Watch: Volcanic island that sank off Hawke's Bay explored for first time

An island created by a volcano once poked out of the sea off the Hawke's Bay coast. The now undersea mountain has just been explored for the first time by scientists, and what they found was important, and beautiful. Shannon Johnstone reports.

Most in Hawke's Bay associate the Hikurangi subduction zone with potentially devastating earthquakes and tsunamis.

But in the past, there's been volcanic activity too. A place called Bennett Knoll, a vast undersea mountain 100 kilometres off the Hawke's Bay coast more than proves that.

Very little has been published about Bennett Knoll before, but it's a fascinating place.

So scientists from GNS Science, NIWA, and the University of Washington decided to pay it a visit in their bid to understand a bit more about NZ's largest fault, and, indirectly, what might have caused a recent 7.3 magnitude earthquake north of East Cape on it.

No human has ever set eyes on Bennett Knoll. There's a reason for that. It's thousands of metres underwater on the edge of the Hikurangi trench.

So sent down in place of humans was a robot called ROPUS. The Remotely Operated Vehicle has been collecting data on the Hikurangi subduction zone, where the Pacific Plate subducts beneath the east coast of the North Island.

At the 10km-wide Bennett Knoll it stumbled upon scientific gold over 12 hours.

Here were octopus, coral and sea sponges, and other life, in abundance, surviving on salt rocks.

Eight engineers from the Canadian Scientific Submersible Facility used ROPOS to download the latest data from the two undersea observatories monitoring earthquakes and slow slip earthquakes (lasting days to weeks).

The data may give new insights into the 7.3 magnitude earthquake that occurred on March 5.

Part of this research included exploring Bennett Knoll seamount, a large sea mountain measuring 10 km across in the Hikurangi trench which is 2500 metres below sea level.

Voyage leader Dr Laura Wallace of GNS Science said Bennett Knoll was created about 80 million years ago by volcanic eruptions. Initially all 10km of it were above sea level, but eroded over tens of millions of years, forming a flat top undersea mountain.

Made of salt rock, areas where hard rock protrudes creates an oasis for sea life.

ROPOS takes rock samples from Bennett Knoll seamount. Photo / Canadian Scientific Submersible Facility, GNS Science, University of Washington

It was one of the more "spectacular" dives they did. Anemones, sea sponges, corals and octopus in abundance - "many more than we'd seen in other places we dived".

"It was really interesting to see all the life on the seamount because usually at those kinds of water depths you don't see such rich sea life.

"It's really because of that hard salt rock that things are able to attach to that allows these diverse ecosystems down there."

Looking at the image of the sealife National Aquarium of New Zealand general manager Rachel Haydon said there looks to be some smooth (Actinostolidae spp) and warty (Hormathiidae spp) deepsea anemones, a pale orange Bottlebrush coral (Thouarella spp) and possibly some hydrozoans, with branching white stems.

Without a scale size, the octopus could be could be one of the three deep water octopus (Benthoctopus) species that occur deep in New Zealand waters: Benthoctopus clyderoperi, Benthoctopus tangaroa, or Benthoctopus tegginmathae.

A triton shell is also visible, along with tubeworms, encrusting sponges and a white little crab.

She said it is an "absolutely incredible array of sea life" and illustrates why it is so important to protect our oceans.

Bennett Knoll is also subducting and will one day be taken down into the Hikurangi trench, a process which will take hundreds of thousands of years.

ROPOS is a Remotely Operated Vehicle with cameras and arms, and it explored Bennett Knoll seamount for 12 hours. Photo / Supplied

Part of how it subducts could be earthquakes, slow slip earthquakes and steady subduction of the plate, Wallace said.

The seamounts are also interesting as they help the scientists understand why we have slow slip events offshore the east coast of the North Island.

Wallace explained that they believe as the seamounts are subducted at the Hikurangi trench that they cause the plate boundary fault to become less smooth and rougher.

Making the plate boundary rougher may help to promote slow slip events happening.

And in some cases, the subduction of seamounts such as Bennett Knoll may help explain why there are shallow, slow motion earthquakes offshore on the North Island's east coast.

The team also collected rock samples which will be age dated and analysed over the next couple of years to understand Bennett Knoll's history and development.

ROPOS is a high-tech vehicle with cameras, manipulator arms which can pick up and move things around, and is connected to the ship with a cable which allows ship operators to operate the vehicle and receive video footage.

"It was really amazing because we spend a lot of time doing research on the offshore part of New Zealand's plate boundary and you're sitting on a ship and you can't really see what is underneath you, so it's kind of amazing to get a really close up look at what's down there," Wallace said.

Bennett Knoll has a similar shape to other seamounts on the Hikurangi Plateau, a big area east of the Hikurangi Trench, that protrude several hundred metres above the sea floor and have a very flat top.