City’s explosive history
Auckland’s volcanic past was temperamental
Auckland’s explosive past has been pieced back together in a volcanic history stretching back hundreds of thousands of years. Two studies published this month reveal that Auckland’s volcanic past was a temperamental one — at one stage there were several large eruptions happening within 4000 years, contrasted with thousands of years or more of silence.
The studies have been published in the Journal of Volcanology and Geothermal Research and in the Bulletin of Volcanology by a team of researchers from the Devora (Determining Volcanic Risk in Auckland) research pro- gramme. They have revealed the Auckland Volcanic Field (AVF) has a complex and episodic eruption history.
The oldest eruption (Pupuke) dates back to approximately 200,000 years ago and the youngest (Rangitoto) only 500 years ago. Mt Wellington, Mt Smart and Mt Eden all erupted relatively recently while Mt Albert and Mt Roskill’s eruptions were much older. One Tree Hill’s explosion, about 60,000 years ago, falls in between. At least half of all the eruptions occurred in the last 60,000 years, which is a relatively short timeframe and indicates an increase in the rate of eruptions overall.
Dr Graham Leonard, a GNS Science senior scientist who led the research team, said researchers used new data to decode Auckland’s volcanic past and understand what sort of volcanic events have occurred and when they happened.
“What we now know about the Auckland Volcanic Field, based on this new research, is that some eruptions flare-up over what is, geologically speaking, a short period of time,” he said.
“For example, there can be six to 10 volcanoes erupting within a 4000-year timeframe.”
On the other hand, the volcanic field had also gone quiet for up to 10,000 years in the past 60,000 years, which Leonard said was “quite a long gap”.
“This new research is exciting because it has allowed us to further define when eruptions have occurred which has helped us flesh out an eruption timeline.”
Researchers used argon-age dating in collaboration with the US Geological Survey to increase the number of reliably, directly dated volcanic centres from 12 to 35, out of a total of 53 in the AVF. A further 13 volcanic centres were also put in their likely place on the timeline using high- precision chemistry by volcanic geochemist Jenni Hopkins of Victoria University.
“We now have an idea of the order and timing of almost all of Auckland’s eruptions, which is an unusual success compared to the state of knowledge on other volcanic fields around the world,” said Hopkins, who completed the work for her PhD.
Leonard said using both new techniques allowed his team to determine that some eruptions in the AVF may be linked and that the field, as a whole, could be either “all on” or very quiet — sometimes for several millennia.
“However, it’s important to note that the Auckland Volcanic Field is temperamental and we can’t use this study to predict a simple likelihood of a future eruption.”
An eruption from one of the volcanoes could blast out an explosion crater 1km to 2km across, destroying everything in it, but planners believe people in its path would be evacuated well before an event.