Sedimentary signatures of cyclic growth and destruction of stratovolcanoes: A case study from Mt. Taranaki, New Zealand

The long-term behaviour of andesite stratovolcanoes is characterised by a repetition of edifice growth phases followed by collapse. This cyclic pattern represents a natural frequency at varying timescales in the growth dynamics of stratovolcanoes worldwide. Around the > 130 ka Mt. Taranaki (Egmont volcano), New Zealand, coastal-cliff successions at 20-40 km distance comprise repeating packages of lithologically and sedimentologically distinctive mass-flow deposits. Varying depositional mechanisms and source properties of these units record growth and collapse cycles of the central edifice. These are used to construct a model for cyclic volcaniclastic sedimentation in the surrounds of stratovolcanoes. During edifice-construction phases, thick packages of tabular, predominantly monolithologic, hyperconcentrated-flow and debris-flow deposits accumulate with intercalated tephra beds. The mass-flow units commonly contain large proportions of fresh pumice or juvenile-lithic andesite. Intervals of quiescence separating eruptive periods are characterised by landscape re-adjustment, accompanied by deposition of fluvial and aeolian sediments, along with steady accretion of medial ash. In contrast, brief episodes of destruction are marked by wide-spread, distinctively clay-rich, polylithologic debris-avalanche deposits and related marginal debris flow units. The growth stages can be terminated by an eruption-triggered sector collapse, or by external forces once the edifice exceeds a critical stable height or profile (dependent on eruptive style and local geo-tectonic conditions). Once the edifice becomes metastable, regional tectonic earthquakes or shallow-level intrusion events are likely triggers for collapse. Although the resulting debris avalanches represent the greatest individual hazard from such andesite stratovolcanoes, their frequency is relatively low compared with other types of mass-flows generated during edifice-growth phases. Accurate forecasts of future hazard from mass-flow events are therefore dependent on recognition of both the frequency of a stratovolcano's growth cycle and its current position in that cycle.