Delving deep into history

Rangitoto last blew 550 to 600 years ago. Photo / Sarah Ivey

Orakei Basin sediment will reveal volcanic activity from 140,000 years ago.

Scientists have probed more than 100m beneath Orakei Basin in a bid to reveal the explosive history of ancient Auckland.

The samples they have just retrieved from ancient deposits of lake sediment will detail, in unprecedented definition, volcanic eruptions that have taken place in the region over a period potentially stretching back 140,000 years.

Cutting-edge analysis of the samples, now to take place in Auckland and Wellington, could ultimately change our understanding of the vast volcanic field that modern Auckland lies upon.

While eruptive activity among the field's 53 volcanoes went back some 250,000 years - and most recently, at Rangitoto, between 550 and 600 years ago - there was much we had yet to learn, said Associate Professor Paul Augustinus, of the University of Auckland's School of Environment.

"We know quite a lot - in particular, we know a lot about the last 50,000 years - but prior to that, we have very little understanding of the nature of the long-term behaviour of the field, and especially eruptive events."

Orakei Basin is seen by scientists as one of the best points in the region to uncover this secret history.

Although it's now a shallow estuary, following the volcanic explosion that formed it, it was for most of its history a deep freshwater lake collecting sediment, volcanic ash and biotic remains. These lake sediments were mostly very finely layered and would be crucial in compiling the most detailed history yet of past eruptions in and around the city area.

Over the past few weeks, researchers on board a drilling rig on the water were able to gather samples to a depth of 105m.

Dr Augustinus said the research would not only shed further light on bigger eruptions reflected in larger, thicker ash layers, but those much smaller events that typically didn't produce obvious layers.

"They're what we call cryptotephras - basically extremely fine, microscopic ash layers, which would've come from eruptions that would still have a huge impact on infrastructure."

Along with better understanding Auckland's volcanic risks, the samples would also help scientists reconstruct the region's climate over the past 100,000 years. These would provide a much-needed comparison to corresponding records recovered from deep below the ice in Antarctica and elsewhere on the planet, at a time scientists were racing to learn lessons from warmer periods in the Earth's past.

Our explosive past

• Scientists have just drilled more than 100m into the bed of Orakei Basin

• The samples they have recovered will allow them to map in unprecedented detail past eruptions among the more than 50 volcanoes in Auckland

• The project will also build a useful picture of the region's climate over more than 100,000 years.