Long-term eruption recurrence rates in monogenetic basaltic volcanic fi elds are difficult to assess because of low eruption frequencies, but they are important because of the spread of human infrastructure into such fi elds. Auckland City, New Zealand, is built on the Auckland volcanic fi eld, a young (<250 ka) basaltic volcanic fi eld. 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, welldated tephra layers from silicic vol canoes 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 "fl are-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 fi eld revealed by paleomagnetic and isotopic ages. In contrast, the fi eld 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.

The Earthquake Flat Pyroclastics form a c. 10 km 3 rhyolite deposit erupted at c. 50 ka from the margin of Okataina Volcanic Centre, immediately following the caldera-forming eruption of the Rotoiti Pyroclastics (c. 100 km 3 ) from vents c. 20 km to the NE. Earthquake Flat Pyroclastics deposits display textural and compositional complexity on a crystal-scale consistent with rejuvenation of a near-crystalline pluton in the upper crust. Quartz and plagioclase crystals are resorbed, whereas hornblende and biotite are euhedral. Fe-Ti oxides indicate large variations in pre-eruption temperatures (702-805 8C). Differences of up to 70 8C within pumice lapilli show that crystals were chaotically juxtaposed during magma stirring and evacuation. Chemical zoning within hornblende crystals is consistent with rimward increases of c. 50 8C. These features are consistent with a convective self-stirring process. Previous isotope studies demonstrate a long (.100 ka) crystallization history for the magma. Resorption of crystals deep in the magma may have produced a Ca-, Fe-and Mg-enriched rhyolite melt that allowed the growth of reverse-zoned hornblende. Microdiorite lithic fragments in the Earthquake Flat Pyroclastics and Rotoiti deposits and a basaltic eruption that immediately preceded the Rotoiti eruption suggest that mafic underplating beneath Okataina Volcanic Centre provided a major thermal and volatile pulse to drive the caldera eruptions.