Transition from effusive to explosive phases in andesite eruptions — A case-study from the AD1655 eruption of Mt. Taranaki, New Zealand

The extrusion of viscous andesite lava forming domes can terminate in explosive activity. To understand the driving forces behind this behaviour, a study of an AD1655 eruption episode at Mt. Taranaki was carried out. We propose that simple changes in magmatic conditions of a single hydrous melt during ascent caused sudden changes in explosivity and gave rise to pumice with highly variable vesicularities and colour. Fractionation of hornblende + plagioclase + clinopyroxene + Fe–Ti oxide at the onset of magma ascent, and step-wise crystallisation of plagioclase ± clinopyroxene in different parts of a single melt within the conduit was controlled by rates of initial rise, capping by an impermeable lava dome, and differential rates of vesiculation and volatile exsolution. This resulted in a vertical stratification in the conduit, comprising a viscous, hypocrystalline lava cap, that overlay alternating zones of grey, brown and grey magma-foams. Horizontal gradients in geochemistry in the conduit are also indicated by different clast textures. The eruption consisted of an initial extrusive phase followed by three pulses of sub-plinian activity. Each phase or pulse, corresponded to individual layers within the conduit. Ejecta included block-and-ash flow deposits, three pyroclastic pumice-flow deposits of alternating grey, brown and grey pumice, as well as fallout deposits dominated by grey pumice. The brown magma foam contained more microlites, had a more-evolved matrix glass, and a higher temperature than the grey magma foams above and below. Its eruption destabilised the sub-plinian eruption column because it was more degassed. It fragmented less efficiently than the grey magma foams due to its lower viscosity, preventing pressure build-up in bubbles. Incomplete mixing at interfaces between brown and grey magma phases gave rise to banded pumices.