How to fragment peralkaline rhyolites: Observations on pumice using combined multi-scale 2D and 3D imaging

Thanks to their alkali-rich compositions, pantellerites have much lower viscosities than other rhyolites. As a consequence, these peralkaline magmas erupt in myriad ways to create volcanic landforms that range from lava domes to ignimbrites and fountain-fed lava shields. However, the mechanisms by which such low viscosity melts are able to fragment in explosive eruptions are poorly understood despite the hazards presented by pantellerite volcanoes, above all in the East African Rift.

Building on her MSci project, Ery Hughes investigated this problem of magma fragmentation under the supervision of Marie Edmonds, Kate Dobson and myself by combining 2D (electron microscopy) and 3D (X-ray microtomography) measurements of pumice samples during my MSci.

Slices through an X-ray tomography volume of a pumice from Pantelleria (Hughes et al. 2017).

We found that pantelleritic pumices from Pantelleria are texturally indistinguishable from calc-alkaline pumices from a range of rhyolitic systems, implying that our peralkaline pumices fragmented in a brittle fashion and that their unusual chemistry had little effect on their syn-eruptive textural evolution. We therefore propose that the observed pumice textures developed in response to high decompression rates and that peralkaline rhyolite magmas can fragment when strain localisation and high bubble overpressures develop during rapid ascent.

Publication

Hughes, E.C., Neave, D.A., Dobson, K.J., Withers, P.J. & Edmonds, A. 2017. How to fragment peralkaline rhyolites: Observations on pumice using combined multi-scale 2D and 3D imaging. Journal of Volcanology and Geothermal Research 336, 179–191. <Open Access>