The intensity and magnitude of the Mangaone subgroup plinian eruptions from Okataina Volcanic Centre, New Zealand

The rhyolitic Okataina Volcanic Centre in the TVZ experienced 12 plinian eruptions in the period between 31 and ∼43 ka and their deposits are known as the Mangaone Subgroup. These pyroclastic deposits total 77 km3. The mean Mangaone eruption volume is 6.0 km3, although erupted volumes alternate between large and small throughout the subgroup and are nearly bimodal (42% are <0.5 km3 DRE, and the remaining 58% are greater). Peak eruption intensities (mass discharge rates) of most of the plinian eruptions have been determined from lithic size distributions and theoretical models of pyroclast fallout from eruption plumes. Values range over more than an order of magnitude, from 6.2×107 to 3.9×108 kg s−1. The total erupted masses (or ‘magnitudes’) of the units varies over a range of a factor of 60, from 2.5×1011 to 1.5×1013 kg. There is no apparent correlation between eruption intensities and the presence of pyroclastic flows: only two of the seven deposits with intensities >108 kg s−1 have associated pyroclastic flows. There is a positive correlation between the intensity and the magnitude of the eruption, supporting previous models that suggest that intensity is positively related to the size of the magma chamber feeding the eruption. When compared with plinian eruptions from other volcanoes, the Mangaone Subgroup deposits fall in the middle of both the magnitude and intensity ranges, although they tend to have somewhat higher intensities at a given mass. Some deposits present inverse grading due to an increase in the eruption intensity as the eruption progressed. We examined temporal variations in column height and intensities in two of the 12 eruptions (units D and F) in detail using individual beds that represent successive chronostratigraphic levels. Minimum eruption durations were estimated for all units from modelled whole-deposit isopach data and compared with those from the individual beds of units D and F; the latter appear to result in more realistic eruption durations. The biggest of the Mangaone Subgroup eruptions had relatively high destructive potentials (areas up to 4200 km2 were buried beneath 1 m of pumice fall). Eruptions like those occurring today could also pose a hazard to major population centres if the wind dispersed them towards such centres.