White Island is New Zealand's most active volcano and is primarily characterised by phreatic and phreatomagmatic eruptions, interspersed by occasional strombolian events. The common occurrence of magma-water interaction at White Island derives from the presence
hydrothermal system that induces alteration of the edifice. Here, we constrain the influence
hydrothermally altered lava flow and four lithified pyroclastic rocks with different grades of alteration were sampled and investigated.
The lava flow is now primarily composed of plagioclase (37%), feldspar (14%) and pyroxene (10%), but alteration has lead to the presence of cristobalite (17%), amorphous silica (14%), gypsum (4%) and jarosite (3%). The pyroclastic rocks do not preserve any primary minerals or glass and consist instead of amorphous silica (66-92%), alunite (1-32%), jarosite (0-4%), gypsum (0-4%) and kaolinite (0-2%).
The lava flow has a low porosity (6.6-15.2%) and is moderately strong under uniaxial conditions (e.g., 110-140 MPa) although the presence of fractures occasionally lowers the strength to 60 MPa. Ultrasonic Vp and Vs velocities approximate 4.6 and 2.8 km/s, respectively, but in the presence of fractures, these values may decrease down to ~1.5 and 1.2 km/s. The altered pyroclastic rocks are more heterogeneous, porous (38-48%) and weaker, with uniaxial compressive strength ranging between 3 and 20 MPa. The Vp and Vs are far slower with values of 1.2-2.0 and 0.8-1.2 km/s, respectively.
The conditions for phreatic eruptions were constrained by fragmentation experiments at temperatures up to 300°C. Rapid decompression (from up to 9 MPa to atmospheric pressure) provided information about the energy threshold and efficiency of fragmentation. High-porosity samples fragment at lower initial pore pressure – following the trend for pristine volcanic rocks. The fragmentation speed of water saturated samples is in the order of 75 m/s, the ejection speed of particles can reach up to 140 m/s. Generally the the fragmentation efficiency of water saturated samples is higher than for dry samples.
This preliminary study suggests that alteration and fluid saturation associated with the presence of a hydrothermal system weaken the rocks, which may cause slope failure and phreatic eruptions.
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