Complex bombs of phreatomagmatic eruptions: Role of agglomeration and welding in vents of the 1886 Rotomahana eruption, Tarawera, New Zealand

[1] Basaltic bombs ejected from a Rotomahana crater during the 1886 Tarawera eruption in New Zealand are of two fundamental types: cored bombs, which have single large fragments, generally of rhyolite wall rock material, as their cores, and loaded'' bombs, which contain abundant small fragments dispersed through the bombs. Similar lithic-bearing bombs are common products of other, phreatomagmatic eruptions worldwide. Cored bombs originate when a large wall rock fragment is entrained in basaltic melt and then ejected with an adhering skin of basalt. In contrast, loaded bombs are constructed by agglomeration of multiple fragments of basalt, among which smaller rhyolitic wall rock fragments are dispersed. Different styles and rates of bomb loading'' are reflected in the bombs' textures and compositions. These textures provide insight into the nature of fragmentation during the eruption, the processes of recycling that produce multiply ejected clasts versus recapitulated'' ones ( those where early formed clasts were reingested in magma and then formed the cores of new bombs), the degree and timescale of interaction of magma with vent walls, and the timescales over which lithic material from the vent walls is incorporated into complex bombs prior to their ejection. Because fragments of rhyolitic pumice are readily refused, their condition constrains estimates of the timing of fragmentation and agglomeration. The presence of nonfused rhyolite pumice within many loaded bombs implies that the processes forming and quenching the bombs cannot have exceeded several seconds. In contrast, other bombs contain patches of compositionally modified basalt with isolated rhyolite-derived feldspar xenocrysts that indicate complete fusing, melting and assimilation of entrained rhyolitic debris. The bomb population overall indicates that conditions within this Rotomahana vent were thermally and physically heterogeneous and that recycling of debris was ubiquitous, both as recycling of large individual clasts and by fallback and aggregation of material which was reingested in magma before being reejected as new bombs.