Miss Grace Cumming1, Dr Karin Orth2, Mr Ralph Bottrill1, Mr John Everard1, Mr Michael Vicary1
1Mineral Resources Tasmania, Rosny Park, Australia, 2University of Tasmania, Hobart, Australia
Skillion Hill, in Southern Tasmania, is a flat-topped hill at the southern edge of a 2.5 km long zone of Cainozoic mugearite/basalt flows and basaltic breccias. The linear volcanic centre is positioned at the northern edge of a north north-east trending line of volcanic centres that span from Droughty Hill (to the south). The volcanic centres consist of strongly evolved alkaline rocks, which generally occur as small plugs, explosive vents, diatremes and scoriaceous flows elsewhere in southern Tasmania. The area has been recently excavated for a building subdivision, enabling the observation of fresh exposures. New geological mapping and textural analysis of samples has revealed a wide variety of volcanic features comparable to eruptive products formed from fire fountaining.
Exposures of “hackly” basalt display an almost clastic appearance owing to a fine network of thin, wispy-lenticular structures (< 0.1 mm thick). These features are parallel to the stratification observed in the scoria deposits above and below. The basalt contains very fine grained, mostly altered, olivine, clinopyroxene, pseudobrookite, magnetite and plagioclase crystals, in an altered glassy groundmass. It is locally xenolith-rich, containing clasts of underlying sedimentary and igneous rocks, anorthoclase-pyroxene, enstatite xenocrysts and some partly fused quartz clasts. An isolated zone within the basalt also contains fluidal ribbon-like clasts. The “hackly” basalt is interpreted to have formed from spatter clasts that underwent agglutination and coalescence during fire-fountaining to form a lava-like appearance. These features indicate rapid accumulation during the eruption, allowing immediate coalescence of hot impacting clasts (as detailed in Sumner et. al., 2004).
Fluidal to ropy and blocky scoria breccia occurs above, beneath and within the “hackly” basalt. The breccia is poorly sorted, coarse grained, massive and generally monomictic with elongate, vesicular, variably flow banded, fluidal lenticular or ribbon shaped clasts (>10 cm across). The breccia contains clasts with distinct morphologies similar to volcanic bombs and the bulk of the material is interpreted as deformed and flattened spatter, which may have started as completely fluid clasts that agglutinated (or partly agglutinated) on impact during deposition. Abundant bread-crust and spindle-shaped clasts with smooth cores are commonly found in the newly excavated trenches and are also interpreted as once partly fluidised volcanic bombs.
Poorly sorted, crudely stratified scoria breccia with alternating finer grained granule rich matrix-supported layers contain fairly oxidised, ragged, angular to blocky shaped clasts which overlie and are intercalated with the “hackly’ basalt and scoria breccia. The crude stratification may have been caused by changes in eruption dynamics; such as eruption column height, fragmentation processes or changes in dispersal patterns during the eruption (McPhie et. al., 1993).
Structural measurements of flow banding and the stratification in the scoria breccia facies, along with the incidence of proximal spatter piles or fountain-fed lavas (formed in an inner-fountain setting) suggest that Skillion Hill represents a partly dissected edifice of a fissure vent or scoria cone. Eruptions likely involved alternating or contemporaneous low viscosity, less volatile rich magma with high effusion rates (to form coalesced spatter), and eruptions with high volatile content and high explosivity (to form agglutinated spatter and volcanic bombs).
McPhie J., Doyle M., Allen R., Allen R.L., 1993. Volcanic textures: a guide to the interpretation of textures in volcanic rocks. Centre for Ore Deposit and Exploration Studies, University of Tasmania, Hobart
Sumner J. M., Blake S., Matela R.J., Wolff J.A., 2005. Spatter. Journal of Volcanology and Geothermal Research. 142:49–65
Grace currently works as a geologist at the Geological Survey at Mineral Resources Tasmania. Grace has spent the most of the last 10 years undertaking geological mapping in a variety of terranes.