U.S. Geological Survey Volcano Hazards Program
U.S. Geological Survey (USGS),
U.S. Department of the Interior
(General Audience)
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While volcanic eruptions are a common occurrence on Planet Earth, the
odds are relatively small that a specific dormant volcano will begin an
eruptive phase within a human lifespan. Certain volcanoes, such as
Mount Stromboli on Sicily and Kilauea in Hawai`i, never really become
dormant. Dormancy of volcanoes such as Vesuvius is highly suspect.
Still, most volcanoes just don’t erupt very often — at least,
not on our timescale. Furthermore, many eruptions are in out-of-the-way
places, such as the wilds of Alaska, where they do not directly impact
many people.
This quiescence typically lulls those who live in close proximity to
volcanoes into complacency with regard to “their” volcano.
Even when warning signs become evident and unmistakable, those who have
lived their entire lives with a sleeping giant are unwilling to accept
the fact that their mountain may be coming to life. This simple and
unfortunate ignorance has led to numerous mass exterminations of human
populations.
Dangers from volcanoes are numerous. Perhaps most feared is the lava
flow. Lava, while popularized in the movies as a relentless killer,
does not often live up to its reputation. While it can be relentless,
sometimes burying entire towns, lava usually moves slowly once it
moves beyond the immediate vicinity of its vent, permitting occupants of
towns in the path of a lava flow adequate time not only to evacuate, but
frequently to pack up belongings as well. Nevertheless, you can’t
take your house (Mobile home owners, you’re the lucky exception.) or
your property with you, and once a lava flow inundates an area, the
neighborhood isn’t good for much of anything for quite a few years.
Lava flows, therefore, while dangerous, are more of a threat to
property than to human life.
Of far greater concern is the pyroclastic flow, a maelstrom of hot
gas, rock and ash which can be ejected without warning from a volcanic
vent or as a result of a volcanic cone collapse. Speed of a pyroclastic
flow can approach 150 km/hr with temperatures up to 800 degrees Celsius.
You cannot outrun a pyroclastic flow, and if you are caught in one, you
stand virtually no chance of survival. A massive volcanic eruption
can result in a towering volcanic column (sometimes referred to
simplistically as an ash cloud) ascending vertically into the
atmosphere to heights that may exceed 40 kilometers. This column will be
sustained until the explosive pressure within the vent or crater is
reduced, whereupon the column will collapse, resulting in powerful and
long-lived pyroclastic flows.
Another volcanic hazard presenting extreme danger is the
lahar, a low-viscosity flow of volcanic ash
and particles suspended in water. Due to the minute size of volcanic
ash particles, lahars are capable of flowing at very high speeds. These
volcanic ash flows acquire a great deal of momentum, enabling them to pick
up large volumes of debris. Lahars formed by recent or active eruptions
become choked with debris, making them capable of destruction on an
enormous scale.
Lahars are most common on the slopes of strato volcanoes, also known
as composite volcanoes because they are made up of layered
aggregations of loose and course material. Lahars can occur with great
frequency for up to 100 years following an eruption. Lahars can be
produced as a consequence of heavy rains, the displacement of rivers or
lakes by volcanic debris, or the melting of snowpack or glacier.
Lahars are especially deadly not only as a result of of their speed and
momentum, but because their small particle size enables them to thicken
into a gooey mess having the consistency of wet concrete. Escaping from a
lahar can only be achieved by moving rapidly to higher ground;
once caught, escape is not possible without assistance or considerable luck.
Volcanic ash is an underrated hazard that can have a dramatic impact over
long distances, sometimes even on a global scale. It is not uncommon
for volcanic ash to be carried downstream by prevailing winds for hundreds
of miles. In the proximity of a volcanic eruption, volcanic ash can
stall jet aircraft engines, collapse roofs and awnings, form a slippery
slurry on roadways, sidewalks and bridges, reduce visibility to zero and
result in lung ailments and skin and eye irritation for those not
adequately protected. Volcanic ash is not like wood ash; it is
made up of nearly microscopic (less than 2 millimeters) particles
of rock and glass that can be mixed with larger particles to appear as
a greyish snowfall. Volcanic ash is highly abrasive; it can
also be corrosive when it is introduced to water, undergoing chemical
changes that can form acidic compounds. Inhaled volcanic ash will form
a paste in the lungs and bronchial passages that resembles wet cement,
leading to suffocation.
Volcanic islands pose two special and significant threats. The first of
these is volcanic explosion (Krakatau Volcano), caused when
sea water or rainwater gradually seeps into the rock structure of the
volcano, becomes trapped and is heated by hot magma to form steam.
Eventually, the entire volcanic cone may be blown apart in a cataclysmic
eruption. The second is a massive landslide with a submarine component,
which can occur if the flank of a large volcanic cone weakens and suddenly
collapses. Either of these situations may cause incredible devastation
in the immediate vicinity of the volcano and may result in a
tsunami,
wreaking destruction both near to and at distant locations from the event.
A massive submarine landslide may create a
mega-tsunami,
a sea wave that may reach hundreds of meters in height when it strikes coastlines.
Other volcanic hazards include volcanic bombs — large,
hot volcanic rocks hurled outward from an erupting volcano — and
other forms of tephra (which encompasses all sizes of airborne volcanic
material including volcanic ash), volcanic gases such as carbon
dioxide and sulphur dioxide, which can be poisonous to plants and animals
in large quantities, earthquakes caused by the underground movement
of magma and
landslides, which can result from the dislodging
of unstable volcanic material and may in rare cases actually trigger
volcanic eruptions (Mount St. Helens).
Destruction and casualties resulting from volcanic hazards can be
minimized when national, regional and local governments implement
building restrictions, monitoring networks and alert systems in
potentially active volcanic zones. It is important that we
understand volcanic processes to have a better
appreciation of volcanism hazards and what we can do to avoid them.
Authored by Kenneth L. Anderson.
Original article published 5 July 2005, updated 7 July
2005.
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