A paleomagnetic study of Rangitoto Island, Auckland, New Zealand

Abstract

A paleomagnetic study has been made of 99 specimens from 11 sites in the recent basalt lavas of Rangitoto Island. The measured natural remanent magnetisation (NRM) had a formation mean direction of 1.9°0E, –62.1°, α95 = 2.6°; thermal cleaning changed the formation mean NRM direction only slightly to 0.2°E, –61.7°, α95= 1.8°. The densities, magnetisations, and magnetic susceptibilities measured for each site are recorded. Since Konigsberger's Qn ratio is large in every case, it follows that the aeromagnetic total force anomaly is symmetrical about the geographic meridian. Paleomagnetic declinations, at each site, when compared with historically recorded declinations of the geomagnetic field, confirm the dating results previously obtained by the l4C method; that is, Rangitoto was an active volcano between A.D. 850 and A.D. 1800, with a maximum of activity between A.D. 1200 and A.D. 1500. It is inferred that near Auckland the geomagnetic field has changed direction at approximately 2.87 × 10-2 deg/year eastwards, and changed little in inclination, during the last 1000 years. © Crown copyright 1986.