Volcanic hazards in Auckland, New Zealand: a preliminary assessment of the threat posed by central North Island silicic volcanism based on the Quaternary tephrostratigraphical record

The City of Auckland (population c. 1 million), built on a basaltic volcanic field active as recently as c. AD 1400, faces an additional volcanic threat: that from several large and productive rhyolitic and andesitic eruptive centres of the central North Island, 140-280 km to the south and southeast. Non-basaltic tephra fallout layers originating from these distal eruptive centres are numerous and widespread in the Auckland region and have primary thicknesses ranging from c. 1 mm to = 0.6 m; ignimbrites up to 9 m thick are also documented but are uncommon. The assessment of volcanic hazards in Auckland is made problematical by the different types of volcanic threat posed by these two spatially distinct source areas, and by the lack of recognition hitherto given to the threat from the distal sources. This paper reviews the Quaternary records of distal volcanism affecting Auckland and outlines current investigations into the assessment of environmental impacts of past eruptions. Our preliminary results indicate that the potential threat to Auckland from the distal volcanic sources has been underestimated and that further research into the impacts of Quaternary volcanism on Auckland's environment and infrastructure is essential. The importance of this threat was underscored in mid-1996 when a small magnitude eruption of Mt Ruapehu necessitated closure of Auckland International Airport for three nights