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During the climactic Plinian phase of the 1886 basaltic eruption of Tarawera, New Zealand, vents along the 17 km fissure erupted explosively with a wide range of dispersal. The 8 km long segment of the fissure which cuts across Mt Tarawera contains approximately 50 vents and includes the sources of both the weakest and most intense activity of the 5 h eruption. We seek to explain (1) what allowed the intensity to reach Plinian values that are rarely achieved by basaltic magma, and (2) what caused adjacent vents to erupt with very different dispersals and intensities despite identical magma composition. AU juvenile clasts stuthed from this eruption have relatively high vesicle number densities (c. 106cm-3) and exceptionally high microlite crystallinities (60-90% of the groundmass), unlike the typical products of weaker Hawaiian and Strombolian basaltic explosions. Textural analysis of juvenile pyroclasts suggests that all the erupted magma experienced the same decompression history through to fragmentation. The Tarawera magma experienced a sudden, large, decompression producing nucleation of bubbles and microlites. The high microlite content was the primary means by which the magma's viscosity increased, which kept the bubble population well coupled to the magma and allowed it to fragment explosively in a manner analogous to that postulated for silicic Plinian eruptions. The main differences at different sites along the Mt Tarawera fissure segment are in the amount and grain size of the wall rock lithic component of the deposits. We suggest that conduit/vent erosion and incorporation of significant volumes of cold wall rock into the eruptive jet prevented some vents from achieving Plinian intensity. Bubble size analysis suggests that coalescence led to open-system degassing, ending the Plinian phase. © IAVCEI2009.

Item Type:	Refereed Article
Keywords:	Tarawera Volcano, Basaltic Plinian, MIcrotextures
Research Division:	Earth Sciences
Research Group:	Geology
Research Field:	Volcanology
Objective Division:	Expanding Knowledge
Objective Group:	Expanding Knowledge
Objective Field:	Expanding Knowledge in the Earth Sciences
Author:	Carey, RJ (Dr Rebecca Carey)
ID Code:	76577
Year Published:	2009
Deposited By:	Earth Sciences
Deposited On:	2012-03-08
Last Modified:	2012-09-10
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