Communication During Volcanic Emergencies An Operations Manual for the Caribbean
To provide a context for understanding the problems caused by
volcanic crises, this part of the manual offers a brief guide
to what volcanoes are and how they behave. More detailed information
can be found in the publications and online sources listed in
Appendix 5.3.
An eruption occurs when molten rock (magma),
reaches the Earth's surface through fractures in the crust.
An accumulation of solidified magma around a fracture constitutes
a volcano. Gases are trapped in magma and, close to the surface,
they form bubbles. How easily the bubbles escape controls the
style of eruption. If the bubbles are able to escape effortlessly,
the magma oozes out as a lava flow, resulting in an effusive
eruption. If the bubbles remain trapped, pressure builds in
the magma, which then explodes at the surface. Trapped bubbles
have a similar effect when opening a shaken bottle of fizzy
drink.
The Lesser Antilles are an arc of islands, most of which are
volcanic. The most common type of volcanism in the region involves
the extrusion of almost solid, hot magma, which accumulates
to form a lava dome. As a dome
grows, it often becomes unstable and collapses. Bubbles still
trapped inside may trigger explosions that hurl out large fragments
of magma, known as ballistic ejecta
or bombs. The collapsing part of
the dome may also disintegrate to form pyroclastic
flows or surges and ash
falls. These and other hazardous volcanic phenomena are
described below, while the various signs detected before an
eruption are summarized in section 1.2.
Lava domes are masses of almost solid magma that accumulate
at the surface. They are very common in the Eastern Caribbean.
All the active volcanoes of the Lesser Antilles have recently
extruded lava domes, while most of the hills surrounding them
are ancient domes produced by now extinct volcanoes. Lava domes
are dangerous because they can collapse or explode to produce
pyroclastic flows and surges and extensive ash fall.
Dome temperatures are typically between 700 and 1000 oC (1300
- 1850 oF).
Although dome growth is normally slow and quiet, the build up
of gas pressure in bubbles can cause periodic explosions.
Lava domes can be kilometres across and several hundred metres
high.
Pyroclastic flows consist of dense mixtures of gases, volcanic
ash and - on many occasions - blocks and boulders of different
sizes. In the Lesser Antilles they occur either when a dome
collapses or during an explosive eruption. They are deadly.
The most recent were generated on Montserrat in June 1997, killing
19 people. In 1902, Pyroclastic flows erupted from Martinique's
Mont Pelée volcano obliterated the town of St. Pierre and killed
up to 29,000 of its inhabitants. Pyroclastic flows appear as
tall, turbulent, grey clouds moving rapidly down slope, and
may glow at night. A pyroclastic flow will destroy or burn everything
its path.
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Dome temperatures are typically between 700 and 1000 oC (1300
- 1850 oF). 
