Geochemistry and groundmass crystallinity controls on lava dome stability at Mount Taranaki, New Zealand

Abstract

The study of lava domes and their properties is inherently important as their collapses pose a significant hazard to infrastructure, human lives and the environment. Mount Taranaki on the west coast of the North Island of New Zealand provides an excellent case study as a large part of the summit lava dome is currently exposed due to a partial collapse. Here, a detailed investigation into the groundmass crystallinity and chemical composition of the Taranaki lava dome is described, with dome samples taken from the current summit dome and several older BAFs. Rock properties are compared to the compressive and tensile strength in deformation experiments, simulating low temperature and low confining pressure conditions. Other parameters such as porosities and densities were also investigated. Porosity is determined to be the main control on rock strength and mode of failure. The groundmass crystallinity does show a relation to rock strength, but a direct control on rock strength cannot be proven as it is linked to a coupling of groundmass crystallinity to porosity. The composition of the rocks did not show any significant impact of the rock strength or any other physical parameters. The results imply that the crystallisation of the groundmass significantly impacts both porosity and density of the dome rocks depending on the resultant groundmass crystal content. Higher crystal content results in less pores, reflecting a slowly undercooled magma experiencing rheological stiffening deeper within the conduit under higher pressure conditions. This is linked to a slower magma ascent rate and results in a lava dome with higher rock strength. On the other hand, a less crystallised rock is more porous and represents magma with a faster ascent rate, resulting in stronger undercooling and solidification under lower pressures, presumably close to the surface. These rocks develop low groundmass crystallinity with high porosities and lower rock strength. The exception to this are low rock strength coulees, which show high groundmass crystallinity with higher and likely shear induced porosities. This implies, that the stability of the dome might shift significantly due to varying ascent rates and the developing rock strength.