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There are many hazards associated with volcanism. This article attempts to give information
on some of the more prominent hazards, or the more catastrophic ones.
It is illustrated throughout with case-study materials.

ASHFALLS
When a person thinks of a volcanic eruption, they always think of a huge cloud of ash sweeping high
into the air. In reality, this ashfall is a relatively minor hazard. It rarely claims lives.

Etna, July 2001. Note the ash-plume.

The most explosive eruptions send out great clouds of ash in enormous so-called 'plumes'.
In fact, scientists use the height of the ash plume to calculate the explosivity of an eruption.
A minor plume, less than 100m in height, is common for a Hawai'ian volcano, while when a
plume exceeds 25 km, the eruption is far more explosive. Examples of these more explosive
eruptions include Mount St. Helens in 1981, Krakatau in 1883, Tambora in 1815, and an
ancient eruption of Yellowstone caldera 2 million years ago. The following brief table is known
as the volcanic explosivity index. It is used by volcanologists to calculate an eruption's force,
as well as its type. These are placed in order of increasing explosivity.

Thus the height of an ash plume is an excellent indication of the power of an eruption. But there are individual hazards based around the ash also. The Mount St. Helens' plume extended over 20,000m into the air, while the eruption of 1956 Bezymianny, in Russia's Kamchatka peninsula, generated a plume 45,000m in height.

The hazard first became evident at Galunggung in 1985. It was found that the ash interfered with the functioning of aircraft, and chaos was barely prevented. The hard and angular particles of ash abraded windshields; fine particles, deposited inside the plane's engines, reacted with water to produce a corrosive acid. It has since been found that it is possible for ash to actually melt inside the engines, creating a sticky fluid that stalls them.

This map illustrates air encounters with the Pinatubo ash-cloud.
Each star illustrates a point of encounter, with the number of events marked nest to it.

In the North Pacific, there is a problem with ash plumes. Each year about 5 eruptions occur along the 2,400-nautical-mile arc from Alaska to the Kuriles. Ash clouds from this segment of the Pacific 'ring of fire' are usually carried to the east and north-east, directly across busy air-routes. On an average of 4 days a year in the North Pacific, volcanic ash is present at the altitudes of jet flight.

In the immediate area, ash may cause lung damage and sclerosis. Where ash is deposited on power lines, it may cause 'shorts'. Additionally, ash may interfere with the radio communications of aid agencies. There have been cases of hot ash falling to the ground and starting small fires.

There are no known ways to end the threat of volcanic ash. All we can do is mitigate them. The hazard to aircraft, for example, can be mitigated by careful monitoring. While volcanic ash clouds are difficult to recognize from an airplane, satellite technology can easily monitor and identify them, and hence Man's growing technological awareness is minimizing this potential problem. In 1991, the United States Geological Survey held the first International Symposium on Volcanic Ash and Aviation Safety. This conference set the groundwork for the formation of a network of Volcanic Ash Advisory Centers (VAACs). These VAACs monitor the entire world and publish regular reports, all of which are sent to every airport in the world. The flight controllers are then able to act on the information, redirecting any flights threatened. (Here at www.vulkaner.no - we receive those messages daily, and therefore we are often able to bring news about ongoing volcano-eruptions very fast.)

The immediate hazards of ash are difficult to mitigate, however. Images abound of people who simply attach a handkerchief over their mouth to prevent the ash getting into their lungs. Although hardly technological, this method is usually successful.

GAS

Magma is molten rock containing dissolved gases released to the atmosphere during eruption. In point of fact, gases are released in other circumstances, for example when magma lies close to the surface.

There are several gases given off by volcanoes. The most prominent is actually harmless - water! H2O is greatly represented in the volcanic gases, sometimes as much as 80%. There are other gases, though, and these are not necessarily so benign.

Carbon dioxide is there, as is hydrogen sulphide. Sulfur dioxide, sulfur trioxide, and Chlorine are all given off.

Sulfur compounds, chlorine and fluorine react with water to form poisonous acids damaging to the eyes, skin and respiratory system even in small concentrations. The acids can destroy vegetation, fabrics and metals. Some of the gases are lethal in themselves; carbon dioxide, for example, can cause death within 10 - 15 minutes when it is only at a concentration of 10%.

Any hazard posed by volcanic gases is greatest immediately downwind from active vents; the concentration of the gases quickly diminishes as the gases mix with air and are carried by winds away from the source. Brief exposure to gases near vents generally does not harm healthy people, but it can endanger those with heart and respiratory ailments, such as chronic asthma.

LAVA

A famous type of volcanic menace is that of the lava flow. Surprisingly, lava is not much of a hazard to people, but more to property. The reason is that most lavas advance at about walking pace. Hawai'ian lavas advance at only a few centimetres per hour.

Lava will be boiling hot. Scientists estimate that the July 2001 lava flows of Etna will take a year to cool down to safe levels once again - a staggering fact!

There are several ways to mitigate the hazard of a lava flow. Surely the most obvious is not to put people in its path in the first place!

There are other ways of mitigating the hazard. One suggestion is to construct barriers and diversion channels. The eruption of Etna 2001 has hit the headlines with this method, but the success of attempts is very difficult to work out. Historically, it is at Etna that these attempts have always been made, and the 2001 efforts show the most concerted efforts of all.

The desperate attempts to redirect lava at Mount Etna in July 2001 have met with little success.

A second technique was first practised in Iceland. This method works on the principle that cooler lava moves slower than hotter lava. Hence Icelandic authorities suggested spraying the surface of the lava with water. An attempt in 1973 at Heimaeyr met with some success, but that was only really because the lava front was already cooling.

A third suggestion has been to disrupt the source area of the lava with explosives. This was attempted on Hawaii during the Second World War, with bombers attempting to blow the vents and redirect the lava flow. Those attempts were not successful.

Basically, the rule with lava flows is - stay out of the way!

PYROCLASTIC DENSITY CURRENTS

Over the last century, there have been over 17,420 deaths due to the phenomenon known as pyroclastic density currents. These are complex phenomenon that are little understood; lateral blasts of gas and solids, sweeping down, capable of even traversing water. Their scale can be awesome and their behaviour is astounding. They hug the ground for tens or hundreds of kilometres, travelling faster than an express train, sometimes leaping over high ridges as they go. The currents can be hot as 800 degrees c, and their heat can be so intense that they fuse the volcanic particles to form a solid sheet of black glass, welded onto the landscape.

One approach considers these currents as 'ash hurricanes' - plumes of white ash and pumice particles, transported by turbulent whirlwinds of gas. These turbulent winds drag the hot pumice and ash particles along with them, while whirling vortices prevent the particles from falling to the ground. Gravity powers this volcanic hurricane - the particles of ash make it denser than the surrounding air, so it tumbles down the slope like a swirling flood of water, gathering momentum as it goes. Like water, it generally follows low ground and valleys, though it can leap over ridges just as powerful floods can. It can only take 1% of solid particles to make the cloud denser than air. The faster the resulting ash hurricane travels, the more vigorously it is stirred and churned, so the more particles it can carry.

Quite how this ash hurricane behaves depends upon the solid-gas ratio. High concentration density flows are known as 'pyroclastic flows', and are confined to valleys. Low concentration flows are called 'pyroclastic surges', which can expand over hill and valley.

The only effective measure of mitigation is evacuation. An illustration of this is Mount Mayon in the Philippines, which has frequently erupted throughout recent history. This volcano has a six kilometre Permanent Exclusion Zone (PEZ), and in light of 2001 eruptions it now has a 7km Extended Exclusion Zone (EEZ). The Filipino police enforce these barriers.

The first ever photograph of a pyroclastic density current.

VOLCANIC EARTHQUAKES

Earthquakes related to volcanic activity may produce hazards which include ground cracks, ground deformation, and damage to manmade structures. There are two general categories of earthquakes that can occur at a volcano; volcano-tectonic earthquakes and long period earthquakes.

The injection or withdrawal of magma causes stress changes in the ground. This results in ground tremors that are known as volcano-tectonic quakes. These quakes can cause land to subside and can produce large ground cracks. Volcano-tectonic quakes don't indicate that the volcano will be erupting but can occur at any time.

The second type of volcanic quake is more interesting. These quakes, known as volcanic tremors, are produced directly by the injection of magma into the surrounding rocks. Observations indicate that they directly precede eruption. For example, the eruption of Mount St. Helens was preceded by volcanic tremor; so was the 1991 eruption of Pinatubo.

VOLCANOES AND CLIMATE

In 1991, the volcano Pinatubo in Indonesia erupted, throwing fine ash and gases high into the stratosphere. About 22 million tons of SO2 combined with water to form acid droplets of sulfuric acid, with an effect of blocking off some of the Sun's insolation. Global temperatures decreased by half a degree.

This is not the worst. The most notable event was in 1815, when Tambora Volcano, Indonesia, erupted. The result was catastrophe on a global scale. In New England, the following year was known as 'eighteen hundred and freeze to death', while Ireland suffered nightmarish famines. Napoleon, en route to Waterloo, encountered terrible weather conditions - possibly one of the main reasons his troops were so exhausted by the time they got to the battle. It is incredible to realise that a volcanic eruption changed the course of the history of all Europe!

Another interesting twist has developed; it is possible some eruptions may impact global air circulation in some ways

.CRATER LAKES AND GAS EMISSIONS

There are three lakes in the world that have menaces most unusual. Lake Kivu in East Africa; and Lakes Nyos and Monoun in Cameroon.

Similar events were seen in 1984 at Lake Monoun, where 37 people died in a similar event which was much less studied than the Nyos event.

Surprisingly, this hazard is actually relatively easy to manage. Whatever the explanation for the specific event, all you have to do is ensure the volcanic gases are released rather than build-up. So simple tubes can be inserted and pushed down into the heart of the crater lake, releasing the gases in small 'spouts'. Simultaneously, efficient monitoring systems are installed to control the procedure.

LAHARS
Lahars are one of the more lethal and dangerous volcanic hazards. Lahars occur where the loose, unconsolidated volcanic ash is caught in a liquid torrent. Lahars differ from normal water-flows in that here, the solid is the driving force - not the liquid. As a result, they have the constituency of wet concrete.

The above case-study is terrifying. Anybody who calls themselves a volcanologist has no excuse for forgetting the human tragedy of a volcanic eruption. To watch the images of Armero's death is to see pain and heartbreak.

Lahars can be generated by an immediate eruption. The 1991 eruption of Pinatubo coincided with Typhoon Vera, and the heavy rains mixed with the immediate ash to generate lahars. These lahars caused significant damage. But most surprising is the fact that they can easily be reactivated; Pinatubo's lahars were reactivated in 1994, and nearly destroyed towns such as Bacalor.

Lahars are difficult to control, but it is possible.

One thing that can be done about lahars is to monitor for them. Near Armero, a network of sensors have been expertly placed. When the next lahars are generated, there will be an hour warning. That's if the lahars travel at the same speed as those in 1985, but that is by no means certain. The truth is, though, that the sensors will enable scientists to know exactly what is happening. When Ruiz awakens, they will have warning, and will be able to evacuate promptly.

TSUNAMI

Sometimes a volcanic eruption can generate powerful, sweeping tsunami. These can range in size from minor events to major ones. These two case-studies illustrate the point;

1902, the island of Martinique. Soon the volcano Mont Pelee will blow her top in a spectacular series of pyroclastic density flows that will cause an unimaginable number of deaths. First, though, the contents of a crater lake are knocked out, and sweep down a river channel, creating lahars.

These lahars hit the sea. The result is an impact wave some 30m high, sweeping outwards like a ripple. The impact wave strikes the town of San Pierre, killing about 30 people. In a few days' time most of the city will die as well, due to the pyroclastic flows; but this first event, had it been understood, would have been efficient warning.

Second example; Krakatau, 1883. A tsunami was generated by pyroclastic flows hitting the water; successive waves were then caused when the volcano blew itself to bits, and collapsed inwards upon itself, creating a vast underwater collapse pit known as a 'caldera'.

Tsunami were observed in the Indian Ocean, the Pacific Ocean, on the American West Coast and the coast of South America - even in the English Channel there were larger-than-average waves! In the immediate vicinity, Indonesia, Java, Sumatra and the Sundra Strait, there were over 36,000 deaths due to these devastating waves.

There are no ways no prevent a tsunami. All you can do is watch for when they're coming, monitor the Earth with remote sensor satellites, buoys and all the science that can possibly be found - with all data collected together into a central agency. The area most at risk is the Pacific. Thus it is here that the world's tsunami warning systems are being most developed and refined.