Fire & water
Extending some 1400 kilometres northeast from the Bay of Plenty, the Kermadec Arc is the longest underwater volcanic ridge on the planet. It is also highly active, creating great calderas on the sea floor and spewing rafts of pumice that drift on the surface for thousands of kilometres. Scientists are only beginning to understand the diversity of life that takes advantage of this unstable environment.
Early on the afternoon of July 31, 2012, Paeroa jeweller Maggie de Grauw was flying home from a holiday in Samoa when she spotted a vast brownish-grey slick on the surface of the ocean beneath the aircraft. She photographed it, and wondered if it was an algal bloom, an oil spill or—recalling a conversation with a friend the week before—a deposit from a volcano. When she got home, an internet search led her to Scott Bryan, a volcanologist at the Queensland University of Technology in Brisbane. She sent her photographs to Bryan, who alerted geologists in New Zealand and the United States.
Nine days later, the crew of a Royal New Zealand Air Force Orion, on a surveillance flight from Samoa to Whenuapai, located the deposit. They estimated that the expanse of floating sediment extended over 250 nautical miles by 30 nautical miles. They took a photograph, noted the latitude and longitude, and reported the sighting.
The Air Force relayed the photographs to HMNZS Canterbury, which was southwest of the mystery deposit on a resupply mission to the Department of Conservation field base on Raoul Island—the main island, and itself an active volcano, in the Kermadec Group. It was just after midday, and I was on the bridge of the Canterbury, recording an interview with NIWA marine geologist Helen Bostock, when Commander Sean Stewart gave the order to alter our course. He was happy to find a geologist on the bridge: Bostock identified the sediment floating on the surface of the blue ocean as likely to be a mix of ash and pumice from an underwater volcanic eruption.
As dusk fell over the heaving grey sea, Bostock and I saw the first streams of pumice floating on the water, pushed by the waves into long, dirty ribbons of flotsam. The crew were soon leaning over the rails of the foredeck with buckets on ropes and cameras, with a team cheering from the bridge each time a piece of pumice was retrieved from the ocean. Most pieces were golf ball to fist sized, but larger chunks of pumice, some the size of soccer balls, were within sight, if not reach.
That evening, we retired to a makeshift laboratory, where Bostock bagged and labelled each sample with the latitude, longitude, time and date of collection. The samples of this freshly minted rock—created when super-heated magma erupts from a vent and is rapidly cooled and depressurised—were heavy with water, and rough and irregular in shape, nothing like the ovoid rocks that wash up on beaches around the main islands of New Zealand. Assuming the four-metre swells and 30-knot winds had dispersed the expanse of sediment that others had spotted from the air, we retired to our cabins.
But later that night, as Lieutenant Tim Oscar started his midnight-to-4am watch, the ship sailed into what he described as something looking like an ice shelf.
Nearly a metre thick, a giant ‘raft’ of loose pumice extended as far either side of the ship as Oscar could see with the ship’s spotlights and lasted for half a nautical mile.
The next day, engineers extracted small chips of pumice from the ship’s water filters—which draw water up from four metres below the sea surface—and Bostock trawled through the bag of debris to retrieve some more samples. (I pocketed a couple of chips to take home as souvenirs.)
We spent the next few days at anchor beside Raoul Island. While the other scientists were tagging Galapagos sharks, photographing the flukes of humpback whales and monitoring the volcanic island’s crater lake, we looked for pumice. Bostock found more fresh pieces on the island’s beaches and the crew of a small inflatable Navy boat collected a raggedy lump the size of a soccer ball floating close to shore. At first, we assumed that the pumice had come from a very recent eruption—some time in the past few days—but a biologist, University of Queensland PhD student Libby Liggins, pointed out the tiny goose barnacles that had already colonised this floating lump of rock. The larvae of this opportunistic species float through the ocean and attach to anything big enough to call home, and the size of the three barnacles on board suggested this lump of pumice had been in the water for at least two weeks.
By the time we left Raoul Island, news of our find had spread—reports circled the globe of a floating raft of pumice, variously described as the size of Tuscany, New Jersey or Canterbury—and American and French geoscientists had used satellite photographs to link the pumice raft to an ash plume visible on July 19 west of L’Havre Rock, a tiny islet in the Kermadec Group that at low tide barely pierces the surface of the sea.
Lieutenant Tim Oscar described the sight as one of the “weirdest things I’ve seen in 18 years at sea”, but according to Scott Bryan, pumice rafts like this one have been sighted in all the world’s oceans, particularly in the Pacific over the past 50 years.
The article that led de Grauw to Bryan described a pumice raft ejected from the Home Reef volcano in Tonga in 2006 that had transported stowaway species thousands of kilometres across the ocean. Over nearly eight months, the raft travelled more than 5000 kilometres across the southwest Pacific to eventually wash up, with its cargo of wildlife, on the east Australian coast.
“Every piece of pumice is a raft opportunity for an organism,” says Bryan. He and his team identified more than 80 species—including algae, barnacles, worms, gastropods and crabs—living on the floating rocks. “There were a whole range of organisms, from photosynthetic and filter feeders, to scavengers and some predators,” he says. Some benthic species, such as bryozoan and barnacles, were firmly attached to the pumice, while other species, such as gastropods and crabs, were free to get on and off as it suited them. Other species, including a marine insect, Halobates, used the pumice as a place to lay eggs.
“We looked at over 5000 pieces of pumice, and every piece had something on it, some up to 10 or 12 species,” says Bryan. We estimated the Home Reef eruption produced about 1014 [one hundred trillion] pieces of pumice, so it’s mind-boggling, astounding, how much biomass can be moved quite quickly across deep oceans in this way.”
Raoul Island, the largest of the Kermadec Group, was built gradually, by many eruptions over more than a million years. When a new volcanic island pushes through the sea surface, it is initially bare rock, bereft of plants, birds and marine species. Over time, species colonise it. It seems pumice has played a role in assembling the unique mix of tropical, sub-tropical and temperate species that now live in the pristine Kermadec waters.
Many marine organisms are opportunistic and will hitch a ride on whatever is available—whether it is pumice, floating kelp or sargassum, or more recent additions to the marine environment such as ships’ hulls and floating rubbish. But unlike biological rafts, which can be consumed by their passengers, or plastic or rubber detritus, which decomposes in the saltwater and ultraviolet light, pumice can float for years.
After the 1883 eruption of Krakatoa, a massive raft of pumice and logs remained afloat for two decades. “Some boats reported travelling through the pumice raft for days on end,” says Bryan, “and in some places the raft was so thick that sailors were able to get out and stand on it.”
In July of the next year, the headmistress of a missionary school in Zanzibar off Africa’s east coast, some 6000 kilometres distant, reported pumice washing ashore, and among the debris, human bones, presumably Indonesian victims of the eruption. Even 20 years after the eruption, a large raft washed ashore 1000 kilometres from Krakatoa, on the Cocos-Keeling atolls between Australia and Sri Lanka.
While it was certainly big in historical terms, geologically Krakatoa was not a huge eruption. When the Sumatran volcano Mt Toba erupted 74,000 years ago, it produced an estimated 2–3000 cubic kilometres of magma, dumping pumice into the straits around the Indonesian islands and Indian Ocean. “That amount of pumice could have easily choked up bays, forming temporary land-bridges between islands that people could have walked across,” says Bryan.
While several pumice rafts have been reported from Tongan volcanoes in recent years, the pumice raft encountered by HMNZS Canterbury is the first confirmed to be from a volcano in the Kermadec Arc, a mostly underwater chain of volcanoes that stretches almost 1400 kilometres along a nor-northeast trending line from White Island to Raoul Island. This line of mountains, most of which were discovered only in the 1990s, is the longest underwater volcanic arc on the planet—three times longer than the Southern Alps—and the most hydrothermally active. The largest of the underwater volcanic cones rises 3300 metres from the seafloor.
East of the volcanoes, the Kermadec Trench forms a 10,050-metre-deep chasm in the ocean floor, a product of the collision between the Pacific and Australian tectonic plates.
On the flanks of many of the arc’s 40 or so underwater volcanoes, diffuse hydrothermal vents leak gas-rich hot water into the surrounding sea, and black smokers jet high-pressure plumes of superheated, mineral-rich water out of the rock. The chimney-like deposits of heavy metals are beginning to attract the attention of mining companies. But for now, they’re home to some of the world’s strangest creatures. The geothermal water, which often contains toxic chemicals such as hydrogen sulphide, would kill most species, but many of the creatures here thrive in the hot, mineral-rich waters. In the dark, deep waters far from the sun’s light, the base of the food chain is not photosynthetic algae, but ‘chemosynthetic’ species that draw energy from chemicals in the hydrothermal fluids. Bizarre-looking tubeworms, tiny forests of stalked barnacles, and clumps of giant mussels and clams live with symbiotic bacteria that turn hydrogen sulphide into food. They, in turn, provide food for predatory starfish and gastropods. Other more mobile species, such as tiny orange shrimps, crabs and eelpout fish, feed on the bacteria and vent animals but steer clear of the superheated water, preferring the warm waters where hot vent fluid mixes with the cold sea.
In a tectonically and volcanically active zone, black smokers and other hydrothermal vents turn on and off as conditions change in the magma beneath them. It means the vent animals have to be adapted to move, says NIWA marine biologist Malcolm Clark. “When the venting switches off, they have got to be able to disperse and re-colonise new areas with similar conditions. The shrimp and vent eels, which are able to survive on the fringes of the venting activity, can move. But the true vent fauna are not mobile—the adults will die and the species have to disperse through their eggs and larvae.”
But the Kermadec waters contain many other species that do not depend on hydrothermal activity. The sub-tropical waters around the island group are also home to sponges and corals, small fish such as grey drummer and mau-mau, large predators such as Galapagos sharks and spotted black grouper, and migrating humpback whales and turtles.
Even if they’re not caught in the eruption plume, or bombarded by rockfalls and underwater landslides, an underwater volcanic eruption can devastate the local biological community.
Clark says that ash in the water, and pumice floating on the surface, can reduce light levels, affecting plant species’ ability to photosynthesise.
“On the seafloor, an ashfall of only one or two millimetres can smother small animals that depend on a clean rock surface to colonise and can clog the feeding mechanisms of filter feeders like corals and sponges. If their tentacles and pores get blocked, they will be unable to feed and they’ll starve.” A massive eruption could also change the temperature and chemical composition over a vast expanse of water, leaching hydrogen sulphide, iron and heavy metals into usually pristine waters. “Animals that live away from the venting are not adapted to those chemical conditions, and if they suddenly get hit by hydrogen sulphide, they’re history. But animals that live around hydrothermal vents are adapted to disperse when changes occur, so their production of eggs and larvae is often quite high to make sure they can cope with disruption to their habitat,” says Clark.
Overseas studies have shown that new vent communities can become well established within five to 10 years. “But non-vent animals, such as deep-sea corals or sponges, which are common on Kermadec seamounts, can be slow-growing and long-lived and are simply not adapted to frequent or substantial natural changes. This means it could take many decades, or even centuries, to rebuild the populations.”
Recruitment of new life in the wake of an eruption relies on benthic animals from the outer flanks of a volcanic system and larvae transported by slow-moving gyres to continue the natural cycle of life and death in the deep sea.
Back in Wellington in late August, Helen Bostock divided up her pumice samples between colleague Richard Wysoczanski and Victoria University of Wellington PhD students Melissa Rotella and Simon Barker. Chemical and X-ray analysis confirmed that the pumice had the same chemical signature as samples dredged from the flanks of Havre, a massive volcano in the Kermadec Arc, on a previous research voyage. The pumice from the raft observed on July 19 was almost certainly from an eruption of Havre.
In early November, Wysoczanski took advantage of a planned trip to the Kermadec and Colville Ridges on the research vessel Tangaroa to map the seafloor around the Havre volcano. This was to be his second encounter with a pumice raft. Back in 2006, on a voyage from Fiji to the Kermadec Islands, Wysoczanski sailed through a pumice raft, the ejecta from what he later discovered was the Home Reef volcano. “It was surreal, beautiful,” he said, “like the floating bits of pumice you use in a bathtub.”
This time, he knew what to expect. In the ocean west of Havre, the Tangaroa encountered floating streams of pumice from the July eruption, and with the help of a collection device that the crew fashioned out of an old bucket and an onion bag, they collected more samples to bring back to Wellington.
Once they arrived over the underwater summit of Havre volcano, they used the ship’s multibeam sonar technology to create a 3D image of the seafloor. A 2002 map had revealed Havre as a five-kilometre-wide caldera. The crater floor was 1500 metres deep, encircled by a 500-metre-high rim. But, says Wysoczanski, they had never seen any volcanic activity, “so we assumed it was dormant, or even extinct”.
Mapping the volcano again revealed a massive new volcanic cone, itself about the size of Rangitoto Island in the Hauraki Gulf, which now rose 240 metres above the crater rim. Inside the caldera, the sea floor was 10 metres higher than the previous sounding in 2002.
When the eruption had run out of steam, pumice and ash in the water column fell to the sea floor, filling the five kilometre-wide crater with a 10 metre-thick layer of pumice, burying the stony and black corals, sponges, squat lobsters, brittle stars and deep-sea urchins that lived there.
The Tangaroa team collected samples of waterlogged pumice from the seafloor, along with shiny dark lava and lumps of sulphur, with some pumice deposits found 100 kilometres west from the eruption vent on the Colville Ridge. Comparison of rocks from the new volcanic cone with rock from the pumice raft, show the cone is the source of the July 19 eruption.
Wysoczanski says this was a big eruption, “probably on a par with the size of the eruption of Vesuvius in 79AD that wiped out Pompeii, or the Kaharoa eruption that built much of Tarawera in 1314, which is a huge edifice”. He estimates the volcano ejected several cubic kilometres of material, in the form of ash, pumice and basalt. But even though this was a massive volcanic event, it took two weeks and one alert airline passenger to notice there had even been an eruption.
People have been able to observe the effects of terrestrial volcanoes throughout human history, but it is only in recent decades, with underwater mapping technology and deep-sea submersibles, that scientists have been able to witness the effect of underwater volcanic activity. “There’s still so much we don’t know,” says Wysoczanski. “Most of the world’s volcanoes are under the sea, and we know very little about their eruptive history.” Observation of this Havre eruption has provided confirmation that a volcano can eject pumice and ash through about 1000 metres of water. “The realisation that the eruption products can get through this much water makes us realise these volcanoes may be a lot more hazardous than we thought,” says Wysoczanski.
Should we be worried? “It’s important to find out more,” he says, “because there are a dozen known active volcanoes within 300 kilometres of New Zealand. So if any of these underwater volcanoes exploded violently, it would certainly be a risk. And if we look at the geological record, sometimes an eruption causes these volcanoes to collapse, and that can generate a tsunami. Some of these volcanoes are bigger than Ruapehu, and they’re closer to Auckland than Ruapehu, so they could be a very real threat.”