Volcanoes - Ministry of Civil Defence and Emergency Management

Volcanic eruptions can result in a number of types
of hazards. The most threatening hazards include
pyroclastic falls (ashfall), pyroclastic density
currents, lava (flows and domes), lahars, flooding,
debris avalanches and volcanic gases. These
hazards can be divided into two categories; nearvent
destructive hazards and distant damaging
and/or disruptive hazards.
NEAR-VENT DESTRUCTIVE HAZARDS
ASHFALLS
Large ballistic projectiles rarely land more than
1-2 kilometres from a vent in either a brittle or molten
state and are capable of starting fires. The impact
of these will cause damage to buildings (including
ignition), with the degree of damage dependent on
mass, temperature and velocity. Projectiles present
a high risk of death or injury to people. In the 1973
eruption of Heimaey (Iceland) incandescent clasts from
0.1 to 2 m in size caused many house fires.
Finer material forms ashfall deposits that, when
thick enough, are capable of overloading roof strength
causing collapse and possible death or injury to people
inside. Since building collapse usually requires ash
thicknesses in excess of 100-300 mm, the area affected
will usually be limited to within a few kilometres or tens
of kilometres of the volcano (except in the case of very
large rhyolite eruptions). Deaths and injuries are also
likely to result from falling branches or other accidents.
A survey of building damage following the 1991
Pinatubo eruption in the Philippines concluded that roofs
failed because the ash load was greater than the vertical
load-carrying capacity of their supporting structure.
Wide-span roofed buildings suffered more damage than
short-span domestic scale construction. Pitch angle is
also critical to the vulnerability to roof collapse. Ash can
obviously slide off steeply pitched roofs. Even moderate
pitches can be less susceptible to collapse than flat ones.
This has been observed in many instances from ashaffected
communities around the world.
PYROCLASTIC DENSITY CURRENTS
Pyroclastic density currents (flows, blasts, and
surges) often travel at speeds up to 900 km/h, and cause
total destruction in the areas they cover. Flows are more
concentrated than blasts and surges and partly follow
valleys in the landscape. Pyroclastic density currents
are usually very hot (at least several hundred ºC) and
can start fires. Pyroclastic surges from hydrothermal
and phreatomagmatic eruptions are cooler (usually less
than 300ºC) because of the interaction with water and
often deposit sticky wet mud. Pyroclastic flows and
surges have been produced by many eruptions from
New Zealand volcanoes and represent one of the most
destructive manifestations of volcanic activity.
People caught in the direct path of a pyroclastic
density current are most unlikely to survive and any
survivors will probably receive severe injuries. Buildings
offer some protection near the slower-moving edge of
the flow but will not guarantee survival as the building
may be destroyed or severely damaged. The best
protection is to evacuate the area prior to the event.
LAHARS
People caught in the path of a lahar have a high
risk of death from severe crush injuries, drowning or
asphyxiation. Lahar events will cause destruction of
buildings, equipment, infrastructure and vegetation
caught in their path. Depending on their densities and
flow velocities, lahars may either destroy structures,
or simply bury them in place. People have survived
lahars by climbing onto the roofs of houses which have
remained intact despite inundation by the lahar.
LAVA
The distance lava travels depends on the
viscosity of the lava, output rates, duration of eruption,
volume erupted, steepness of the slope, topography and
obstructions in the flow path. Basalt (eg. Auckland)
flows have low viscosity (flow easily) and have been
recorded to travel more than 50 km from a volcano
but usually only flow 5-10 km. Andesite flows (eg.
Ruapehu) are more viscous and rarely travel more than
5 km and may be quite "blocky". Dacite and rhyolite
lavas (eg. Taupo) have high viscosity and typically form
short, thick flows or domes.
Lava flows will seldom threaten human life
because of their slow rate of movement. The steep fronts
of flows may became unstable and can collapse, causing
small pyroclastic flows. Lava flows will cause total
destruction of buildings and other infrastructure in their
path. Land use possibilities will be significantly altered
on a new lava flow.
SECTOR COLLAPSES AND
DEBRIS AVALANCHES
Debris avalanches are one of the most hazardous
volcanic events. Debris avalanches can travel many
kilometres from the summit area. Debris avalanches do
destroy everything in their path. As debris avalanches
can occur with little or no warning and can travel at
high speeds, prior evacuation is the only safe option
for areas that might be affected if an avalanche is
anticipated. They may be triggered by magma bulging
the flanks of a cone (eg Mount St Helens, exposing the
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TEPHRA
June 2004