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

3A1_2G-O4Room B2Date/Time: July 23 9:30-9:45

The magmatic-hydrothermal transition: Observations from the 2012 eruptions of White
Island and Tongariro, New Zealand
Bruce W Christenson1 , Arthur Jolly2 , Karen Britten2 , Agnes Mazot2
1National Isotope Centre, GNS Science, New Zealand, 2 Wairaki Researcch Centre, GNS Science, NewZealandE-mail: b.christenson@gns.cri.nz

The 4 August 2012 phreatomagmatic eruption of White Island was the first significant activity from this volcano
since February 2001. Although small in size and duration, the eruption was followed by the building of a 20 m hightuff cone, and subsequent extrusion of a small lava dome. Some 6 months prior to the eruption, low temperature(100 °C - 200 °C) fumaroles on the main crater floor began to show increasing trends in CO2 /CH4 , CO2 /H2 O, C/S
and CO/CO2 ratios, constituting shifts from hydrothermal to increasingly magmatic signatures. These changes
were also accompanied by a progressive isotopic shift in δ 15 N2 from +2.9 ‰to +5.5 ‰. This is the heaviest
δ 15 N signature yet encountered in the Taupo Volcanic Zone, and provides new insights into the TVZ magmatic
N2 end-member composition. These anomalies were coupled to an increase in harmonic and broad spectrum
tremor during the same period. Together, these observations provide an unequivocal context for recognising
future eruptive activity, but also criteria for constraining hydrothermal system response, and gas-magma transport
behaviour in this volcano.

Similarly, and just two days after the White Island eruption, the 6 August 2012 phreatic eruption of Te Mari,
Mt Tongariro marked the re-awakening of this volcano which last erupted in 1896. This eruption was accompanied
by precursory volcano-tectonic earthquakes, and by marked changes in chemical signatures of gases emitted fromthe Lower Te Mari fumaroles. Fumarolic gas sampled from the western rim of Lower Te Mari crater on 22 May 2012
showed no significant changes from baseline values. Earthquake activity commenced on 13 July, and intensified
over the period of 18-21 July with approximately 120 located hybrid and volcano-tectonic events. Fumarolic gases
collected on July 21 revealed significant changes in gas signatures from the Lower Te Mari fumaroles by this time,with key ratios of CO2 /CH4 and N2 /Ar increasing from 5,400 to 91,200 and 91 to 1,245 respectively, and increasing
further to 121,000 and 1,300 respectively by 27 July. During this time there were no significant changes in fumarolictemperatures or pressures, suggesting that permeabilities in the hydrothermal system were low, and were throttlinggas transfer from below. Baseline C/S mole ratios for Te Mari fumaroles are close to 5. Immediately prior to theAug 6 eruption, C/S had fallen to ca. 3, and post eruption fell to < 1, consistent with extensive degassing of a small
magma volume. A second, short-lived pulse of CO2 -rich gas was recognised in late September, but this did notlead to further eruption activity. The current model for this activity involves open-system degassing of shallowly
emplaced dike intrusion(s) beneath the volcano.

©IAVCEI 2013 Scientific Assembly. All rights reserved.

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