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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.

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The INternational focus group on Tephrochronology And Volcanism (INTAV) of the International Union for Quaternary Research (INQUA) has conducted an intercomparison of tephrochronology laboratories with electron-beam microanalytical data on volcanic glasses submitted from 27 instruments at 24 institutions in 9 nations. This assessment includes most active tephrochronology laboratories and represents the largest intercomparison exercise yet conducted by the tephrochronology community. The intercomparison was motivated by the desire to assess the quality of data currently being produced and to stimulate improvements in analytical protocols and data reporting that will increase the efficacy of tephra fingerprinting and correlation.Participating laboratories were each supplied with a mount containing three samples for analysis: (1) rhyolitic Lipari obsidian ID3506, (2) phonolitic Sheep Track tephra from Mt. Edziza, British Columbia, Canada, and (3) basaltic Laki 1783 A.D. tephra. A fourth sample, rhyolitic Old Crow tephra, was also distributed.Most laboratories submitted extensive details of their analytical procedures in addition to their analytical results.Most used some combination of defocused or rastered beam and modest beam current to reduce alkali element migration. Approximately two-thirds reported that they routinely analyze one or more secondary standards to evaluate data quality and instrument performance. Despite substantial variety in procedures and calibration standards, most mean concentrations compare favorably between laboratories and with other data. Typically, four or fewer data contributions had means for a given element on a given sample that differed by more than +/-2 standard deviations from the overall means. Obtaining accurate Na 2 O concentrations for the phonolitic tephra proved to be a challenge for many laboratories. Only one-half of the data sets had means within +/-1 standard deviation of the ~8.2 wt% Na 2 O value obtained by other methods. One mean is higher and 14 are lower. Three of the data set means fall below 7 wt% Na 2 O. Most submissions had relative precision better than 1-5% for the major elements. For low-abundance elements, the precision varied substantially with relative standard deviations as small as 10% and as large as 110%. Because of the strong response to this project, the tephrochronology community now has a large comparative data set derived from common reference materials that will facilitate improvements in accuracy and precision and which can enable improved use of published data produced by the participating laboratories. Finally, recommendations are provided for improving accuracy, precision, and reporting of electron-beam microanalytical data from glasses.