Influence of porosity and groundmass crystallinity on dome rock strength: a case study from Mt. Taranaki, New Zealand
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Lava domes pose a significant hazard to infrastructure, human lives and the environment when they collapse. Their stability is partly dictated by internal mechanical properties. Here, we present a detailed investigation into the lithology and composition of a < 250-year-old lava dome exposed at the summit of Mt. Taranaki in the western North Island of New Zealand. We also examined samples from 400 to 600-year-old block-and-ash flow deposits, formed by the collapse of earlier, short-lived domes extruded at the same vent. Rocks with variable porosity and groundmass crystallinity were compared using measured compressive and tensile strength, derived from deformation experiments performed at room temperature and low (3 MPa) confining pressures. Based on data obtained, porosity exerts the main control on rock strength and mode of failure. High porosity (> 23%) rocks show low rock strength (< 41 MPa) and dominantly ductile failure, whereas lower porosity rocks (5–23%) exhibit higher measured rock strengths (up to 278 MPa) and brittle failure. Groundmass crystallinity, porosity and rock strength are intercorrelated. High groundmass crystal content is inversely related to low porosity, implying crystallisation and degassing of a slowly undercooled magma that experienced rheological stiffening under high pressures deeper within the conduit. This is linked to a slow magma ascent rate and results in a lava dome with higher rock strength. Samples with low groundmass crystallinity are associated with higher porosity and lower rock strength, and represent magma that ascended more rapidly, with faster undercooling, and solidification in the upper conduit at low pressures. Our experimental results show that the inherent strength of rocks within a growing dome may vary considerably depending on ascent/emplacement rates, thus significantly affecting dome stability and collapse hazards.
KeywordsCompressive strength Tensile strength Porosity Density Crystallinity Major element composition Andesite Block-and-ash flow Lava dome
We would like to acknowledge and thank the following people for their contribution: Geoff Lerner, Manuela Tost, Mirja Heinrich, Christopher Schmidt, Jie Wu and Elisa Piispa for assistance during the field trips as well as a large quantity of rocks that had to be carried. We also thank Raffaello Cioni, Mike Heap and two anonymous reviewers. This paper benefited greatly from their very constructive feedback and their contribution is much appreciated.
Funding for this study was kindly awarded by the R.N. Brothers memorial award via the University of Auckland Council. SJC was supported by the Quantifying Multiple and Cascading Volcanic Hazards project of the Natural Hazard Research Platform of NZ.
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