AN ENHANCED -1800-YEAR RECORD OF RECENT VOLCANIC ASH-FALL EVENTS FOR NORTHERN NEW ZEALAND FROM THE ANAlYSIS OF CRYPTOTEPHRA

Abstract

The history of volcanic eruptions in North Island, New Zealand, based on the visible stratigraphic record of tephras is not fully representative of the type, frequency and magnitude of eruptions that have occurred in the past or that are likely to occur in the future, Relatively small-scale eruptions from the region's andesitic volcanoes can produce widespread fine tephra falls with considerable impact, such as during recent eruptions of Mt Ruapehu ( 1995-1996) and Mt Ngauruhoe ( 1974-1975), and their threat is therefore likely to be underestimated. The aim of this study was to develop an enhanced stratigraphic record of recent ash-fall events in the Auckland and Waikato regions through the application of cryptotephrostratigraphic techniques. Cryptotephras in sediment cores from two peat bogs and two lakes were quantified and characterised for the period since deposition of the AD 233 ± 13 Taupo Tephra. A complex record of primary and reworked rhyolitic and andesitic tephra-fall is revealed. A stratigraphic and geochemical protocol for distinguishing primary from likely reworked tephras was developed and enabled the identification of ten post-Taupo cryptotephra-fall events. The rhyolitic Okataina Volcanic Centre-derived Kaharoa Tephra (AD 1314 ± 12), a key chronostratigraphic marker for human settlement in New Zealand, was found in all sites together with four pre-historic Mt Ruapehu-derived tephras of the Tufa Trig Formation (Tf4, Tf5, Tf6, and Tfl4). In the Waikato Region, an additional Tufa Trig tephra (Tf8) was detected, as well as ash fallout from historical eruptions of Mt Ruapehu ( 1861, 1945 and 1996) and Mt Ngauruhoe (1975). These findings represent a considerable extension to the known geographical ranges of tephra-fall events and demonstrate that cryptotephrostratigraphic techniques can enhance the historical and geological record of ash-fall events as a guide to future volcanic hazard assessment for countries such as New Zealand.

Date of Award	2009
Original language	English
Awarding Institution	University of Plymouth