Credit: White Island Tours

I'M STANDING ON the crater rim of White Island, New Zealand's most active volcano. Clouds of sulphurous steam veil the marbled green waters of the lake below. The entire area hisses and boils like a witch's cauldron. With an acidity of about pH 0.2, the crater lake is one of the most acidic environments on Earth. Yet even here primitive algae and bacteria thrive.

I've visited New Zealand twice before and been blown away by the scenery, featured in films from Lord of the Rings to The Piano. But this trip, I'm on a mission to some of the geological hot spots, which have produced this remarkable terrain. New Zealand is a Mecca for geologists. Processes that elsewhere on the planet have been frozen by the unrelenting aeons are still in action here.

It is one of the few places on Earth where a plate boundary - the line demarking the collision of two massive tectonic plates - is accessible on land, rather than buried out to sea. The colliding plates meet in a subduction zone, where the thinner oceanic crust of the Pacific Plate is pushed underneath the Indo-Australian Plate.

As the Pacific Plate dives into the mantle, minerals under intense heat and pressure release water, which permeates up into rocks of the overlying plate, causing these to melt. This melted rock forms subterranean magma chambers in the crust, instigating volcanism on a widespread and spectacular scale.

"We're surrounded by volcanoes in New Zealand and it's exciting to work in an area that is so active - it's one of the reasons I became a volcanologist," says Jan Lindsay from the Institute of Earth Science and Engineering at the University of Auckland. "The area north of Auckland is peppered with old volcanoes. There's also Little Barrier Island in the Hauraki Gulf, and the Coromandel Peninsula is formed by old volcanoes too."

The most recent, and most massive, eruptions in New Zealand have all been found in what volcanologists call the North Island's 'Taupo Volcanic Zone' (TVZ), a 250-kilometre-long frontier of intense volcanism, stretching from the central Taupo region north to White Island, 48 km out from Whakatane in the Bay of Plenty.

White Island and Mt Ruapehu are the two most active volcanoes, venting ash, gas and occasionally lava. Ruapehu is found on the southern side of Lake Taupo. It erupted as recently as September 2007, accompanied by a 2.9 magnitude earthquake, and even more spectacularly between 1995 and '96.

White Island is a stratovolcano, a cone-like volcano, mostly submerged by the ocean and created about 150,000 years ago as magma formed by the subducting Pacific Plate punched through the seafloor. It has been erupting gas, steam, lava and rock fragments since at least 1826, when the first Europeans set foot on the island.

Getting out to the island summit of the marine volcano isn't always easy - access by boat is restricted in bad sea conditions, and on the night I arrive in Whakatane, on the mainland, the wind picks up and the swell is rising. I'm told the skipper will make the call about leaving in the morning.

Nervous, but packed and ready, I discover at 8:00 am that the trip is off. To bide my time while hoping the weather will improve, I drive 100 km southwest to Rotorua, New Zealand's most famous geothermal region, and do what every tourist here does.

A light mist of rain plays on my face as I steam my skin in a thermal spa. I lazily move through the three pools, from nicely warm, through nicely warmer to satisfyingly scalding. The soak may cost money, but the energy heating the water is free: all of the heat that permeates the area originates in the TVZ's geological activity.

On the way back to Whakatane, I drive past the sparkling waters of Rotorua's lakes. Their circular shape gives them away for what they are — scars of prehistoric volcanism on a shattering scale. Periodically in the past 1.6 million years, so-called supervolcanoes in the TVZ have created these calderas, remnants of eruptions so large they blasted their volcano to pieces.

In 2007, a paper in the Geological Society of America Bulletin suggested that these large eruptions are triggered by sequences of smaller eruptions within the chamber complex, which occur in cycles every 1,000 to 3,000 years. And by large, we're talking vast - somewhere between 300 and 500 km3 of magma is produced in a supervolcano eruption.

Catastrophic eruptions such as these are the classic "high consequence, low probability event", says volcanologist Brad Scott with New Zealand's Institute of Geological and Nuclear Sciences in the city of Taupo. "The return period is about 100,000 years," he says, making the chance of it happening on our watch unlikely.

The last big eruption here - in 1886 in Tarawera, 24 km southeast of Rotorua - led to the loss of 108 lives. But as eruptions go, that was well short of caldera forming, Scott points out. For an eruption on that sort of scale, you have to go back about 22,500 years.

Scott's job is keeping an eye on the activity of the North Island's volcanoes. To do so, he and other members of New Zealand's 'GeoNet' volcano hazard monitoring network use a bunch of high tech toys to measure ground shaking, deformation and changes in the composition of steam and lava. "For volcanoes, we are getting to the point where we can say, 'OK, things are changing'," he says. "But we can't yet do what society expects, and say, 'it's going to erupt at two o'clock this morning'."

OVERNIGHT, THE WEATHER improves, and at 7:45 am next morning I get the call; the skies are blue, the wind has dropped, and the trip to White Island is on.

Despite the calmer conditions, the boat bucks like a rollercoaster cart, and 10 minutes out from the volcano I'm baptised with sea spray as the crew issues us with hard hats and gas masks. I quickly learn the mask isn't just for show: sulphurous steam jets from vents in the crater walls, and unlike Rotorua, which smells like a rotting swamp, the volcanic steam is acrid, and difficult to describe, but maybe something like a Coke factory on fire.

Along with a group of about 10 others, we pile off the larger boat that has brought us here and into an inflatable dinghy that takes us the final stretch to a tiny, rusty quay. Once off the quay, I'm transported to a land unlike anything I've seen before. The island is roughly circular and about two kilometres across, rising up in a mountainous peak with a small crater lake in the centre - the base of the volcano, hidden from our view, is found hundreds of metres below the waves.

Eruptions have gouged out scars on the 321-metre-high, yellow and grey cliffs. The lake itself is the site of periodic eruptions from 1975 to 2000 that deposited 12-metres-worth of fresh material on the lake floor.

The ground is warm to touch, and periodically the sulphur-tainted steam catches in my eyes and throat. Milky, acidic streams host bright green algae and create wide stains out to sea where they meet the ocean. On the banks of the streams, crystals of sulphur catch the sunlight and tempt me to move in with my camera — until a puff of corrosive steam sends me scuttling backwards.

Coming here is a little like playing truant from school. It's exciting, but you get the feeling you should be somewhere else. Scott has been to the island well over 400 times, and he agrees it can be unsettling. "There are times I've felt very uncomfortable," he tells me after my visit.

Dotted about the volcano floor are stout wooden pegs - some of Scott's "toys". Any change in the height of the pegs indicates ground swell from rising magma under the surface. Activity is also monitored by a seismometer high up on the cliff. About eight times a year, scientists check the venting steam for changes in temperature and levels of dissolved gases. Increasing sulphur dioxide indicates the pressure is dropping and magma is on its way up.

"White Island provides an excellent chance to test monitoring techniques, as the volcano is frequently active and you can get a result for an experiment in a couple of years," says Scott. "At a less active volcano you may wait 10 to 20 years."

Public safety is also a consideration. The most recent eruption here was in July 2000. "We were really fortunate it occurred during the night. If it happened during the day when a bunch of tourists were there, they probably would have been injured or killed," he says.

The crater lake is another good indicator of the volcano's condition. Colour changes are closely monitored. "If that lake turns pink, you won't see me here," says Kris Dale, my guide from White Island Tours, a stocky, 27-year-old ex-teacher.

While I'm wearing several jackets (it's Autumn during my visit) and a gas mask, Dale seems quite comfortable in just a T-shirt and shorts. He points to a small pile of broken volcanic debris called tephra, which we should shelter behind if the volcano erupts. If that were to happen, our boat would head out to sea to avoid any potential tsunamis, he says - which doesn't do much to reassure me.

White Island was mined for sulphur in the early 20th century and we finish our trip by climbing through the ruins of the factory. In 1914, 12 factory workers were killed in a debris flow of ash and hot rocks when the crater wall collapsed. Beside these grim remains, a few hardy shrubs called ice plants are some of the island's only plant life, although around the corner, pohutukawa trees grow, and colonies of young gannets stretch their wings before their maiden flight across the Tasman Sea.

Back on the boat, I chat with skipper Paul Kingi, a Maori from the Tainui and Takitimu tribes. Kingi's built on an impressive scale; he offers me his seat in the cabin and my feet dangle above the floor. He tells me that, traditionally, the Maoris believe that volcanoes can be male or female. The Maori name for White Island is Whakaari, which means "that which can be made visible" - possibly a reference to the shifting veils of steam that periodically envelop the volcano.

The Maori legend that details the creation of the island tells of two sisters, Whakaari and Moutohora (the Maori name for Whale Island). Whakaari prepared fire while her sister looked for food. But when Moutohora returned, Whakaari was gone. In a rage, Moutohora searched for her sister. She found her, but Whakaari swerved violently eastwards and made a huge jump. As she did, she dropped rocks at Whakatane's harbour entrance. The leap carried her out to sea where she sits today.

The three pinnacles of rock beside White Island are known as Te Paepae Aotea (the Volkner Rocks) "the gateway to Aotea", the early Maori name for New Zealand (also known as Aotearoa). The pinnacles were one of the first things that voyagers in the sacred canoe Mataatua saw as they travelled from Hawaiki, the legendary homeland of many Polynesian cultures, now thought to be the Society Islands of Tahiti.

"Two brothers from Hawaiki, Taukata and Hoaki, came to New Zealand in search of their sister and landed at Whakatane. Descendants of Toi-te-hautahi, an earlier traveller who settled in Whakatane in 1125, invited the visitors to a feast," Kingi tells me.

Unfortunately the two brothers didn't enjoy the food that was on offer; so instead, they introduced Toi's descendants to the kumara, or sweet potato. The chief of Toi's descendents, Tama Ki Hikurangi, made the decision to build a waka (a canoe), and with the help of Hoaki they sailed back to Hawaiki, partly in order to bring back more kumara. From that return trip to Hawaiki, the story goes, the great migration of the Maori to New Zealand began.

Many Maori legends involve tales of volcanoes. The brothers were descendants of Maui, the great voyager and creator or discoverer of New Zealand. Another legend relating to White Island or Whakaari tells how the volcano arose from the deep after Maui first touched fire.

Maui picked up the fire in both hands, but was so tormented by the pain that he dived under water to relieve his agony. Whakaari rose from the water in the place where Maui shook fire from himself.

"A lot of people think that these are myths — they're not," says Kingi. "There was such a person as Maui. He went back to his people and said 'Hey, look at the big chief I am, I have fished this land from the sea.'"

Leaving the TVZ, and the Pacific Plate boundary that creates it, I drive 300 km northwest to a very different volcanic landscape — that of Auckland, New Zealand's largest city.

Volcanism in the Auckland region differs from the Taupo Volcanic Zone because here, instead of being generated from melting crust they are found over a hot spot fed directly from material in the Earth's mantle. The mantle is a thick, partly melted goo on which the crust sits like the burnt coating on a toasted marshmallow. The lava sourced from the mantle is basalt, it's less viscous than the silica-rich lavas produced by TVZ volcanoes, and tends to form shallow-sided, 'shield-style' volcanoes like those of Hawaii.

One such shield volcano is the island of Rangitoto, eight kilometres off Auckland's coast, where I walk across lava fields just 600 years old. Rangitoto is the latest and largest eruption of the Auckland Volcanic Field, a dormant network of around 50 volcanoes that form the hills and headlands on which the city is built.

As I climb Rangitoto's 260-metre-high summit, solidified lava scrunches underfoot. The island is a biological conundrum: mangroves grow directly on the lava, alpine mosses grow at sea level, unique and bizarre hybrids thrive.

Despite the island's youth, massive pohutukawa trees loom near the summit, another example of how quickly life can penetrate newly formed land. "Plants may take a while to get established [on volcanoes], but once established they soon grow," says Scott. Its only eight years since the last eruption at White Island, and plants are well established in the crater, he points out.

Peering closer, I notice some of the adaptations life has made here. Unable to get water from soil, the fragile mosses are thick with hairs that reduce moisture loss. Ferns curl their leaves protectively to prevent evaporation in the harsh dry climate.

Although Rangitoto itself is unlikely to erupt again, the hot spot in the Earth's mantle is the city's ticking time bomb. Predicting just when and where the next volcanic belch will occur is problematic; what's certain is that there will be another eruption, and that it will strike somewhere entirely new.

"The Auckland Volcanic Field (AVF) is the meeting point for a bevy of reawakening volcanoes," says Scott. "They are just sitting there resting, waiting to wake up." This makes raising awareness of the risk of living with this sort of volcanism harder to communicate, Scott says. "What's worrying with reawakening volcanoes is the juxtaposition of society and volcano. If you stand back and throw a dart at a map and say 'that's where the volcano is going to be', you're going to have around 200,000 people who are going to be affected within a five kilometre radius."

Volcanologist Graham Leonard from the Institute of Geological and Nuclear Sciences in Lower Hutt, Wellington, is one of the experts on the Auckland Field. According to him, pinpointing the next eruption in Auckland is like "shooting at the surface from 80 km down … chances are it's going to hit somewhere different".

I ask Leonard which type of volcano is the bigger risk to society. "That's a good question, and it depends what you mean by risk," he says. "In Taupo, the risk of an eruption is low — about one in 1,000 in any given year. At Ruapehu, the chance of an eruption is about one in 10 in any year, but the vulnerability is much smaller.

"Eruptions happen in Auckland every few thousand years, but the last - 600 years ago - was very different in terms of the chemistry and the volume of lava, and that probably elevates the risk [that another will follow]. Because the impact would be so huge across the city, we put a lot of effort into planning for that."

From the summit of Rangitoto, I spy other volcanic islands dotted across the Hauraki Gulf. Their sharp, angular forms reveal their fiery genesis. One of these, Motutapu, is one of several sites dotted around Auckland equipped with seismometers, forming part of the Auckland Volcano-Seismic Monitoring Network. Over the past 30 years this network has checked for the warning murmurs that indicate fresh volcanic activity.

In March 2008 New Zealand's civil defence agencies ran Exercise Ruaumoko, a simulated national emergency based around a volcanic eruption in Auckland. Public education based on the program is underway.

"One of the key vulnerabilities associated with a future AVF eruption is the expected short warning period days rather than weeks," says Lindsay. It would take between one to 19 days (from 50 km depth), or between two and 38 days (from 100 km depth) for rising magma from the hot spot to reach the surface, she points out. "We're not going to feel any earthquakes until the rising magma hits 30 km deep."

Descending from the volcano, I wash off the sweat of the trek in the cool waters surrounding Rangitoto's young slopes. I take in the blue skies, the black basalt and the milky-white, sediment-rich waters. Living with risk, I decide, comes with perks, too.