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på
norsk her: 
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.
| In 1902, the volcano Santa
Maria in Guatemala spewed out great clouds of ash. This ash
swept out and over the local area, and as the ash piled up on
the roofs, many buildings collapsed. Scientists estimate that
1cm of ash on a roof may add as much as 19kg/m2 in weight! |
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.
| DESCRIPTION |
PLUME HEIGH |
VOLUME OF ASH |
EXAMPLE |
| 1) Non-explosive |
<100m |
1,000m3 |
Kilauea,
Hawaii |
| 2) Gentle |
100 - 1,000m |
10,000m3 |
Stromboli |
| 3) Explosive |
1 - 5km |
1,000,000m3 |
Galeras,
1992 |
| 4) Severe |
3 - 15km |
10,000,000m3 |
Nevado del
Ruiz |
| 5) Cataclysmic |
10 - 25km |
100,000,000m3 |
Galunggung,
'82 |
| 6) Paroxysma |
l >25km |
1km3 |
St Helens,
'81 |
| 7) Colossal |
>25km |
10km3 |
Krakatau,
1883 |
| 8) Super-colossal |
>25km |
100km3 |
Tambora,
1815 |
| 9) Mega-colossal |
>25km |
1,000km3 |
Yellowstone |
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.
During a 1989-1990 series of eruptions, Redoubt
Volcano, Alaska, spewed enormous clouds
of ash into the air. On December 15, 1989, a 747 (KLM flight
867) flew into the ash cloud.
An accident was barely prevented; had a crash occurred, all
231 passengers would have
been lost. The eruption of Mount Spurr in 1992 posed similar
problems. |
The hazard became most notable in 1991 with
the catastrophic eruption of Mount Pinatubo.
Ash from this volcano travelled more than 5,000 miles to the
east coast of Africa, interfering
with some 20 aircraft as it did so. In total, the ash from this
eruption damaged over 40 planes, causing damage of over $22
million. |
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.
Update by www.vulkaner.no to this article,
December 13th, 2011
from an article at earthquake-report.com
Airborne Volcanic Object Imaging Detector (AVOID)
The AVOID system (Airborne Volcanic Object Imaging Detector)
is a device meant to provide real-time images of hazards ahead
of a jet airplane. Information is supplied to the cockpit from
two infrared cameras that are tuned to detect hazardous volcanic
ash particles in the airspace up to 100 km ahead of the aircraft
day or night. At normal flight cruising altitudes and speeds
this will give a pilot-7 – 10 minutes warning of a potential
dangerous encounter with an ash cloud.
AVOID software is used to convert the image signal into ash
concentrations. Combined with GPS and airspeed data, “ash
dosages” can be quickly determined and displayed in real-time.
The information can also be relayed back to air-traffic control
centres or to other aircraft not equipped with the AVOID system.
Eyjafjallajokull
volcano eruption cost to airline industry
Iceland’s Eyjafjallajokull volcano ash generated the closure
of much of Europe’s airspace during more than a week. The
International Air Transport Association reported that more than
10 million passengers were affected and that more than 100,000
flights had to be cancelled. The damage to the airlines was
valued to be $1.7 billion
. |
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.
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A common gas produced during Hawai'ian eruptions
that is potentially harmful to human health
is sulfur dioxide. Even small concentrations of sulfur dioxide
can combine with water to form sulfuric acid, which can attack
skin, cloth, metal, and other materials.
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When a volcanic plume mixes with atmospheric
moisture, acid rain results. Acid rain can significantly retard
the growth of cultivated or natural plant life downwind of a
vent that
degasses over a long period of time.
The sulfur dioxide emitted from Kilauea's
summit during typical non-eruptive periods affects
a relatively small area downwind of the summit. Similarly, the
gases produced during short-lived eruptions affect only a limited
area, although their odor may be detected many miles from the
vent. The continuous emission of volcanic fumes during Kilauea's
Pu'u 'O'o-Kupaianaha
eruption, however, resulted in persistent volcanic haze and
acid rain conditions in the
South Kona district on the leeward side of the island.
This digital shaded-relief map shows the usual wind conditions
on the island of Hawai`i. Moderate to strong trade winds carry
gases and volcanic fog (vog) from Kilauea Volcano around the
southern tip of the island where the gas tends to accumulate
on the leeward or "kona" coast. During these usual
conditions, vog often becomes trapped by daytime (onshore) and
night-time (offshore) breezes (double-headed arrows). During
the day, onshore sea breezes carry vog up the slopes of Hualalai
and Mauna Loa volcanoes,
and into the topographic saddle between Mauna Loa and Mauna
Kea. When the landmass cools in the evening, cooler, denser
air and vog flow back down to the coast. However, when the trade
winds are light or absent or when winds blow from the south,
much of the vog stays on the eastern side of the island where
it sometimes moves into the city of Hilo.
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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.
| The precise speed of flow depends upon the chemical composition
of the lava. The volcano Nyiragongo
in Zaire has lava with an unusual chemical composition. In 1977,
five fissures released the lava
lake in the summit crater, draining it within an hour; a wave
of fluid lava, travelling at 30 - 100 km/h, killed 300 people. |
Even when lavas do not move this fast, they are still a menace in that
they inexorably advance, pressing forward. In 2001, an eruption of Etna
has illustrated this perfectly. The village of Nicolosi can only watch
and wonder whether the lava will cease advancing before it reaches them.
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In 1986, a small Hawai'ian community fell foul
of lava flows from Kilauea Volcano's Pu'u
O'o vent. These lava flows advanced upon Kalapana Gardens,
and the residents could only
watch and wait in horror. Paradise had turned badly wrong, and
the entire community
was soon destroyed, inundated under the advancing front.
This image shows a Kalapana house bursting into flames as
the lava advances into it. The intense heat of the lava is notable.
Hawaii perfectly illustrates the hazards of
lava. In the last hundred years, only one person had
died on Hawaii due to lava flows. In the same amount of time,
5% of the land has been
freshly covered with lava.
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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!
|
The scientists of the Hawai'ian Volcanological Observatory
have produced a 'hazard map' showing the areas most likely to
be inundated beneath lava.
|
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.
| Historically, these flows have killed thousands. One of the
most famous occurred in 1902, on the Caribbean island of Mont
Pelee. An awesome pyroclastic flow swept into the town of
San Pierre, a boom-town of nearly 30,000. Almost everyone was
killed; there were but three survivors. |
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
|
Alistair Dawson, an earth scientist at Coventry University,
and Kieran Hickey of St. Patrick's College in Maynooth, compared
Edinburgh's meteorological records from 1770 to 1988 with the
activity of volcanoes around the world. They found that the
city, famed for its strong winds, was buffeted by most gales
in the winters that followed three of the biggest eruptions.
For two winters after the eruptions at Tambora
in April 1815 and Krakatau
in August 1883, both in Indonesia, Edinburgh endured force 7
or stronger gales on 70 days a year - twice the usual frequency.
After the El Chichon Volcano erupted in Mexico during March
and April 1982, there were gales in Edinburgh on more than 50
days.
Dawson says that the precise mechanisms by which eruptions
increase the frequency of storms need to be investigated. Particularly
fierce volcanic explosions shoot large clouds of ash more than
20 km into the stratosphere, blocking out the Sun. This can
cool the lower atmosphere and increase the air movement between
the equator and the poles.
|
.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.
|
Lake Nyos was a lake situated
inside a volcanic crater. Over the decades it had become supersaturated
with carbon dioxide released from underground volcanic events.
Normally convection cells in the water would transfer the gas
to the surface and remove it in safe levels, but in the Cameroon
the surface lake-water is constantly heated, therefore less
dense. Convection cells are not generated, and the gas collects
at the lake bottom.
The events of 1986 are a mystery. All that is certain is that
on August 21, a poisonous cloud about 50 m thick poured down
Nyos valley, killing 1,200 in Lower Nyos village. Five or six
inexplicably survived, and told of watching the rest of the
village fall dead around them. The clouds continued onwards,
travelling 16 km before being dissipated. In total, 2,000 were
killed; a tragedy compounded by August 21 being a market day,
with a multitude of visiting traders.
These are the facts, and nobody can dispute them. What is more
controversial is the proximate or trigger cause. Some scientists
theorise some chilly rain generated a limited amount of convection,
and that this caused a slight decrease in pressure. A slight
decrease would be all needed for the lake to empty itself.
Other scientists wondered whether the rain might have triggered
a small landslide, disturbing the lake; or perhaps a seismic
wave, an earthquake, had done that? There is, however, no geological
evidence for this landslide. Certainly there was no volcanic
explosion.
Probably the best explanation is the so-called "limnic
eruption" theory propounded by J. C. Sabroux. He theorised
that it was possible for the gases to build up to a point where
the pressure had to be released. His explanation fits the facts
most efficiently.
|
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.
In 1985, the volcano Nevado
Del Ruiz, in Colombia, erupted. The event took place
on 14 November, and 28,000 lives were lost as a result.
To quote one newspaper report;
[The] instant release of heat produced millions of cubic
metres of meltwater which poured down local slopes…
The wave of mud probably
exceeded 30m depth in narrow sections and travelled at 50kph.
About 50km from its source, the lahar emerged from the confines
of the valley and spread more thinly over more gently-sloping
ground at the mountain foot. Here lay Armero with 28,000
inhabitants. Survivors maintained that the first wave of
mud was icy cold but the torrents became hot as the content
of freshly erupted lava increased. Within seconds most of
the buildings in Armero and some neighbouring communities
had been flattened or buried along with their inhabitants…
A new geological horizon, 3-8m of mud, had been instantly
laid down.
Following these horrific events the world was bombarded
with some of the most sickening images of all time. Surely
the most heart-rending was a little girl, stuck in the mud,
unable to be rescued. There were desperate attempts, but
the world could only watch as her life slowly ebbed away.
With that child died the hopes of Armero.
Today, Armero has returned. The village still exists, is
still inhabited. The terror could happen again.
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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.
|
In Japan, there has been a significant amount of investment
in lahar defences.
This network of dams and dykes captures and
redirects the lahar flows. Similar structures have been built
in the Philippines to protect the people of Bacalor from lahars.
However, it is in lahar defenses that the distinction between
an Economically More Developed Country (EMDC) and an Economically
Less Developed Country (EMDC) becomes of importance. Look at
the Japanese structures; towering and powerful structures, capable
of withstanding the elements. Compare them to the dykes of the
Philippines. These structures are actually themselves composed
of loose, unconsolidated volcanic material - in short, come
bad weather or rain, and it is quite possible that the defenses
will themselves generate lahars!
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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.
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