Under the volcano
By Garry Booth
The deadly eruption of New Zealand’s White Island volcano at the end of last year, and the Philippines’ Taal volcano this month, are explosive reminders of how quickly a normal day can turn into a headline catastrophe.
White Island, also known as Whakaari, erupted on 9 December when dozens of tourists were on the island. The official number of fatalities is put at 17, with two missing presumed dead. Many more are in intensive care with severe burns.
White Island is a remote spot and although no volcano has impacted severely on a major population centre in modern times, it isn’t hard to imagine the human and economic toll that will ensue when it does inevitably happen.
Residents of the Philippine capital Manila were put on notice of a hazardous eruption on 12 January after Taal emitted a huge plume of ash, triggering the mass evacuation of 8,000 people from the immediate area. A weak flow of lava then began seeping out of the volcano, located some 70km (45 miles) south of the city, prompting the alert level to be raised from 3 to 4, out of a maximum of 5.
A number of big cities live under the threat of a major eruption, according to experts.
Over the next 100-200 years, about half of the earth’s volcanoes in eruption are expected to have reawakened after several generations at rest, according to Willis Research Network member, and Director of the UCL Hazard Centre, Dr Christopher Kilburn.
Moreover, about one percent of the world’s population lives in 15 cities within range of an active volcano. “Populations near these volcanoes are rarely aware of how much their livelihoods are exposed to a natural hazard; similarly, international bodies rarely appreciate the exposure of their activities in such districts,” Dr Kilburn says. “These cities include major hubs for international trade. Volcanic unrest is thus a direct threat not only to the urban population, but also to the stable flow of international business.”
Naples: top of the list
Bill McGuire, Professor Emeritus of Geophysical & Climate Hazards at UCL, puts the sprawling city of Naples in Italy at the top of his list.
“To the east is Vesuvius, which hosts more than half a million people in the most dangerous zona rossa (red zone). Vesuvius has not erupted since 1944, so another eruption at any time would not be a surprise,” McGuire explains. “On the southern edge of the city is the giant caldera known as Campi Flegrei, which is active and recently restless. The caldera is located in an area of high population density, so that even a small eruption would be very damaging and costly.”
Robert Muir-Wood, chief research officer at RMS, also identifies Naples as the area most at risk but adds the 600,000 population city of Kagoshima in Japan, which is located right on the edge of a volcanic crater and directly opposite the Sakurajima volcano.
Muir-Wood also highlights the U.S. west coast city Seattle which is situated 100km from the 4,400m volcano Rainier. The mountain has almost three cubic kilometres of ice sitting on top of it which will melt creating enormous floods in any significant eruption, he predicts.
The major perils associated with volcanoes depend on what kind of volcano it is. “On Hawaii the magma is very hot, and does not erupt explosively, but pours out of fissures. Where the magma is cooler and more viscous, eruptions tend to be explosive,” Muir-Wood says.
“For a city located tens of kilometres from a volcano the chief hazard is windblown ash fall, which can lead to roof collapse and rusting of machinery, for example. Closer in, the main danger is pyroclastic flows: avalanches of red-hot ash, caused when an eruption column blasted up into the air collapses and then flows over the terrain.”
Pyroclastic flows can ride uphill and extend for 20km or even further from the volcano and they typically kill anyone in their path. Alternatively, tsunami hazards can exist when a volcano collapses into a lake or the sea.
Bill McGuire agrees volcanic ash is likely to be the main hazard from an erupting volcano, along with lava flows and pyroclastic flows. “In addition to ashfall from Guagua Pichincha, parts of the Ecuadorean city of Quito are also under threat from volcanic mudflows (lahars) from Cotopaxi. The parts of Peru’s economic hub Arequipa closest to the El Misti volcano are at risk from lahars and possibly also lava flows,” McGuire says.
Predicting and planning
As with earthquakes, a big problem with volcanoes is that they develop very quickly and frequently take people by surprise.
Large eruptions tend to be better forecasted because geochemical signatures and seismic monitoring warn that a new pulse of magma is rising under the volcano. But smaller more local steam and ash eruptions are harder to anticipate.
“Often, the rates of increase of earthquakes and ground swelling can be used to estimate the start of an eruption quite accurately. We are still not, however, very good at predicting other critical parameters such as eruption size, duration and climax,” McGuire says. “In addition, most of the world’s 1,500 active or potentially active volcanoes remain unmonitored or poorly monitored at best.”
Long term forecasting for active volcanoes is even more problematic. Like earthquakes, volcanoes do demonstrate characteristic return periods, which can give some guide to when they might next become active. But with long dormant volcanoes (thousands to tens of thousands of years), the timing of a return to activity is impossible to guess at.
Eruptions cannot be prevented, so mitigation strategies rely on reducing their impact, says Dr Kilburn: “Feasible options for large urban centres are to prepare robust evacuation procedures and to avoid major development in exposed areas. In the case of ash fall, buildings can be designed to inhibit the accumulation of ash on their roofs (similar to well-established strategies against the accumulation of snow).”
Years without a summer
A big volcanic eruption can have wide ranging consequences, affecting cities around the world. The ash cloud from Iceland’s Eyjafjallajokull explosion seriously disrupted air traffic over Europe in 2010, for example. But Eyjafjallajokull was a relatively modest event in the Geological Record.
The 1815 eruption of Tambora in Indonesia severely affected weather across the Northern hemisphere and resulted in 1816’s “Year Without a Summer”, devastating harvests. It’s thought that a volcanic super-eruption, such as that which occurred at Toba (Sumatra) 74,000 years ago, almost wiped out the ancestors of homo sapiens.
A similar event today would be just as catastrophic, Dr Kilburn says, even putting climate change into reverse gear. “Ash and volcanic gas forced into the upper atmosphere would significantly block incoming sunlight and cause average world temperatures to drop several degrees Celsius for many years. Global crop failure would result and, in the worst case, the polar ice caps may grow sufficiently for cooling to continue after the ash and gas have dissipated, producing conditions that might herald a new Ice Age,” he says. “Although not expected to occur soon, a super-eruption is virtually inevitable at some time in the future.”