Communication During Volcanic Emergencies An Operations Manual for the Caribbean
Typical temperatures are 300 - 650oC (570 - 1200
oF).
Pyroclastic flows normally travel very rapidly, with velocities
in the range 50 - 150 km/h (30 - 90 mph).
They can travel tens of kilometers. They can move over the sea.
Accompanying clouds of ash can reach altitudes exceeding 10
km (33,000 ft), often trigger lightning strikes, and may disrupt
air traffic.
Pyroclastic flows tend to move along valleys and into depressions,
but they can also overflow valley sides.
They are often silent.
Fig 3. Pyroclastic flow - St. Pierre 1902
Pyroclastic surges are pyroclastic flows that consist mostly
of gas. They may form at the volcanic vent or as the dilute
outer parts of a pyroclastic flows break away from their more
dense bases.
Surges can travel tens of kilometres at velocities of 50 - 150
km/h (30 - 90 mph).
They can move over topographic highs and across the sea.
They are silent.
Hot surges contain volcanic gases and may reach temperatures
in the range 300 - 650oC (570 - 1200oF).
Cold surges contain mostly water vapour and have temperatures
lower than 100oC (212oF). These normally
occur when the gas is dominated by heated groundwater, and are
associated with phreatic and phreato-magmatic eruptions (see
pages 9 & 10).
Associated terms:
Ash
Ash cloud
Tephra
Fig 4. Ash damage
Tephra is the term used to describe
all volcanic debris expelled into the atmosphere from a volcano.
The fine fraction is known as ash, which is formed either by
explosions or when lava domes disintegrate into pyroclastic
flows. Ash fall accumulating on structures can add sufficient
weight - especially when wet - to cause collapse. Ash mixes
easily with water to form mud, making surface travel difficult
and providing the source material for lahars
(see page 9). Ash may stay in the atmosphere for months, causing
long term health problems. In large quantities, it can also
contaminate water supplies, destroy crops and, if ingested,
kill grazing animals.
Ash can accumulate at rates of up to tens of centimetres an
hour.
30 cm of wet ash is sufficient to cause collapse of unreinforced,
flat roofs.
It can bring down power and telephone lines.
Gritty ash may cause vehicles to lose traction, while reduced
visibility may contribute to preventing road travel.
The long-term presence of ash in the atmosphere may exacerbate
respiratory problems and cause illnesses such as asthma and
silicosis.
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Typical temperatures are 300 - 650oC (570 - 1200
oF). 
