Transition from effusive to explosive phases in andesite eruptions — A case-study from the AD1655 eruption of Mt. Taranaki, New Zealand
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- 作者：Thomas Platz ; Shane J. Cronin ; Katharine V. Cashman ; Robert B. Stewart ; Ian E.M. Smith
- 关键词：banded pumice ; sub-plinian ; permeability ; hornblende ; Mt. Taranaki ; lava dome
- 刊名：Journal of Volcanology and Geothermal Research
- 出版年：2007
- 出版时间：1 March 2007
- 年：2007
- 卷：161
- 期：1-2
- 页码：15-34
- 全文大小：3170 K
文摘
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.