Spatial change in lava flow properties & an estimate of lava flow rheology: case studies from the Auckland Volcanic Field

Abstract

Understanding the rheology of lava flows within Auckland City is essential in risk-based decision making, in land-use planning, and emergency management. Little work has previously focussed on estimating the rheological properties of the lava flows of Auckland Volcanic Field. Properties measured that have been used to solve three-phase rheological equations include the bulk rock and groundmass composition, groundmass crystallinity, crystal aspect ratio, and the volume fraction of phenocrysts, groundmass crystals, groundmass glass, and bubbles. To this end, surface and borehole samples from Rangitoto, and Mt. St. John were analysed. These various properties, as well as the viscosity values output, have been plotted versus distance and linear regression analysis performed in order to understand their spatial nature. Results from the various properties studied showed distinct differences between the surface samples of Mt. St. John and Rangitoto, and between the borehole and surface samples of Mt. St. John. In both spherical and elongated bubble shape calculations, the differences in the properties resulted in the Rangitoto surface samples outputting a higher viscosity than that of the Mt. St. John surface. The Mt. St. John borehole samples were found to have a higher viscosity than the surface samples. These differences in viscosity are primarily due to the chemical composition and amount of bubbles present within the samples. Higher SiO2 was primary responsible for the difference in viscosity between the two volcanos, while the higher borehole viscosities were due to larger and more crystals at depth. Bubble shape influenced the viscosity by orders of magnitude, with spherical bubbles increasing viscosity. The large effect that bubble shape has highlights the need for future research to better account for and constrain the shape of the bubble. Both Rangitoto and Mt. St. John lavas have a high degree of groundmass crystallinity, indicating that shallow degassing occurred at both vents. The shallow degassing allowed for crystallisation to occur at depth, increasing the crystallinity of the lava. The chemistry of the groundmass has a much higher concentration of SiO2, overall indicating that the residual melt was more evolved than the bulk rock. It is the groundmass that represents the liquid portion of a lava flow. These findings highlight the role that degassing has in determining viscosity as well as the need for viscosity studies to use groundmass and not bulk rock in their melt viscosity calculations.