IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

1W_1E-P9Date/Time: July 21 Poster

Experimental approach to constrain phreatic eruption processes on White Island, New
ZealandKlaus Mayer1 , Bettina Scheu1 , Yan Lavallée2 , Ben Kennedy3 , H. Albert Gilg4 , Michael Heap5 , Mark
Letham-Brake3 , Cristian Montanaro1 , Laura Jacquemard5 , Noémie Pernin5 , Donald B. Dingwell11LMU Munich, Germany, 2 University of Liverpool, Great Britain, 3 University of Canterbury, NewZealand, 4 TUM Munich, Germany, 5 University of Strasbourg, France
E-mail: klaus.mayer@min.uni-muenchen.deWhite Island is New Zealand’s most active volcano and primarily characterised by phreatic and phreatomagmatic
eruptions. A phreatic eruption on August 2nd, 2012 ended an eleven year quiescence. More than 100 years ofintense hydrothermal activity from magmatic fluids and groundwater has created a weak and unstable volcanic
edifice highly susceptible to sector collapses and landslides. Here, we constrain the influence of alteration onphreatic eruption conditions and on the stability of an edifice subjected to an active hydrothermal system. A
hydrothermally altered lava flow and four lithified pyroclastic rocks with different grades of alteration, together withunconsolidated material and sulfur- and iron-rich crusts from the crater-fill were sampled and investigated.
The lava flow is primarily composed of plagioclase, K-feldspar and pyroxene, but alteration has also lead to thepresence of cristobalite, amorphous silica, gypsum and jarosite. The pyroclastic rocks do not contain any primaryminerals or glass and consist instead of amorphous silica, alunite, jarosite, gypsum and kaolinite.
The lava flow with a low porosity (6.6-8%) was found to be moderately strong (110-140 MPa) when deformed inuniaxial compression tests, although we find that the presence of macroscopic fractures can lower the strength to
60 MPa. The altered pyroclastic rocks are more heterogeneous, porous (32-48%) and weaker, with unconfinedcompressive strengths ranging between 3 and 20 MPa. Our uniaxial experiments also show that the alteredpyroclastic rocks are weakened when saturated with water.
Conditions for phreatic eruptions were constrained by fragmentation experiments on dry and water-saturated
samples due to rapid decompression (from 9 MPa to atmospheric pressure) at temperatures of up to 300°C.
This provided information about the energy threshold, fragmentation efficiency, the maximum speed and evolutionof particle ejection velocities. The fragmentation threshold ranges between 5.1 MPa and 3.8 MPa for samples
with a connected porosity of 32% to 48% , which is in agreement with the trend for pristine volcanic rocks. The
fragmentation efficiency generally increases with higher applied energy but also if the sample is saturated. The
particle ejection velocity after fragmentation increases with the applied pressure and porosity. For experiments ondry consolidated samples, where the fragmentation occurs at an argon gas overpressure of 6.5 MPa and 300°C,
the ejection speed (45 m/s) is significantly lower than for fully water-saturated samples (145 m/s). Unconsolidated
samples with connected porosities of 35% show a similar effect of sample saturation on the ejection.
Our study suggests that hydrothermal alteration and fluid-saturation associated with the presence of a hydrothermal
system weakens the rocks (over a range of strain rates), which may result in slope destabilisation, lateral/sector
collapse and further phreatic eruptions.

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