Constraining the Eruption History of the Rangitoto Volcano using Palaeomagnetic Data

Rangitoto is an island volcano situated outside the city of Auckland, New Zealand. The volcano is the youngest and largest volcano in the monogenetic Auckland Volcanic Field (AVF), with the last eruption occurring about 550-500 calibrated years BP, a date determined from studying historical records. The eruption history of Rangitoto is unknown, however all other volcanoes in the AVF have a brief eruption history. In February 2014 a core spanning 127 metres in length was recovered, consisting of 53 lava flows varying in thickness from 1 to 15 metres. Radiocarbon dates taken from marine sediments found at the bottom of the core, underneath the Rangitoto’s lava flows, suggest that there was early activity as far back as 6000BP, after which Rangitoto may have been dormant until the main shield building phase at around 600BP. Magnetic mineralogy analysis has also shown that much of the core is a reliable recorder of the past geomagnetic field. 156 samples have been analysed for palaeodirectional data and 21 acceptable palaeointensity estimates have been accrued from a range of depths throughout the core length. The collected palaeomagnetic data are used to reconstruct variations in the geomagnetic field, which in turn are used to constrain the eruption rate. Preliminary results suggest that the palaeomagnetic data are incompatible with a short eruption duration of the shield building phase implied by the radiocarbon data (under 100 years) and more compatible with a longer duration of shield building for Rangitoto Island. We discuss alternative explanations for this discrepancy and potential implications of our results in regard to improving hazard planning in Auckland.