Large-Scale Phreatomagmatic Silicic Volcanism- A Case Study From New Zealand

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Journal Article: Large-Scale Phreatomagmatic Silicic Volcanism- A Case Study From New Zealand

Pyroclastic deposits of the 20,000 year old Wairakei Formation occur in the region surrounding Lake Taupo in the central North Island of New Zealand. Recent advances in the correlation of both proximal and distal equivalents of the Wairakei deposits show that they were the result of one large eruption. The ash layers once covered most of New Zealand; they are also present in deep sea cores from the SW Pacific and on the Chatham Islands. The dispersal and grain-size parameters of the ash-fall members indicate that their vent area was within the Taupo volcano-tectonic depression which, for a long period, has contained a large lake. The Wairakei eruption is interpreted to have been phreatomagmatic throughout. Each phase of the eruption sequence generated its own characteristic deposits. The sequence of events can be summarized as follows: (1) Eruption and deposition of a white, fine-grained, bedded, fall deposit; (2) eruption and deposition of a medium-grained, pumice-fall deposit; (3) generation of ultra-fine, wet, ash clouds, with a fall deposit accumulating largely as aggregates and accretionary lapilli accompanied by base surges; (4) formation of water-cooled pyroclastic flows and deposition of an extensive ignimbrite; (5) return to a dominantly vertical eruption column and deposition of a medium-grained bedded ash fall, immediately followed by another wet, ultra-fine fall deposit charged with accretionary lapilli. Finally, (6) an ignimbrite less widespread than that of phase (4) was produced. Co-ignimbrite fall deposits of fine vitric dust and medium-grained pumice were laid down over a huge area. There may have been a short hiatus in activity between phases (4) and (5).

Stephen Self

Published Journal 
Journal of Volcanology and Geothermal Research, 1983

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Stephen Self. 1983. Large-Scale Phreatomagmatic Silicic Volcanism- A Case Study From New Zealand. Journal of Volcanology and Geothermal Research. (!) .