IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

4W_1D-P23

Date/Time: July 24 Poster

Identifying transitions in activity and precursors to major eruptive events at Mt. Taranaki,New Zealand
Kimberly Genareau1 , Shane J. Cronin2 , Thomas Platz3
1Lehigh University, U.S.A., 2 Massey University, New Zealand, 3 Freie Universitat Berlin, Germany
E-mail: kig210@lehigh.edu

Mt. Taranaki, located in the western North Island of New Zealand is an andesite stratovolcano which has been in astate of quiescence for the last >200 years. Its record of >100 eruptions over the last 12,000 years and its upwind
position with respect to the largest populated areas of the country means that it presents a significant hazard withany re-awakening. Volcanic activity at Taranaki is characterized by periods of hundreds of years dominated byrepeated lava dome growth and collapse, interspersed semi-regularly at 300 year intervals with large sub-Plinian
explosive events. In addition to widespread tephra falls, Taranaki has produced hazardous pyroclastic density
currents in the form of surges, block-and-ash flows, and column collapse pyroclastic flows that reach run-out
distances of up to 15 km from the summit. This study focuses on the sequence of pyroclastic deposits thatstratigraphically precede the 1655 AD Burrell Lapilli unit, which was the latest sub-Plinian eruption of Taranaki.
This eruption culminated a period of repeated lava effusion and dome growth that lasted roughly 1000 years.
The pre-Burrell pyroclastic sequence is comprised of several, fall, flow, and surge units and these tephras wereanalyzed using a combination of scanning electron microscopy, stereo-scanning electron microscopy, secondary
ion mass spectrometry (SIMS), and electron microprobe analyses. SIMS analyses of oscillatory-zoned plagioclase
phenocrysts from the Burrell Lapilli show increasing Li contents in crystal rims compared to cores. Profiles acrossphenocrysts show that Li increases substantially even when An contents increase, decrease, or remain relatively
constant, indicating that Li variations reflect properties of the ascending magma parcel rather than local chemical
gradients at the melt-crystal interface. In addition, thin (10-20 microns), decompression-induced breakdown rims on
amphibole crystals and extensive groundmass crystallization of plagioclase microlites in the indicate changes in thevolatile content of the magma leading up to the Burrell explosive phase, and thus, Li trends may allow quantificationof the timescale during the final stage of magma ascent prior to the Burrell sub-Plinian explosion. Microtexturaland geochemical trends in the pre-Burrell sequence will be compared to those of the subsequent Burrell Lapilli
unit to highlight the evolution of magma properties during transitions from effusive to explosive activity. Quantifying
physical parameters within conduits and understanding how they vary over time to cause dome collapse events andexplosive activity will allow researchers to more accurately model (and ideally, with ongoing analysis of samples
collected during an eruption crisis, forecast) volcanic behavior. This will lead to improved hazard assessment
strategies for >100 andesitic volcanoes worldwide that show regular variations in effusive and explosive activity likethose observed at Taranaki.

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