Geophysical constraints on the structure and formation of Onepoto, Orakei, Pupuke and Tank Farm maar volcanoes, Auckland Volcanic Field

Abstract

ABSTRACTWe report on a geophysical study of maar volcanoes in the Auckland Volcanic Field which was performed in 1975, whose results remain relevant. Modelled gravity and magnetic anomalies indicate that substantial funnel-shaped magnetised basaltic bodies underlie the mud-filled craters at Onepoto and Orakei Basin. These bodies may represent magma that solidified within the craters after initial explosive eruptive phases that involved direct interaction between magma and surface or near-surface water. The estimated volume of residual magnetised basalt beneath the crater is roughly equal to the ejected quantity of basalt at Onepoto and about 3 times larger at Orakei. Magnetic anomaly data show that the Lake Pupuke crater also partially filled with basaltic material after an explosive phase than in turn followed an initial effusive eruptive phase. In contrast, there is no magnetic indication of a substantial magnetised basaltic body beneath Tank Farm crater, so it appears that magma supply ceased at the end of the explosive phase.