A view across Hauraki Gulf to Rangitoto, part of the Auckland Field. It last erupted in ~1350 A.D. Image: Sids1 / Flickr. The city of Auckland might be in one of the most volcanically interesting locations on the planet. The largest city of New Zealand is built on top of an active volcanic field, with at least 50 scoria cones, maars and shield volcanoes dotting the landscape of the city and into the Hauraki Gulf (be sure to read this great summary of all things Auckland Field from GNS Science). Considering that a major metropolitan center is built on this volcanic field, we know surprisingly little about the full history of volcanic activity in the Auckland area that reaches back 250,000 years. The largest single feature of the Auckland Field is the island of Rangitoto (pdf link), on the north end of the field. The island is thought to have formed during a period of intense volcanic activity in ~1350 A.D. (only 50 years after the eruptions that formed the modern Tarawera edifice to the south of Auckland in the Taupo Volcanic Zone). This eruption of Rangitoto helped build much of the island with 3 cubic kilometers of basaltic material (a significant portion of the evidence Auckland Field activity) -- both as lava flows and explosive activity. However, it was thought that this eruption at Rangitoto was the only eruption within the Auckland Field in the last ~10,000 years. The Auckland Field is still closely monitored by GNS Science, with an array of seismometers watching for any changes in its currently quiescent state. A new study in the Journal of Volcanology and Geothermal Research seems to suggest that the Auckland Field and Rangitoto have been more active during the last few thousand years that previously appreciated. Phil Shane and others from the University of Auckland and University of Plymouth (UK) dated tephra (volcanic material from explosive eruptions -- ash and other fragments) from lake sediments in order to find evidence of smaller volcanic eruptions from the Auckland Field. Remember, the Auckland area has been heavily developed, so evidence of many eruptions, especially the small explosive eruptions that dust ash over the landscape, have been erased by human activity (along with normal weathering). However, these fine ash layers are preserved in undisturbed lake sediment, so by sampling the fine layers of ash in cores taken from the bottom of these lakes, you can look at the evidence of eruptions that is lost elsewhere. By carbon-dating layers of lake sediment and identifying ash from well-dated eruptions (like the ~230 A.D. eruption of Taupo or the ~5.5 ka eruption from Whakatane on the Okataina caldera), the ages of Rangitoto tephra layers could be interpolated. What Shane and others found was that Rangitoto produced numerous small eruptions for ~1000 years before the voluminous eruption at ~1350 A.D. Now, it wasn't as easy as that to be sure that the tephra layers were from Rangitoto -- the lake that they sampled the sediment was a volcanic feature itself (a maar), formed 207,000 years ago. There could be the potential that ash from Lake Pupuke could be the source of the fine layers of basaltic ash in the sediment. However, by compositionally fingerprinting the ash, Shane and others could confirm that the ash was sourced from Rangitoto, not Lake Pupuke. They also examined peat cores from nearby areas to help with identifying the extent of some of these ash layers. What does this mean for Auckland? Well, the previous interpretation of Rangitoto was that it was a monogenetic shield volcano, like Mauna Ulu on Kilauea. This means that after an eruption begins from a specific vent, it might erupt from months to a few years, but then go quiet. The next eruption would likely not be sourced from the same vent as before, but possibly somewhere else within the Auckland Field. Shane and others finding instead suggest that Rangitoto puffed away for a thousand years before the final voluminous eruption that formed much of the island on which it sits. For Auckland, this is relatively bad news as the volcanic risk models for the area has been focussed on short eruptions (weeks to years), but if the tephra analysis from Lake Pupuke is correct, then an eruptive center in the Auckland Field could be active for hundreds if not a thousand years (intermittently, of course). Those of you who think a lot of volcanic mitigation know that this changes the game for the city of Auckland, because if a new eruption could produce small explosive eruptions and lava flows for hundreds of years, how the city copes with such activity is very different than an eruption that only lasts a few months. That being said, this news is not catastrophic for Auckland, either. Think about all the people living around Etna on Sicily, a place that has survived through persistent volcanic activity like we might expect from the Auckland Field for thousands of years. A hypothetical 500 year eruption sequence in the Auckland Field would clearly have a dramatic effect on the city, but depending on the location of the vent, it is by no means a death sentence for the city.