Volcanoes - Extreme Events: A Physical Reconstruction and Risk Assessment - page 204

volcanoes can also consist mainly of lava flows such as those found in the Hawai-
ian Islands (Bell, 1998).
Bryant (2005)notes that explosive volcanoes produce five main types of erup-
tions. These are as follows.

(1) Strombolian eruptions which consist of moderate amounts of fluid lava

material of all sizes that are thrown a few hundred metres up in the air.
The lava is released in the air as bombs every few seconds. Events like
this can continue for years. These volcanoes have symmetrical cinder
cones and produce moderate lava flows. The magma is an intermediate
stage between basaltic and acidic magma extrusions.

(2) Vulcanian eruptions expel large blocks of very viscous, hot magma that

may be thrown more than 10 km in the air. In extreme cases viscous
material can be thrown as high as 40 km. The activity is explosive and
can span over periods of months. Vulcanian eruptions rarely produce
flows. Tephra and block cones develop during the eruption. Between
eruptions lava solidifies in the vents of the volcano and creates a plug.
The strength of this plug determines the amount of pressure that can
build up from underneath before another eruption occurs. Vulcanian
eruptions are more acidic in nature than Strombolian eruptions.

(3) Surtseyan eruptions produce large dust clouds that can rise several kilo-

metres in the air. These fine dust clouds are produced as hot, fluid
magma comes into contact with seawater. Static electricity, resulting
in lightning, occurs as fine-grained dust particles rub together in the
rising cloud. The coarser-grained particles form rings around the vent
rather than cones or ash sheets as is found in other types of eruptions.
Due to the contact between magma and water, these eruptions are also
violently explosive and are termed phreatomagmatic.

Plinian (Vesuvian) eruptions cause more than 1 km 3 of magma to be pro-
jected up to 25 km into the atmosphere at speeds of between 600 and
700 m s − 1 by a continuous jet stream and thermal expansion. The erup-
tion becomes starved of material as the magma chamber empties and
themagma changes in character. When this occurs the volcano usu-
ally collapses internally under its own weight to form a caldera. Such
eruptions are even more explosive than Surtseyan eruptions.

(4)

(5) Peleean eruptions occur when the ash column from an explosive vol-

canic eruption collapses under gravity. Such an event can result in a
debris avalanche of ash and hot gas that can travel down the slopes of
the volcano at speedsupto60kmh − 1 . This debris avalanche is termed
apyroclastic flow or nuees ardentes. The debris left from such flows is
called ignimbrite.