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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.

Attachments

Long-lived shield volcanism within a monogenetic basaltic field: The conundrum of Rangitoto volcano, New Zealand. Cover
Long-lived shield volcanism within a monogenetic basaltic field: The conundrum of Rangitoto volcano, New Zealand.