Image Credit: B.Hayward Conundrum of Auckland’s Rangitoto volcano Hazard scenarios for monogenetic (clustered) volcanism must cover a range of eruption periods as revealed by a study of Rangitoto. By drilling through the structure of Rangitoto – the youngest and largest volcano in the monogenetic Auckland volcanic field – the eruptive history of the basalt shield volcano has been revealed. A massive shield-building phase occurred from 650 to 550 years BP (Before Present), erupting uniform subalkalic basalts. Four batches of magma were erupted in sequence. However, they lack genetic connection. Two show a drop in incompatible trace-element abundance and a rise in the ratios of incompatible elements with decreasing age – all consistent with progressive partial melting at the source. Activity from 550 to 500 years BP was explosive and less voluminous, producing summit scoria cones. It was a period of greater diversity in magma compositions, including more mafic subalkalic basalt, and alkali basalt, pointing to the sourcing of magmas simultaneously from different mantle depths. The make-up of Rangitoto supports growing evidence that major periods of activity in “monogenetic” basalt fields occur at centres with multiple eruptions. Visiting Rangitoto soon? Check out our field guide! You can find it here: https://www.devora.org.nz/auckland-volcanic-landscape/under-a-blood-red-sky-a-guide-to-rangitoto/ Tags:basalteruptionmagmamonogeneticpublicationsRangitoto
