Variability of plinian fall deposits: examples from Okataina Volcanic Centre, New Zealand

Plinian eruptions are intense, sustained, open-vent volcanic outbursts that produce extensive beds of generally well-sorted and generally highly vesicular pumice fallen from eruption plumes up to 55 km high. Their deposits tend to be almost homogeneous throughout their thickness, although inverse grading commonly occurs. Here, we detail departures from this near homogeneity that we observed among the Mangaone Subgroup, a group of 12 plinian pumice-fall deposits from Okataina Volcanic Centre (one of the dormant rhyolitic volcanoes in the Taupo Volcanic Zone, New Zealand). We propose that plinian eruptions may pass through seven states, ranging from inception (commencement of activity, or renewed activity following a shutdown), waxing, steady discharge, climax, and waning leading to column collapse or shutdown (decrease to zero intensity). Some deposits include intercalated water-flushed ash-rich beds having a bimodal grain size. Other examples of non-homogeneity include phreatoplinian ashes that may alternate with the plinian pumice as though modulated by a delicate balance in water flux, and still other eruptions may switch completely from dry to wet conditions when the column wanes towards the latest stages of the eruptions. Pink pumice occurs in some deposits and is totally absent in others. We infer that it oxidized during temporary storage in or around the vent. Lithics are ubiquitous, and concentrations of them may indicate vent-wall collapses and explosive ejection of some of the resulting debris. Some lithic-rich horizons neither coincide with apparent variations in the plinian discharge nor interrupt it, as is the case with some rain-flushed bimodal beds. The events that produced these incongruent beds proceeded synchronously with, but independent of, the main plinian discharge, in an isolated portion of the vent system (such as a subsidiary vent). Other variations of density, vesicularity and crystallinity among the eruption products record the pre- and syn-eruptive history of the magma. A local inverse relationship between pumice density and crystallinity of one eruptive unit is inferred to reflect a control by crystals on vesiculation and fragmentation.