Abstract

Sedimentary response to tephra deposition from substantial volcanic eruptions is generally immediate and dramatic. An important exception is the A.D. 1886 eruption of Mount Tarawera, New Zealand, which blanketed >200 km2 with ≥50 cm of basaltic scoria and ash fall erupted from a fissure through the Tarawera dome complex. This fall deposit has suffered very little rilling or other surface erosion, and there was no immediate downstream redistribution of tephra by lahars or dilute floods. We attribute this lack of early response to the high permeability of the tephra deposit and gentle relief in the areas of substantial accumulation. Numerous small lakes occur among the dome complexes of Okataina caldera, and one direct result of the eruption was constriction of Lake Tarawera's outflow, which caused the lake to rise substantially. Eighteen years after the eruption, collapse of a tephra bank that had controlled lake level triggered a breakout flood from this raised intracaldera lake, triggering a decades-long period of intense tributary erosion in the upper catchment and damaging stream-bed aggradation outside the caldera. This abrupt increase in sediment yield from an initially stable posteruptive landscape has no documented precedents, yet may be a common posteruptive sedimentary response in caldera complexes of temperate to humid regions.

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