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Eruption cloud
The cloud of gases and ash created by the explosive eruption of a volcano.
Explosive eruption
A violent eruption of sticky (acid) lava that involves the sudden ejection of huge volumes of ash and gas. Very little lava flows at the start of an explosive eruption.
There are two reasons for an explosive eruption. First, the more acid and full of gas the magma
is, the harder it is for it to flow quickly out of the vent of the volcano before the gases in it expand. Second, the old plug that is sealing the vent may be difficult to dislodge, so that more pressure has to build up in the magma chamber below in order to break through the plug.
It may help to imagine an explosive volcano as a soft-drink bottle. The magma chamber is the bottle, the volcano is the neck of the bottle, and the cap is the plug blocking the vent. If you shake
the bottle vigorously and then undo the cap quickly, a large amount of liquid comes out, propelled
in part by the gas in the
liquid. To begin with, the froth explodes out in all directions, and only later on does it flow down the side of the bottle. There are three types of explosive eruption:
1. Whengasandashare thrown out of the vent in the form of a dark, swirling cloud over the volcano. This is the least violent type (see: Vulcanian- and Strombolian-type).
2. Whencloudsexplode sideways because the vent remains blocked. Gases, ash, and rock behave like
a fast-moving liquid that rolls down the side of
the volcano, rather like a glowing avalanche (see: Pelean-type).
3. Whengasesshootupinto the air in a giant column (see: Plinian-type).
 Explosive eruption—An explosive eruption occurs when gas-rich magma reaches the surface.
Near the surface the reduced weight on the magma allows the gas bubbles to grow.
Explosive eruption
 Eruption—A spatter cone near the top of the summit complex of Hawaii. Eruptions like this contribute to the gently sloping cones typical of Hawaiian-type volcanoes.
 Eruption—The initial phase of the Mount St. Helens eruption was a Pelean-type, but later the eruption changed and became a Plinian-type. The result of the Pelean-type eruption was to blast part of the cone away.
At the surface gas bubbles expand explosively and send magma out as a fine spray, which cools to ash.
Deep in the vent the magma is under pressure, and gas bubbles are tiny.
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