Hazardous Volcanic Events
There are several kinds of events caused from volcanic action
that can be harmful to life and property. These include lava flows,
lahars, ash falls, debris avalanches, and pyroclastic density currents.
List of Volcanic Hazards
Mitigation of hazards is an
important goal of the volcanological community, including the U.S. Geological Survey .
safety procedures for conducting hazardous scientific studies on
Pyroclastic Density Currents (pyroclastic flows and surges)
Structural Collapse: Debris flow-Avalanches
Dome Collapse and the formation of pyroclastic flows and surges
Tephra fall and ballistic projectiles
Many volcanoes around the world have been targeted for
and several of the most notorious volcanoes have been designated as
for concentrated hazards research. Within striking range of 30,000,000 people around it, including Mexico City, Popocatepetl
should be on the Decade Volcano list.
Access Montserrat for an on-going hazards mitigation drama.
Pyroclastic Density Currents
Pyroclastic density currents
are are gravity-driven,
rapidly moving, ground-hugging mixtures of rock fragments and hot
gases. This mixture forms a dense fluid that moves along the
ground with an upper part that is less dense as particles fall toward
the ground. The behavior of the fluid depends upon the solids
concentration relative to the amount of hot gases (i.e.,
solids-gas ratio). High concentration density flows are called
"pyroclastic flows" and are essentially nonturbulent and
confined to valleys. Low concentration density flows are called
"pyroclastic surges" which can expand over hill and valley like
hurricanes. Temperatures may be as hot as 900 degrees Celsius, or as cold as
see "base surges" in section on Hydroclastic Processes).
Pyroclastic flows and surges are potentially highly destructive owing to their mass, high temperature, high velocity
and great mobility. Deadly effects include asphyxiation, burial,
incineration and crushing from impacts. Many people and
the cities of Pompeii and Herculaneum were destroyed in 79 AD
from an erupion of Mount Vesuvius; 29,000 people were destroyed by
pyroclastic surges at St. Pierre, Martinique in 1902; >2000
died at Chichónal Volcano in southern Mexico in 1982 from pyroclastic
surges. The only effective method of risk mitigation is evacuation prior to
such eruptions from areas likely to be affected by pyroclastic
Lahars are part of
the family of debris flows that are fluids
composed of mixtures of water and particles of all sizes from
clay-size to gigantic boulders. The abundance of solid matter
carries the water, unlike watery floods where water carries the
fragments. Debris flows have the viscous consistency of wet
concrete, and there is a complete transition to watery floods.
Lahars are composed of volcanic particles and originate directly
or indirectly from volcanic action. Lahars can form by hot
pyroclastic surges or flows entering watershed systems or flowing
over snow and ice, by eruptions through crater lakes, by heavy
rains on loose volcanic debris -- that is, any process by which
volcanic particles can become saturated by water and move
downslopes. They can move with velocities as low as 1.3 m/s to as
great as 40 m/s on steep slopes (1 m/s = 2.55 miles per hour).
They are known to have travelled as far as 300 km (1 km = 0.63
miles). Lahars have destroyed many villages and lives living on Indonesian
volcanoes because most people live in valleys where lahars flow.
The 21,000 lives lost at Armero, Colombia, was from a lahar that
formed during the eruption of Nevado del Ruiz in 1985. It was
generated by meltwater from the interaction of pyroclastic surges
with snow and ice, from a very small eruption.
Lahars can transform into regular floods as they become
increasingly diluted with water downstream. This phenomenon was
first discovered at Mount St. Helens where hot pyroclastic surges
transformed to lahars, which further transformed to hyperconcentrated
streamflow and then to normal stream-flow turbulence (floods).
The eruption of Mount St. Helens on May 18, 1980 started with a
relatively small volcanic earthquake that caused collapse of the
north side of the volcano because it was oversteepened and therefore
unstable. When the landslide occurred, it decreased the pressure on
the pressurized interior of the volcano which expanded explosively to
form a lateral blast that devastated the countryside north of the
volcano. Most of the
debris flow avalanche was diverted down the North
Fork Toutle River, but some moved directly northward over a 300 meter
ridge and down into the next valley. Since the 1980 Mount St. Helens
eruption, dozens of volcanoes that have given rise to avalanches
have been discovered.
For example, 40 avalanches exceeding 1 Km3 in volume, and 22 with a volume
of less than 1 km3, are now known from the Quaternary alone, and 17
historic volcanic avalanches have been identified. The hilly
topography north of Mount Shasta in northern California is now known to
be the result of a have debris-flow avalanche. Some are known to
extend up to 85 km from their sources and to cover tens to more than
1000 km2 in area.
Lava flows rarely threaten human life because lava usually moves
slowly -- a few centimeters per hour for silicic flows to several
km/hour for basaltic flows. An exceptionally fast flow (extremely
rare) at Mt. Nyiragongo, Zaire (30-100 km/hour), overwhelmed about 300
people. Major hazards of lava flows -- burying, crushing, covering,
burning everything in their path. Sometimes lava melts ice and snow to
cause floods and lahars. Lava flows can dam rivers to form lakes that
might overflow and break their dams causing floods. Methods for
controlling paths of lava flows: (1) construct barriers and diversion
channels, (2) cool advancing front with water, (3) disruption of source
or advancing front of lava flow by explosives.
Tephra falls and Ballistic Projectiles formed on Land
Tephra consists of pyroclastic fragments of any size and origin.
It is a synonym for "pyroclastic material." Tephra ranges in
size from ash (<2 mm) to lapilli (2-64 mm) to blocks and bombs (>64
mm). Densities vary greatly, from that of pumice (<0.5)) to solid
pieces of lava with density about 3.0. Blocks from basement material
may exceed 3.0. Material may be juvenile (formed of magma involved in
the eruption ) or accidental (derived from pre-existing rock).
Tephra fall and ballistic projectiles endanger life and property
by (1) the force of impact of falling fragments, but this occurs only
close to an eruption, (2) loss of agricultural lands if burial is
greater than 10 cm depth, (3) producing suspensions of fine-grained
particles in air and water which clogs filters and vents of motors,
human lungs, industrial machines, and nuclear power plants, and (4)
carrying of noxious gases, acids, salts, and, close to the vent, heat.
Burial by tephra can collapse roofs of buildings, break power and
communication lines and damage or kill vegetation. Even thin (<2 cm)
falls of ash can damage such critical facilities as hospitals,
electic-generating plants, pumping stations, storm sewers and
surface-drainage systems and sewage treatment plants, and short circuit
electric-transmission facilities, telephone lines, radio and television
transmitters. When dispersed widely over a drainage basin, tephra can
change rainfall/runoff relationships. Low permeability of fine ash
deposits leads to increased runoff, accelerated erosion, stream-channel
changes and hazardous floods. In contrast, thick, coarse-grained
deposits closed to the source can increase infiltration capacity and
essentially eliminate surface runoff.
Many of the hazards of tephra falls can be mitigated with proper
planning and preparation. This includes clearing tephra from roofs as
it accumulates, designing roofs with steep slopes, strengthening roofs
and walls, designing filters for machinery, wearing respirators or wet
clothes over the mouth and nose because tephra can contain harmful
gases adsorbed on the particles as acid aerosols and salt particles.
Magma is molten rock containing dissolved
gases that are released
to the atmosphere during an eruption and while the magma lies close to
the surface from hydrothermal systems. The most abundant volcanic gas
is water vapor; other important gases are carbon dioxide, carbon
monoxide, sulfur oxides, hydrogen sulfide, chlorine, and fluorine. The
gases are transported away from vents as acid aerosols, as compounds
adsorbed on tephra and as microscopic salt particles. Sulfur
compounds, chlorine and fluorine react with water to form poisonous
acids damaging to the eyes, skin and repiratory systems of animals even
in very small concentrations. The acids can destroy vegetation,
fabrics and metals. Atmospheric veils of dust or acid aerosols caused
by large-volume explosive eruptions can effect regional or global
Most volcanic gases are noxious and smell bad, but they
can cause mass fatalities. An rare case of mass deaths by volcanic
gases in 1986 at Lake Nyos, in Cameroon, West Africa. Tons of carbon dioxide
spilled out of Lake Nyos, and flowed silently down a canyon and through 3
village occupied by 1700 people. They and 3000 cattle died instantly
from lack of oxygen.
Carbon dioxide emissions are now being monitored
Mammoth Mountain, California.
A tsunami is a long-period sea
wave or wave train generated by a sudden displacement of water. Tsunamis
travel at very high speeds through deep water as low broad waves and
build to great heights as they approach the shallow bottom of shores.
Most are caused by fault displacements on the sea floor, but many have
been caused by volcanic action. The eruption of
Krakatau in 1883
produced tsunamis that killed 36,000 people. The pyroclastic flow
generated by this eruption displaced the water that
initiated the tsunamis.
Copyright (C) 1997, by Richard V. Fisher. All rights reserved.