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The violence of an
eruption
The separation of gases from the magma has very important effects on the nature of the volcanic eruption. The role of gases depends both on the amount of silica in the magma and on the amount of gas dissolved in it.
To understand this, a volcanic eruption can be compared with a bottle of soda. If the soda bottle is opened slowly, the carbon dioxide gas in the drink (which remains dissolved while the bottle contents are under pressure) can escape in a gradual way; when the cap is finally taken all the way off, the soda remains inside the bottle. If, on the other hand, the bottle is first shaken vigorously and then the cap removed quickly, the gas comes out of solution very quickly,
and the result is an explosive eruption of a mixture of liquid and gas.
In the case of quiescent lavas the basalt magma reaches the surface through long fissures, and the gases are able to escape easily. The lava is also runny, and this makes it easy for the gases to come out of solution. On the other hand, the magma such as that found at Mount Saint Helens (dacite lava)
is so sticky that the gases cannot readily escape, and so the gases are not released until the lava is almost at the surface. As
a result, they burst from the magma just
as it is rising up the vent, and there is an explosive eruption of lava spray which cools in the air to form ash.
(Below) The Mountains of the Moon National Monument in Idaho, in the northwestern United States, are a classic example of a region of cinder cones. The cones are built on pahoehoe and aa lava, showing that the material flowing to the surface was basalt.
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