Drilling begins to study Rangitoto's explosive history

Recent volcanic ash studies by the team resulted in the discovery that Rangitoto was more active for much longer than previously thought. Photo / Thinkstock

Rangitoto's explosive history - and what it means for risk to Auckland in the future - will become clearer following a study drilling up to 150m into the island's surface.

Drilling, which began at a clearing on the island three days ago, will gather dozens of core samples revealing the volcano's eruptive history.

The study team, led by Associate Professor of Geology Phil Shane and researchers from Auckland University's School of Environment, have so far drilled to a depth of 75m and have found numerous thin lava flows.

"This shows how the volcano was built-up with time," Dr Shane said.

"It's surprising the number of lava flows recorded. We think we've still got another 80-100 metres to go."

From samples taken, scientists will learn more about the duration and frequency of eruptions on Rangitoto over the past 1000 years, information that will help hazard and risk planners such as Civil Defence significantly improve models of future volcanic activity.

"While we can't predict the future, our research so far raises questions about if there was future activity in Auckland, how long would it last and could it be for longer periods rather than just months?"

Recent volcanic ash studies by the team resulted in the discovery that Rangitoto was more active for much longer than previously thought.

As with other small basaltic volcanoes around the world, the island was thought to have erupted only once or twice about 500 years ago.

However, studies of volcanic ash in sediments on the floor of Lake Pupuke show the island erupted intermittently or semi-continuously from about 1,500 years to 500 years ago.

The study is funded by EQC and is a collaboration with Massey University.