Violent Basaltic Explosive Volcanism: A Case Study Of The 1886 Tarawera Eruption, New Zealand

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AbstractEAR-0125719HoughtonUniversity of HawaiiBasaltic volcanism is most typically thought to produce effusion of lava, with the most explosive manifestations ranging from mild Strombolian activity to more energetic fire fountain eruptions. However, some basaltic eruptions are now recognized as extremely violent, i.e. generating widespread phreatomagmatic, subplinian and Plinian fall deposits. These eruptions are particularly dangerous because the ascent rate of basaltic magma prior to eruption can be very rapid (giving warning times as little as a few hours) and because their precursors may be ignored or misunderstood. The main question addressed in this proposal is: what are the conditions that cause basaltic magma to adopt an eruption style more typical of chemically evolved, highly viscous magma? Possible mechanisms (acting singly, or in concert) are: (1) interaction between magma and water, (ii) very rapid ascent producing a delayed onset of degassing then exceptionally rapid "runaway" vesiculation at shallow levels in the conduit, (iii) microlite crystallization and degassing of the magma during ascent leading to increased viscosity. Our goal is to establish the triggering mechanism for the Plinian phase of the 1886 Tarawera eruption. This proposal will test the hypothesis that this phase was a consequence of either rheologic changes during magma ascent that accompanied early (pre-fragmentation) interaction between the basaltic melt and water-bearing rhyolitic units forming the conduit walls or late-stage magma water interaction.