The nature, origins and distribution of ash aggregates in a large-scale wet eruption deposit: Oruanui, New Zealand

This study documents the processes and products of volcanic ash aggregation in phreatomagmatic phases of the 25.4 ka Oruanui supereruption from Taupo volcano, New Zealand. Detailed textural and stratigraphic relationships of aggregates are examined in six of the ten erupted units, which range from relatively dry styles of eruption and deposition (units 2, 5) to mixed (units 6, 7, 8) and dominantly wet (unit 3). Aggregate structures and grain size distributions shift abruptly over vertical scales of cm to dm, providing diagnostic features to identify deposits emplaced primarily as vertical fallout or pyroclastic density currents (PDCs). The six categories of ash aggregates documented here are used to infer distinct volcanic and meteorological interactions in the eruption cloud related to dispersal characteristics and mode of emplacement. Our field observations support the notion of Brown et al. (2010, Origin of accretionary lapilli within ground-hugging density currents: evidence from pyroclastic couplets on Tenerife. Geol. Soc. Am. Bull. 122, 305–320) that deposits bearing matrix-supported accretionary lapilli with concentric internal structure and abundant rim fragments are associated with emplacement of PDCs. However, on the basis of grain size distributions and field relationships, it is inferred that these types of ash aggregates formed their ultrafine ash (dominantly < 10 μm) outer layers in the buoyant plumes of fine ash lofted from PDCs, rather than during lateral transport in ground-hugging density currents. The propagation of voluminous PDCs beneath an overriding buoyant cloud – whether coignimbrite or vent-derived in origin – is proposed to generate the observed, concentrically structured accretionary lapilli by producing multiple updrafts of convectively unstable, ash-laden air. The apparent coarsening of mean grain size with distance from source, which is observed in aggregate-bearing fall facies, reflects a combination of multi-level plume transport and enhanced proximal fallout of fine ash (< 250 μm) by aggregation. Gravitational fallout and melting of abundant ice in the clouds was likely to have contributed a key source of liquid water for wet aggregation in near-source areas. In contrast, deposits from relatively drier eruption phases are aggregate-poor in proximal areas, yet develop loosely-bound particle clusters and mm-scale massive ash pellets > 100 km from vent. It is inferred that ambient meteorological conditions play a more important role in ash fallout in these cases. Entrainment of moist air, and distal subsidence and melting of ice carried by the plume, are both likely to have contributed to the observed features of late-stage aggregation in the drier phases of eruption. These observations suggest that proximal, column-influenced aggregation processes, which weaken with distance from source, are overprinted by secondary, later-stage aggregation mechanisms farther downwind.