The North Island sits atop the remnants of the biggest ever "volcanic outpouring" in the history of the planet, new research has found.
About 120 million years ago, when dinosaurs roamed the Earth, a "giant plume of hot rock detached itself from the core-mantle boundary, about 3000km below the Earth's surface, and rose rapidly to the surface as a superplume", Victoria University of Wellington scientists said on Thursday.
While it's been known for decades the Earth's mantle - located between the core and the crust - can churn like a lava lamp, evidence for superplumes such as this one has been scant.
"Melting of this rock near the surface could then be the cause of prolific volcanism, such as that observed in Iceland or Hawaii," said study co-author Te Herenga Waka-Victoria University of Wellington geophysicist Tim Stern.
"Even larger volcanic outpourings have happened in the geological past, of which the biggest known occurred in the southwestern Pacific in the Cretaceous Period during the time of the dinosaurs, forming a continent-sized underwater volcanic plateau."
The fact something huge happened in the Pacific 120 million years ago was first proposed in 1989, when scientists looking in the western Pacific for older rocks from the Jurassic period couldn't find any - nothing was older than 120 million years.
Just how big it was has been hard to prove, as in the millions of years since it happened, the drifting tectonic plates have broken it apart. Prof Stern said geologists recently have been close to giving up on the theory altogether.
But the new research confirms separate plateaus of volcanic rock found beneath the North Island, and beneath the ocean floor near American Samoa, Tokelau and the Solomon Islands, were once joined - meaning there was once an outpouring of volcanic rock covering "a region over 2000km across".
They figured it out by measuring the speed at which seismic waves travel through the layers of rock beneath the ocean. The one beneath the North Island is known as the Hikurangi Plateau.
"The key observation in the new study is that seismic pressure 'P' waves - effectively soundwaves - triggered by either earthquakes or man-made explosions travel through the mantle rocks beneath the Hikurangi Plateau much faster than are observed beneath most of the sea floor, reaching speeds of nine kilometres a second," said Prof Stern.
"A peculiar feature of these high speeds is that they are equally high for seismic vibrations travelling in all horizontal directions, but much lower for those vibrations travelling vertically upwards."
The data from the Hikurangi Plateau matched that from the Manihiki Plateau north of Samoa and the Ontong-Java Plateau north of the Solomon Islands (previously believed to be the world's biggest volcanic plateau), proving they were all created at the same time and part of the same superplume, which would have had dramatic effects on the planet.
Prior research has found the Earth's magnetic poles, which are created by the churning of the mantle and regularly reverse polarity, stood virtually still for millions of years after the enormous mushroom-shaped superplume. There was also a massive rise in temperatures afterwards, which would have released massive amounts of carbon into the atmosphere. Sea levels rose about 250m above where they are today, previous research has found, partly due to the greenhouse effect but also because the ground beneath the Pacific Ocean was literally pushed upwards.
There is also evidence that at least half of the world's oil supply was created in the years after the superplume, and mountain ranges in the Americas were pushed up as the newly-raised Pacific crust eventually went down beneath its neighbouring plates.
"It is an intriguing thought that New Zealand now sits on top of what was once such a powerful force in the Earth," said co-author Associate Prof Simon Lamb.
The research was published on Thursday in journal Science Advances.