Experimental Calibration of Phreatic and Hydrothermal Explosions: a Case Study on Lake Okaro, New Zealand

Abstract

Phreatic and hydrothermal eruptions often occur with little or no warning representing a significant hazard in geothermal areas. They occur at a range of temperatures and pressures within varying rock types. These eruptions can lead to increased local permeability and the development of shallow hydrothermal resources. A range of mechanisms including heating or decompression allows hydrothermal/supercritical fluid to rapidly flash to steam, triggering an eruption.

Previous studies have focused exclusively on either physical characteristics of eruptions or experimental modelling of trigger processes. Here, a new experimental procedure has been developed to model phreatic fragmentation based on shock tube experiments for magmatic fragmentation by Alidibirov & Dingwell (1996). Water saturated samples are fragmented from a combination of argon gas overpressure and steam flashing within vesicles. By integrating physical characteristics of porosity, permeability and mineralogy with analysis of these experimental results a model of phreatic fragmentation is proposed, to aid future hazard modelling in geothermal areas.

The phreatic explosion forming Lake Okaro, Taupo Volcanic Zone, was used as a case study. Eruption was triggered within the Rangitaiki Ignimbrite, therefore serving as the experimental sample material. To evaluate alteration effects, both original material and hydrothermally altered samples were analysed. Experiments were performed at room temperature and 300°C and pressures from 4 to 15 MPa, to reflect the conditions at the study location while also assessing the effect of water saturation on fragmentation. First analyses of grain sizes reveal a clear shift to smaller grain sizes with saturated samples (independent of pressure/sample type) possibly reflecting improved efficiency in the conversion of energy, most likely in combination with strength reduction due to saturation. We provide herewith a first parameterisation of conditions for phreatic and hydrothermal eruptions and offer an explanation for the reduction in grain size associated with phreatic eruptions.