Abstract:
Long-term eruption recurrence rates in monogenetic basaltic volcanic fields are difficult to assess because of low eruption frequencies, but they are important because of the spread of human infrastructure into such fields. Auckland City, New Zealand, is built on the Auckland volcanic field, a young (<250 ka) basaltic volcanic field. In the absence of abundant material for radiocarbon and isotopic dating, an eruptive chronology based on basalt tephra layers deposited in maar lakes was developed. Interbedded, well-dated tephra layers from silicic volcanoes some 220–270 km to the south were used as age constraints. The basalt tephra layers reveal a pattern of activity not evident from the temporal-spatial distribution of volcanic landforms. Twenty-four basalt tephra layers over the last 80 k.y. represent an average frequency of one per 3.5 k.y. Recurrence times vary from <0.5 k.y. to 20 k.y. and show no temporal trend. The tephra record shows a major “flare-up” in explosive activity at 32 ± 2 ka during heightened activity at 34–24 ka. This was related to a period of simultaneous eruptions from several volcanoes across the field revealed by paleomagnetic and isotopic ages. In contrast, the field has been relatively quiet during the last 20 k.y., punctuated by the construction of a shield volcano at 0.7 ka. Thus, the surface manifestation of magmatism varies greatly with time, complicating long-term volcanic hazard forecasting. Despite the high risk from future basalt activity because of proximity, ash fall from distant andesite volcanoes is the most frequently occurring volcanic hazard in Auckland City.
