Rapid emplacement of a mass via pyroclastic flows, or edifice failure, generates volcanic tsunamis. Physical modelling demonstrates that the efficiency of tsunami generation is influenced by the angle the mass enters the ocean. Efficiency decreases with increasing slope angle from 20° to 60°, before increasing to a maximum at 90°, which corresponds to a mass falling directly into the ocean without interacting with the slope (impact tsunami). Further, in the case of surging pyroclastic flows or regressive failures, successive closely spaced events may generate larger tsunami waves than a single event of comparable volume. It is difficult to assess if physical model results are meaningful for real world tsunami events due to limited observational data. Two New Zealand palaeo-events - pyroclastic flows from Mt Tarawera and edifice failure at Whakaari (White Island) - can be linked to tsunami deposits, which constrains numerical simulations of the source mechanisms. The Mt Tarawera event involved multiple pyroclastic flows entering a lake during the AD 1314 ± 12 Kaharoa Eruption. The interaction of multiple closely spaced pyroclastic flows is necessary to generate the 6-7 m maximum wave height inferred from near source tsunami deposits. Tsunami deposits in the Bay of Plenty, dated to 2962 ± 52 BP, are consistent with edifice failure at Whakaari. In this case a single event with a volume of 0.23 km~3 is sufficient to account for the tsunami deposits. Hence, if the failure was regressive, the successive stages were sufficiently close together to be indistinguishable from a large single event.
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