Overlooking disaster

Volcanic disasters have been few and far between in the news over the past decades, and, as a result, have slipped beneath many (re)insurers' radar. Unfortunately, there is no reason to suppose that state of affairs will continue.

There are around 1511 potentially active volcanoes all over the world, with an average of between 20 and 30 erupting with varying intensities every year. Some are in out-of-the-way places such as Siberia. Others are in more populated areas. More are biding their time closer to major population centres.

Volcanoes can be found on or close to both constructive and destructive plate boundaries. The former are where new land is being created as tectonic plates push apart, such as the mid-Atlantic ridge, which runs through the middle of the Atlantic and which has given birth to Iceland. Destructive plate boundaries are better known as subduction zones. In these areas, one plate is being forced down under another, as the sea floor is pressed into the interior of the earth. As the rocks heat up miles below the surface, they melt and bubble upwards as they are lighter than the surrounding rock. As this molten rock (or magma) rises, it seeks a weak spot to allow it to escape - and a volcano is born. The presence of a number of these subduction faults around the rim of the Pacific has given rise to a long chain of volcanoes that forms the so-called 'ring of fire' (above right) that runs from Antarctica up to South America, along the eastern coast of the US and Canada, through Russia and Japan, into Indonesia and then around to New Zealand and back down to Antarctica.

Europe has a far smaller number of volcanoes and these are mostly confined to southern Italy and several Greek islands. However, there are also a number of dormant volcanoes in the Massif Centrale of southern France, in the Eifel region of Belgium and Germany and in Turkey. Although these have not erupted in many thousands of years, this does not necessarily mean that they are extinct.

The uncertainty that exists when trying to assess whether a volcano is dormant or extinct is a very real problem. Most people are accustomed to thinking of time in terms of a few years or decades. However, volcanoes tend to have a lifetime that consists of thousands and sometimes even millions of years, with wide gaps between eruptions that can lull people who settle nearby into a false sense of security. As a result, there have been a large number of settlements and even cities being built close to mountains that later turned out to be volcanoes.

The most famous example of this kind of catastrophic mistake occurred in AD 79, when the people living around Vesuvius suddenly realised the truth about the mountain when the volcano erupted with massive violence for the first time in centuries. When the eruption was over a few days later, the cities of Pompeii, Herculaneum and Stabiae had been buried in ash and an estimated minimum of three to four thousand people had been killed, both from ash fall and pyroclastic flows.

One of the biggest problems of living around a volcano is that it can have a very long reach. The popular image of the main threat from a volcano is lava bursting out from the sides of the mountain and running down the nearest slope. In fact, apart from in Hollywood movies, very few people have historically ever been killed directly by lava flows as they tend to be slow moving and therefore easy to avoid.

Multiple risks

Unfortunately, volcanic dangers come in many forms, many of which are just as deadly as lava and have a longer range. When a volcano erupts, it often sends an eruption column of ash and superheated gases into the air. If this column weakens then it can lose power and collapse, forming a pyroclastic flow, a cloud of very hot gas, ash and dust that flows down the sides of a volcano at speeds of up to 100km/hour. Such flows are fatal for anyone they catch and are almost impossible to avoid.

The AD 79 eruption of Vesuvius contained several pyroclastic flows, but the phenomenon was not well known until it was observed in 1902, when a flow was emitted during an eruption of Mont Pelee on the island of Martinique.

The flow rolled down the mountain and engulfed the city of St Pierre, killing 29,000 people in a matter of minutes. A total of two people survived the event.

Another volcanic threat again comes from the ash that is thrown up by a volcano into the atmosphere. This can be spread over a massive area, depending on the strength of the eruption. When Mount St Helens erupted in Washington State in 1980, it sent ash over a huge swath of the western and central US states. As well as causing damage to cars and crops all over the area, later resulting in insured losses of $27m, it also formed a serious hazard to aviation. Volcanic ash can melt inside jet engines, coating the fan blades in volcanic glass and causing the engine to choke and stop. So far, there has not been a case of a major airliner crashing as the result of passing through an ash cloud, but there have been some very narrow escapes. One such case occurred in 1982 when a British Airways Boeing 747 flew into an eruption cloud over Java which caused all four engines to fail. The plane rapidly descended from 11,000 metres to just 2,000 metres, until the crew was able to restart the engines and regain full control.

Ash in the upper atmosphere can cause a different type of damage as well, by partially blocking out the sun and acting as an atmospheric pollutant.

The eruption of the Tambora volcano in Indonesia in 1815 vented so much ash into the sky that the following year became known as 'the year without a summer'. Harvests failed all over the northern hemisphere due to the reduced global temperature and an estimated 92,000 people starved to death in Indonesia alone.

Falling ash close to the volcano forms another threat. Volcanoes also emit water vapour during an eruption and this can result in rain falling around the mountain. As this mixes in with the ash and loose debris such as blasted trees and earth, this can form a lahar: a torrent of hot mud that pours off the volcano and spreads out, following the force of gravity.

Lahars can be immensely destructive. In 1985, the Nevado del Ruiz volcano in Columbia erupted, sending a series of pyroclastic flows across the snow and ice that had built up at its peak in the centuries since it had last been active. As these melted, a series of lahars were generated which poured off the mountain and into the valleys close to it. Tragically, these lahars then went on to almost totally destroy the town of Armero, which was situated 50 kilometres from the mountain, killing more than 23,000 people in the space of a few hours.

One of the biggest potential volcanic killers however, comes from volcanoes that are close to the sea. When a volcano's magma chamber empties itself in an eruption, then the entire structure can be weakened to the point of collapse. When this happens to an island or coastal volcano, then the effects can be catastrophic. The volcano can explode from the forces generated by cold water encountering hot magma and collapse in on itself, forming a caldera. As the sea rushes in to fill the void it then rebounds to form a tidal wave or tsunami. The famous eruption of Krakatau in Indonesia in 1883 sparked off a series of tsunamis that killed at least 36,000 people and devastated large parts of Indonesia.

With such a wide range of possible risks from volcanoes, this begs the question of why people still live anywhere near them. There are a number of reasons for this, including, as mentioned earlier, the fact that often a mountain isn't identified as being a volcano until long after a settlement has been built close to it. Even then, there are benefits to volcanoes.

Volcanic ash eventually breaks down in the ground to form very rich soil that can benefit agriculture. In Japan and Iceland, technology has been developed to harness geothermal energy from volcanic fields to generate electricity and heat entire cities.

And because centuries can pass between eruptions, entire cities have grown up close to volcanoes. Despite the fact that the danger from Vesuvius is well known, an estimated 3.75m people live within 30 kilometres of the summit, many in the city of Naples. The volcano last erupted in 1944 and is overdue for another bout of activity.

In New Zealand, the city of Auckland sits in an area dotted by 50 volcanoes, one of which last erupted 800 years ago. Although these volcanoes are small cinder cones, which tend to erupt once and then become extinct, the field itself is still active - and will erupt again in the future, adding to the number of cinder cones. Almost 30% of New Zealand's population lives in and around the city.

In Mexico, at least 40,000 people live in the shadow of the volcano Popocatepetl, which lies just 40 miles from Mexico City, one of the largest urban centres in the world with a population of over 20m.

The US is also at risk from numerous volcanic dangers. The Cascades range in the western of the country contains 11 potentially active volcanoes.

Mount St Helens rumbled back into life in 2004 and concerns are growing about the likelihood of a volcanic eruption in the Three Sisters area of Oregon, which contains three dormant volcanoes. In Washington State, Mount Rainier has been described as being the most dangerous volcano in America, due to its history of regularly producing catastrophic lahars.

The mountain, which contains the largest snow cap on any US mainland volcano, is just 60 miles away from the cities of Seattle and Tacoma. Parts of the latter are actually built on debris from previous lahars.

Warning signs

With such large populations at risk, warning systems are a vital part of any local infrastructure, coupled with detailed plans for evacuating the communities around a volcano. Although long-term predictions of volcanic eruptions are not possible due to the inability of scientists to see so many miles beneath the surface of the earth, short-term predictions have been more successful. As the magma moves upwards to the various magma chambers beneath a volcano, the pressure it exerts sets off a series of earthquakes in the rocks above and around it. These earthquakes, coupled with an increase in the amount of gases that a volcano produces, can be an important sign of an impending eruption.

The danger comes in being able to confirm that an eruption is imminent, as the wholesale evacuation of an entire town or city is an immense operation that have massive economic repercussions. In 1982, an increase in activity at the Long Valley caldera in California prompted the United States Geological Survey to issue a notice of potential volcanic hazard. However, no eruption occurred and many of the local ski resorts near Long Valley threatened to sue for loss of trade.

On the other hand, there have been many cases where warnings were given and eruptions did occur. In 1991, the Pinatubo volcano in the Philippines erupted for the first time in centuries. More than 200,000 people were evacuated from the area following a steady stream of warnings from scientists.

The 359 people who died during the eruption were mainly made up of those who chose to refuse to be evacuated.

The consequence of ignoring warnings can be terrible. Nevado del Ruiz first showed signs of life the previous year, but the initial eruption was small-scale enough that the local authorities chose to take little or no action. When the mountain finally erupted and generated the lethal lahars that buried Armero, no emergency evacuation plans were in place.

This was despite the fact that the town had been built on the site of other towns that had also been destroyed by lahars in 1595 and 1845.

Inability to learn from past events can the biggest volcanic risk of all.