Researchers say they've discovered the "missing link" between active onshore and offshore volcanoes in New Zealand.
And it could help them understand volcanic hazards off the coast of New Zealand, and their potential to generate tsunamis.
Around 5 million years ago, the deep seabed began stretching and splitting in two off the North Island, forming the Havre Trough. On the west side is the Colville Ridge, while the Kermadec Ridge is to the east. This area continues onshore at the Bay of Plenty into the Taupō Volcanic Zone (TVZ).
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Now new research led by GNS Science experts shows rifting and volcanic activity in the TVZ is "strongly related" to these two ridges, which continue to move apart.
"For a long time we've known the TVZ is stretching by a few millimetres a year," research leader Dr Fabio Caratori Tontini says.
"We also knew the Havre Trough was similarly expanding, but scientists could not understand why the boundaries of the TVZ did not follow the Colville and Kermadec Ridges offshore.
"Our research has shown that actually, the Havre Trough is made of two distinct sections: a dormant western half and a much more active eastern half, representing two separate stages of evolution."
Co-author Dan Bassett, of GNS Science, says the research shows the currently active Kermadec arc volcanoes in the eastern half align perfectly with the TVZ.
Dr Caratori Tontini says this will help scientists fine-tune what they know about volcanic hazards in the TVZ, their link to submarine volcanoes in the Kermadec arc, and their potential to generate tsunamis that could reach New Zealand's coastline.
"This reminds us of the awesome forces that shaped New Zealand's unique landscape - and how they will continue to do so in the future," Dr Caratori Tontini says.
Their findings have been published this week in the journal Nature Geoscience.