4 km, a nd has a maximum thickness of 11 m, Deposits from hydrothermal eruptions are typically very poorly sorted, matr ix-supported, and may contain hydrothermally altered clasts that derive fro m within the geothermal reservoir. Their lithologies and alteration mineral ogies are useful guides to subsurface conditions. Hydrothermal eruptions do not require any direct input of either mass or energy derived directly fro m a magma and, thus, differ from both phreatic and phreatomagmatic eruption s. Many hydrothermal eruptions in a hot water field start very close to the ground surface and result from the rapid formation of steam due to a sudde n pressure reduction. This steam provides the energy necessary to brecciate , lift and eject fragments of the host rocks as a flashing front descends a nd water nearby in the reservoir boils. A rock brecciation zone accompanies this front, and both precede the descent of the eruption surface. A hydrot hermal eruption continues until the steam is produced too slowly to lift th e brecciated rocks. There is no genetic difference between the small erupti ons induced by exploitation and those which occur as a geothermal system ev olves naturally and whose effects may penetrate to much greater depths. Hydrothermal eruptions do not need the presence of either field-wide cap ro cks or pressures within a reservoir that exceed that provided by a hydrosta tic column of water very close to its boiling temperature. (C) 2001 Elsevie r Science B.V. All rights reserved.