A traverse across the caldera floor of Havre Volcano at a depth of about 1600m.

The world's largest deep ocean volcanic eruption in the past century happened 1000 kilometres north east of the North Island.

A gigantic pumice raft - 400 square kilometres in size - that was found floating in the ocean near New Zealand in 2012 showed the underwater eruption had occurred.

The "complex" eruption involved 14 aligned vents causing a "massive rupture". Named Havre, the volcano was only discovered in 2002.

SUPPLIED The Havre volcano is located about 1000 kilometres north east of the North Island.

Studying it has helped increase understanding of powerful deep ocean volcanic eruptions and how magma rises from the crust to the surface.

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It's been called a "scientific gold mine" by researchers.

SUPPLIED This image shows the high resolution seafloor topography of the Havre caldera with the new 2012 erupted lavas in red. The volcano has a depth of 1519 m, and top at 650m below sea level.

University of Tasmania volcanologist Dr Rebecca Carey said: "We knew it was a large scale eruption, approximately equivalent to the biggest eruption we've seen on land in the 20th Century.

"Having the pre-eruption map of Havre volcano allowed us to know exactly what and where the new eruption products were on the submarine edifice.

"This event is a scientific gold-mine as for the first time there are quantitative constraints on submarine eruption dynamics, and the role of the ocean in modulating those dynamics."

SUPPLIED University of Tasmania volcanologist and lead author of a report about the volcano, Dr Rebecca Carey.

Carey said 80 per cent of the world's volcanoes were on the sea floor, making understanding them important.

They provided heat and chemicals for the ocean which also supported life.

AN INTERNATIONAL EFFORT

University of Tasmania researchers joined international collaborators including Woods Hole Oceanographic Institute for the two-year study.

The team used submersibles including a remotely operated vehicle and an automated underwater vehicle to map, observe and collect samples from Havre volcano.

PASSIVE ERUPTION

"There are conceptual theories about how deep volcanic eruptions should be relatively passive, forming lava flows due to the huge amount of pressure from the overlying water column," Carey said.

"This is the first event of high silica magma composition where we are able to provide the constraints that test whether the hydrostatic pressure did suppress explosivity.



"We were able to demonstrate that the eruption was very complex, involving more than 14 aligned vents that represent a massive rupture of the volcanic edifice.

"We were also able to demonstrate that 80 per cent of the volume of the pumice was delivered to the pumice raft and efficiently dispersed into the Pacific Ocean landing on Micronesian island beaches and the East Australian seaboard.

"The record of this eruption on Havre volcano itself is highly unfaithful - it preserves a small component of what was actually produced, which is important for how we interpret ancient submarine volcanic successions that are now uplifted and are highly prospective for metals and minerals."'

DATA'S WIDE INTEREST

Carey said data collected for the study also had much interest from the broader scientific community.

"The eruption blanketed the volcano with ash and pumice and devastated the biological communities. Biologists are very interested to learn more about how species recolonise, and where those new species are coming from.

"We also discovered new infant hydrothermal systems, and observing how they recover after such a large event is of importance."

There was a decade worth of science to do based on the 2015 voyage to study the volcano, she said.

The study, The largest deep ocean silicic volcanic eruption of the past century, was published online in Science Advances.