The Auckland Volcanic Field
Auckland is the only large city on earth, built on top of a volcanic field. It lies on an active basaltic volcanic field that contains at least 53 volcanoes formed over the past 200,000 years. It sits on continental crust approximately 400 km west of the Hikurangi trench, where the Pacific Plate is being actively subducted below the Australian Plate, and approximately 200 km west of the active volcanic arc.
Unlike other New Zealand volcanic fields, each eruption has occurred from a new location and the volcanoes do not erupt twice, otherwise known as monogenetic. There is one exception to the rule, Rangitoto which has erupted at least twice and has led to further scientific debate on whether the field is truly monogenetic. Recent research from Rangitoto suggests there is now evidence for multiple events of contrasting chemistry from essentially the same centre.
Rangitoto is the most recent volcanic eruption, which was witnessed by Māori living on Motutapu Island, occurring approximately 600 years ago and produced Rangitoto. The oldest volcano is Pupuke at around 190,000 years old. The gaps between eruptions have ranged from approximately 50 to 10,000 years. A simplistic approach of averaging the 53 volcanoes across the 193 ka history gives an average rate of one eruption every 3,600 years. New data shows that at least 26 Auckland Volcanic Field centres have erupted in the last 60,000 years, dropping the average to every 2,300 years. This is only a statically average and there are thus no grounds on which the duration of the next eruption can be forecast.
The one thing that is certain about the Auckland volcanic field is that its next move is totally uncertain. This is due to the fact that globally there is not much knowledge of what happens before an eruption within a volcanic field. White Island which erupted in December 2019, had its alert level elevated to level 2 in the weeks before as there was observed increases in the amount of SO2 gas output, volcanic tremor and geysering within the crater. The Auckland volcanic field does not have the signals and monitoring luxury of that Whakaaki / White Island or Mt Ruapehu has, so the only modern day method that can be used is earthquake recording.
Rising magna will cause volcanic ground tremors and based on recent examination of crystalised rocks at existing volcanoes, Auckland’s magna commences its journey from around 70 to 90km below the earth’s surface. The speed at which it rises to the surface and signals it will generate is not entirely known but most likely small earthquakes will be recorded in days to weeks before magna reached the surface. The recent awaken of an Icelandic volcanic field on the Reykjanes Peninsula after 800 years was an interesting sneak peak of what could happen under Auckland in the weeks leading up to an eruption. The Icelandic eruption had as many as 40,000 volcanic tremors, alerting volcanologists and locals for what was to come.
For Auckland's volcanic field, once an ongoing volcanic signal is confirmed rising to the surface, a 5km evacuation zone will be put in place by the Auckland Emergency Management group with mass evacuations of the suburbs at most risk. The most recent study modelled that 320,000 Aucklanders would most likely have to evacuate in the days leading up to an eruption and it would take around 49 hours to execute the evacuation. The most dangerous stage of the volcanic eruption is within the first few days as magna will come into contact with either ground or salt water triggering a violent phearotmagmatic eruption. A large eruption from the new volcano could last up to months and possibly years with ongoing hazards from volcanic ash and lava flows a persistent hurdle of all Auckland residents to live a normal and healthy lifestyle during this period.
For more than 13 years, DEVORA (DEtermining of volcanic VOlcanic Risk in Auckland) has been focused on preparing Auckland for a volcanic eruption – and the ongoing disruption it will cause. The chances of the next eruption occurring in our lifetime are low but eventually the magna will once again rise to the surface in the Auckland volcanic field.
One Tree Hill
Mt St John
Approx. years old