Eruption patterns in monogenetic volcanic fields

Abstract

How often do new volcanoes erupt in volcanic fields such as Auckland? Geophysical research has shown that five volcanoes, geographically spread throughout the Auckland volcanic field, may have erupted at the same time.

These volcanoes are Puketutu, Crater Hill, Wiri, Mt Richmond and Taylor Hill. This multiple volcanic event, which occurred about 30,000 or more years ago, has been recognised from magnetic measurements on volcanic rocks which have recorded an unusual perturbation of the Earth’s magnetic field. The first high-precision radiometric dating of the Auckland volcanoes is currently being undertaken to confirm the date of this event and to provide other dating information crucial for understanding the timing of eruptions in Auckland. 

Planning for the re-awakening of volcanic fields such as Auckland has not previously envisaged the possibility of contemporaneous eruptions from several widespread volcanoes. The results of this research identify that the possibility of such a scenario needs to be considered in hazard assessment and mitigation planning for Auckland City and also have important implications for cities elsewhere in similar settings.

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Technical Abstract

Five monogenetic volcanoes within the Quaternary Auckland volcanic field are shown to have recorded a virtually identical but anomalous paleomagnetic direction (mean inclination and declination of 6.17 and 351.0⁰, respectively), consistent with the capture of a geomagnetic excursion. The volcanic rocks also record that the paleointensity of the geomagnetic field was 8-21 µT, i.e. on average about 20% of normal values, thus confirming the occurrence of an excursion event. High-precision 40Ar – 39Ar dating of these volcanoes appears to confirm that a single excursion event has been recorded by the volcanic rocks, though the exact age of the event is currently subject to confirmation. Based on documented rates of change of paleomagnetic field direction during excursions, the narrow spread of directions implies that this volcanoes may have all formed within a period of only 50-100 years and possible erupted contemporaneously. These temporally linked volcanoes are widespread throughout the Auckland volcanic field and appear not to be structurally related. Hence these results provide the first clear evidence for widespread multiple eruptions in the Auckland field and by analogy for multiple eruptions in any other similar monogenetic volcanic field. This finding challenges conventional risk calculations for monogenetic fields such as Auckland since the general paradigm for their reawakening is that only a single new volcano or group of closely spaced vents is created, typically at intervals of several hundred years or more. The work therefore has critical importance to volcanic hazard assessment in Auckland, and also elsewhere, since the impact of renewed eruptive activity could be significantly under-estimated.