SUBMARINE HYDROTHERMAL ACTIVITY WITHIN THE OFFSHORE TAUPO VOLCANIC ZONE, BAY OF PLENTY CONTINENTAL-SHELF, NEW-ZEALAND

The onshore Taupo Volcanic Zone (New Zealand) is a region of vigorous, and relatively well studied, geothermalism and hydrothermalism distri buted at 20 known sites. Within the offshore extension of the Taupo Vo lcanic Zone, new data acquired in 1988 and 1989, and a review of surve y data since 1966 has delineated three main areas of submarine hydroth ermal activity (SHA) on the Bay of Plenty continental shelf. Of these, the most intense and extensive area of SHA is associated with the act ive late Quaternary faulting of the offshore Whakatane Graben. Within this latter area, there are four major hydrothermal vent sites; the pr eviously reported Calypso vent site, and three others newly located. T hree distinct plume forms are recognisable within the water-column, an d are related to the state of buoyancy of the hydrothermal fluid, alth ough the character of the plume can change in the vertical since the o verlying water mass is density stratified. Some SHA has proved ephemer al over a 22 year period which is here related to a cycle of conduit o pening driven by secular extension, and conduit collapse and closure c aused by seismic shocks. Mathematical modelling (phase-plane analysis) of the hydrothermal plumes provides a method of analyzing plume forms from known initial parameters.