ROTORUA, New Zealand—If you head east from my parents' home in New Zealand, you'll travel through rolling hills for a while. Then, as you crest a rather unremarkable climb, an unexpectedly spectacular view opens up before you. Mokoia Island is small, bushy, and brooding, and it sits at the center of a wide blue lake in what appears to be a large valley.

But that's no valley. From the distant view of that crest, the only obvious clue lies in a large hill, grandiosely named Mount Ngongotaha, off to one side. It is not attached to the valley walls and stands alone, a land-locked cousin to Mokoia Island.

The view is from the collapsed wall of the caldera of the Rotorua Volcano. Mokoia and Ngongotaha are the remnants of eruptions that are slowly re-filling the huge volume of rock that was blasted out of Rotorua in the distant past. This process is called caldera-repairing. The town of Rotorua sits right inside the caldera and is surrounded by evidence of the energy stored just beneath the surface. The town abounds with hot springs, boiling mud, and, yes, the sulfurous farts of the gods.

Rotorua is not alone. An hour's drive away, the town of Taupo sits on the edge of an even more spectacular caldera. Its largest eruption put down a 200m thick layer of ignimbrite—a frozen foam of volcanic ash, rock, and gas—over much of the central north island. A thousand kilometers farther away, the ash was still 18cm thick. Auckland, New Zealand's largest city, sits on top of no fewer than eight volcanic vents, all powered by the same churning pot of molten rock. Go south and you'll find a string of volcanoes and vents: Tongariro, Ngauruhoe, Ruapehu, and Taranaki. All are home to popular recreational spots, farmers, villages, Iwi, and city-folk alike. And, in the case of Taranaki, some areas are even covered in cows.

Taupo, despite its massive caldera, actually has a history of much smaller caldera-repairing eruptions.

"It's had 28 eruptions," Brad Scott, a volcanologist from a New Zealand research institute called GNS Science, told me as he pointed to a chart. "Of which these 19 here—we could have sat in a cafe on the Taupo lakefront and watched." For the eruption that formed the caldera, however, he said, "a cafe in Sydney would have been more appropriate."

Approximately half of the population of New Zealand lives in the shadow of one or more volcanoes, built by the conjunction of the Pacific and Australian plates. And that makes understanding volcanoes and knowing how to live with them kind of important. Volcanoes tend to do what they've done in the past. So, by studying previous eruptions—what sort of rock, the chemistry of the ash, the size of the eruption, and other details—you can come up with a pretty good indication of what the volcano will do in the future.

The unfortunate part, of course, is that this does not tell you when and where a volcano will erupt. At present, there is no way to know. But with a good enough understanding, you can determine when a volcano is getting restless and therefore may blow.

Education Images/UIG via Getty Images

Education Images/UIG via Getty Images

When should you leave town?

Volcano monitoring generally depends on detecting seismic activity. The general idea is that, as molten rock moves toward the surface, it has to fracture and shift the rock above it, creating a series of small earthquakes. A solitary earthquake doesn't tell you much because the molten rock is always moving around. Instead, scientists look for patterns in the movement. If there are a string of earthquakes that are sequentially closer to the surface or systematically change location, "that's when we start to get concerned," understates Scott.

This monitoring requires a network of seismometers. Earthquakes generate two waves that travel at different speeds. So each seismometer can provide an estimate of the distance to the epicenter by measuring the time difference between the fast-moving p-wave (pressure wave) and the slower s-wave (shear wave). Two seismometers at two different locations provide two distances, creating a circle of possible epicenters. Add a third seismometer, and a unique location is obtained.

Our global network of seismographs is dense enough that more than three seismometers usually record most earthquakes, providing even better precision. Alongside seismic activity, geologists use GPS to monitor the motion of the landscape and look for drift, swelling, and rapid uplift. These latter are used to corroborate long-term trends in magma motion, while earthquakes are about the here and now.

But detecting seismic activity is only the start. After the 2011 Christchurch earthquake, the New Zealand government decided that its scientists needed to provide information much faster than they had previously. GNS Science is now required to provide an initial report on seismic events within five minutes (down from 20 minutes pre-Christchurch). To provide this speedy response, a computer algorithm continuously gathers data from a network of seismographs, examining the frequency and amplitude of the continuous shaking each device experiences.

Software extracts the regular rhythm of traffic noise from nearby cities. It removes foot stamping and cheering from the local stadium (Scott tells me that you can tell how well the All Blacks are doing from the raw seismograph data for some stadiums). In the case of Okataina and Taupo, nearby geothermal fields also provide an overlapping background of seismic activity, which must be taken into account. At the end of all that subtraction, the seismic data can be used to understand the processes from regional, geothermal, and potentially volcanic activity.

Within three minutes, that preliminary report is publicly available (there's an app for that, and it's fascinating to check out). For earthquakes above a certain threshold, the system notifies the nation's on-call geologist, who has less than 20 minutes to get to their desk, analyze the data, and update and confirm the report. Scott shows me his schedule, which has him on duty one week in every month.

(I had never considered the possibility of having an on-call geologist. I'm envious.)

Kill the skiers

While that rapid reporting helps with the response to earthquakes, seismic activity alone doesn't really allow volcanic eruptions to be accurately predicted. Scott recounted the 2012 Tongariro eruption: "We start getting seismicity. This is different, [normally] we get five or 10 earthquakes a year. We're not getting five or 10 in a week... So, at Tongariro, we went to level 1. And we started engaging with the community, saying 'Hey, this volcano is waking up. We haven't seen this before, so now it's an active volcano.' The community started doing some preparative stuff: setting up phone trees for elderly people, stuff like that... But then, at night on the 6th of August, it let rip. None of us saw that coming."

Level 1 refers to a volcanic activity scale: 0 is sleeping, and all but two of New Zealand's volcanoes are currently at a 0. Level 1 indicates unrest—earthquakes. Level 2 means you should be worried, but nothing is erupting yet. Levels 3-5 are all about the volcano erupting and the scale of the eruption. All of this is scaled to the size of the volcano, though. So interpreting the activity level means you need to know about your local volcano.

Luckily, Tongariro (like Ruapehu) is a relatively small volcano in terms of how many rocks it throws and how far it throws them. Ruapehu, on the other hand, presents a different danger. Its eruptions are often accompanied by lahars—fast-moving, hot volcanic ash and mud that flows down the mountain, burying and baking everything in their path. The ski field is unlikely to get hit by any flying rocks, but the skiers are in real danger.

Ruapehu has an automated warning system that uses air-pressure sensors to identify any explosion that accompanies an eruption. This signal is combined with seismic data to determine that an eruption has occurred and there is, very likely, a lahar on the way. The system provides skiers at the top of the field 90 seconds to get out of the valleys and to the top of a ridge (you have a bit longer if you are farther down the slopes). All the infrastructure runs along ridge lines, reducing its vulnerability to lahars.

The ski field runs two lahar exercises every year: one planned and one blind. "Every year, we kill about 15 percent of the skiers in these exercises," says Scott ruefully. "And we know the valleys where this is most likely to happen. And we just sit there and watch the people, and—the ones that do something silly—you ski over to them and ask them to complete a questionnaire. And we’ve found absolutely no systematics: they're locals, they're overseas visitors, they're young, they're old. There's just no common factor. And we've done it for about 15 years now. And we just cannot nail it. We just persistently kill about 10 or 15 percent [of skiers] during each test."