Characteristics and consequences of lava dome collapse at Ruawahia, Taupo Volcanic Zone, New Zealand

The eruption of large, rhyolitic lava domes may be accompanied by the formation of large block and ash flows. This may be linked to the style of dome extrusion—whether it forms a series of individual lobes, flows or spines (exogenous) or grows by internal inflation (endogenous). Lava domes can transition from one extrusive style to another as a result of a change in extrusion rate or the formation of facilitating structures such as shear zones. How this change can affect large rhyolitic lava domes is unclear as there are few historically recorded rhyolitic dome eruptions. Here, we present structures at Ruawahia lava dome (a well exposed ~ 700-year-old lava dome), how these facilitating structures enable exogenous extrusion at Ruawahia dome, and link this to collapse episodes along the fringes of the dome during growth. Ruawahia dome is part of the Tarawera dome complex, a chain of domes running parallel to regional structures across the Okataina caldera complex in the Taupo Volcanic Zone, New Zealand. Ruawahia dome consists of (1) a high porosity (44–52%), crystalline (65% DRE), locally brecciated carapace facies with rare bread-crusting and ‘ropey’ flow textures; (2) a core facies of dominantly low to moderate porosity (20–25%) with elongate vesicles that mark weak flow bands; and (3) thin (< 5 m thick) interior breccia zones. Flow bands at Ruawahia are complex and do not fit with hypothesised flow band orientations attributed to a single phase of exogenous or endogenous dome growth. Inward dipping flow bands on ramp structures on the flow surface suggest a flow-like (coulee) morphology; however, steeply dipping and multidirectional flow bands on the edges of the dome challenge this hypothesis. Widespread block and ash flow deposits have been sourced from the leading dome fronts to the NW and SE; these collapse events left behind inflated and bread-crusted outcrops on these dome fronts, suggesting syn-eruption collapse events that led to expansion of a hot, pressurised dome interior. We consider Ruawahia erupted from multiple, aligned vents, either as lobes confined within the crater of a pyroclastic cone formed during the initial Plinian phase of the eruption or those able to flow down the cone flank. The confined lobes formed steep internal breccia zones as individual dome lobes extruded past one another. Lobes that were able to overcome the pyroclastic cone rim (or where the vent was outside the crater) were able to flow down the flanks as bulldozing, thickening flows with dominantly ductile interiors and brittle exteriors; these flows collapsed as the front thickened, possibly due to a decrease in gradient, producing widespread block and ash flows. The removal of lava associated with collapse generated a decompression event which resulted in fragmentation, cracking and vesiculation in the hot interior of the lava flows. These events left behind a re-vesiculated and bread-crusted lava flow front and produced block and ash flows with abundant breadcrust bombs that reached the base of Tarawera.