DELAYED SEDIMENTARY RESPONSE TO THE AD 1886 ERUPTION OF TARAWERA, NEW-ZEALAND

Sedimentary response to tephra deposition from substantial volcanic er uptions is generally immediate and dramatic. An important exception is the A.D. 1886 eruption of Mount Tarawera, New Zealand, which blankete d >200 km(2) with greater than or equal to 50 cm of basaltic scoria an d ash fall erupted from a fissure through the Tarawera dome complex. T his fall deposit has suffered very little rilling or other surface ero sion, and there was no immediate downstream redistribution of tephra b y 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 th e dome complexes of Okataina caldera, and one direct result of the eru ption was constriction of Lake Tarawera's outflow, which caused the la ke to rise substantially. Eighteen years after the eruption, collapse of a tephra bank that had controlled lake level triggered a breakout f lood from this raised intracaldera lake, triggering a decades-long per iod of intense tributary erosion in the upper catchment and damaging s tream-bed aggradation outside the caldera. This abrupt increase in sed iment yield from an initially stable posteruptive landscape has no doc umented precedents, yet may be a common posteruptive sedimentary respo nse in caldera complexes of temperate to humid regions.