Phreatic eruptions are caused by rapid evaporation of hydrothermal fluids to steam with the resulting pore overpressure leading to fragmentation of overlying rocks.
We investigate White Island (New Zealand) and the active volcanic sites Solfatara and Monte Nuovo in Campi Flegrei (Italy); further our study will involve Valley of Desolation and Wotten Waven (Dominica). All of these sites are characterised by intense hydrothermal alteration and have high potential of future phreatic, phreatomagmatic and magmatic eruptions. Here, we constrain the influence of alteration on phreatic eruption conditions and on the stability of an edifice subjected to an active hydrothermal system. White Island was chosen as a first case study, where we worked on hydrothermally altered lavas, four lithified pyroclastic deposits with different grades of alteration, unconsolidated material and sulfur- and iron-rich crusts from the crater-fill.
The low porosity (6.6-8%) altered lava was found to be moderately strong (110-140 MPa) when deformed in uniaxial compression tests. The altered pyroclastic rocks are more heterogeneous, porous (32-48%) and weaker (3-20 MPa). Conditions for phreatic eruptions were constrained by fragmentation experiments due to rapid decompression (from 9 MPa to atmospheric pressure) at temperatures ≤ 300°C. This provided information about the energy threshold, fragmentation efficiency, the maximum speed and evolution of particle ejection velocities. The fragmentation threshold increases with decreasing porosity. Higher applied energy and water saturation of the sample improves the fragmentation efficiency. The particle ejection velocity after fragmentation rises with the applied pressure and porosity. For fragmentation experiments at 6.5 MPa and 300°C on dry consolidated samples, the ejection speed (45 m/s) is significantly lower than for fully water-saturated samples (145 m/s). Our study suggests that hydrothermal alteration and fluid-saturation associated with the presence of a hydrothermal system weakens the rocks, which may result in slope destabilisation, lateral/sector collapse and further phreatic eruptions.
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