Defining Volcanic Regimes at Mt. Taranaki, New Zealand

Abstract

Active stratovolcanoes commonly undergo periods with heightened rates of eruption. These often show a particular, narrow range in eruption characteristics and scales and thus may be categorized as “regimes” of volcanic activity. Over the past ~1000 years, the Maero Eruptive Period (MEP) at Mt Taranaki produced a series of episodes involving lava dome growth and collapse from its summit crater. This millennial record provides an excellent test for the concept of volcanic regimes in hazard estimation. Deposits from this period were studied in detail to provide a precise record and a new age model. The MEP consists of 11 temporally distinct eruption episodes, with a restricted range of style and magnitude compared to earlier eruptions at Mt Taranaki. MEP eruptions included single dome growth/collapse events, repeated dome growth/collapse sequences, directed-blast eruptions, and two sub-Plinian eruptions. Paleomagnetic methods were used to analyse the youngest summit dome and deposits from its collapse, producing a new age of 1780-1800 AD for Taranaki’s latest eruption. Detailed mapping and age determinations showed that the repose periods during the MEP were between <10 yrs to ~150 years, with the 200 year gap from the latest eruption being the longest yet. These methods were also used to show that the primary hazard during the MEP resulted from block-and-ash flows deposited on the northwest flanks of the volcano. These domes were still hot during collapse. Analysis of deposits of an older larger eruption in the same area revealed it to be part of one of the largest-known eruptions from Taranaki at 11,500 years BP, and far larger than the events of the MEP. This research showed for the first time that large scale eruptions at Taranaki may produce pyroclastic flow hazards out to 24 km from the volcano, around 9 km further than they have ever been recognized before. The methods profiled here and the concept of volcanic regimes provided a more robust evaluation of the timing and hazards associated with long periods of dome growth at Taranaki, with results applicable to similar stratovolcanoes around the world.