The eruption history of Rangitoto: reappraisal of a small New Zealand myth

Abstract

This paper reviews the evidence for the widely held opinion that Rangitoto has erupted several times, once very recently. Such a history is inherently improbable. The magma from basaltic volcanoes solidifies in the vent after an eruption, so simple mechanical factors make it unlikely that eruptive activity could recommence after a long interval. The absence of relevant Maori traditions is also inconsistent with a very recent eruption. Widely accepted as evidence of multiple eruptions are a sample of wood on Rangitoto supposedly from a tree overwhelmed by a lava flow, and early historic descriptions which have been taken to indicate that 150 years ago the island was almost completely barren. More recently, palaeomagnetic measurements have been interpreted as indicating a series of eruptions spanning the period 1000 – 1810 AD. None of this evidence stands critical examination. There is great uncertainty about the nature of the wood sample, and it should be discounted. A survey of historic records shows that there was diverse and luxuriant vegetation on the island during the first half of the nineteenth century. And practical and statistical aspects of the palaeomagnetic data mean that they can be interpreted as relating to a much more short-lived event. I therefore prefer the view that Rangitoto was formed as the result of only a single eruptive episode. Several lines of evidence agree in dating the eruption at around 1400 AD.