Viscosity of Evolving Magmas: A case study of the Glass House Mountains, Australia
Final Report Abstract
1. The difference in viscosity of the evolving magmas is not as large as expected as the trend in viscosity depends not only on SiO2 content but is a function of polymerisation. Thus the variation in viscosity is not shown as a clear function of SiO2 content but needs to be shown as a function of melt structure (e.g. non-bridging oxygens vs tetrahedra). 2. As there is little or no information on the eruption temperature of the lavas of the Glass House Mountains volcanoes it is difficult to estimate the amount of magma involved. Further, although the mafic lavas near Maleny are present, the lavas and/or pyroclastics of the rest of the Glass House Mountain volcanoes appear to have been removed by erosion. Taking the best case scenario, the volume flow rate for a frictionless cylindrical volcanic conduit can be calculated using literature values of the average eruption temperature of each composition melt, the viscosity of the present melt at that temperature, and the calculated density of the present melt and the assumption that the source of the magma is the upper mantle with a density of 3300 kg m^-3. The mafic lavas around Maleny are known to cover 200 km2, and to be made up of ~10 m thick layers to a total depth of 180 m. The present calculations indicate that 68 days of continuous eruption with a 2.5 m radius vent are needed to produce 36 km3 of basaltic lava, with 4 days eruption time needed to cover the surface with a 10 m thick layer. These calculations would infer that the basaltic eruptions were of short duration, or that the vent radius was smaller than 2.5 m. A reduction of the radius by 15% would halve the flow rate. Doubling the radius of the vent would increase the volume flow rate by more than an order of magnitude. 3. The combination of low temperature viscosity data with low temperature heat capacity data allows the use of the Adams-Gibbs equation to extrapolate the viscosity of the Glass House Mountains magma to higher temperature with great confidence. Based on the small basalt flows observed in the Maleny region it is suggested that the discharge rate of the more viscous melts would be very low and perhaps, as has been previously suggested the Glass House Mountains are the remnants of unextruded magmas which formed plugs and laccoliths. The effect of water and perhaps bubbles would, however, decrease the viscosity. 1 wt% water will decrease the viscosity of comendites by 2 or more orders of magnitude in the temperature range of interest. This would increase the discharge rate by 2 orders of magnitude. In the case of the comendite compositions (as well as the dacite and andesite) such an increased discharge rate would still remain very low when compared to that of the basalt melts.
Publications
- Viscosity of evolving magmas: a case study of the Glass House Mountains, Australia, Bull. of Volcanology, 83, 2021
Webb SL
(See online at https://doi.org/10.1007/s00445-021-01495-8)
