Frequency of eruptions

The oldest volcanoes erupted about 190,000 years ago (Pupuke Moana & Te Kopua-o-Matakerepo /Onepoto Basin), but over half of Auckland’s eruptions occurred in the last 60,000 years, meaning that the eruption rate has accelerated.

Our research shows that the chance of a future eruption is roughly 10% over the next 50 years, based on a median (middle) and modal (most common) gap of about 500 years between eruptions over the last 60,000 years. The mean (average) is not included in these statistics as the few, less common larger gaps of about 10,000 years skew the mean value, leading to an incorrect and misleading number.

Our scientists study the pattern of ash layers in lake sediment cores to figure out how often volcanic eruptions affect a certain area. We discovered that sometimes ash layers from Auckland volcanoes were close together in the cores, meaning there were lots of eruptions within a short time, and sometimes there were large gaps between ash layers, meaning that there was a long time gap between eruptions. This irregularity in time gaps makes it impossible to predict when the Auckland Volcanic Field will erupt again.

Some eruptions are only preserved as scattered grains of ash in tiny layers. Known as cryptotephra, these layers get their name from crypto, the Greek word for ‘secret’, and are made of glass shards so small that scientists have to use a special microscope to see them. Cryptotephra layers are sometimes the only evidence that an eruption ever happened.