Inside a volcano
What happens inside a volcano?
Underneath active volcanoes are magma chambers, containing
molten rock (magma). The magma is less dense than the solid
rock surrounding it, and so the magma tends to rise to the
surface through cracks or conduits in the solid rock.
Sometimes the magma contains large amounts of dissolved
gasses. If this is the case, when the magma reaches the
surface, the gasses will suddenly come out of the magma,
causing pumice and ash to be violently thrown into the air.
This type of eruption is not unlike the sudden release
of carbon dioxide from a carbonated beverage can that
has been shaken up, spewing soda pop foam. Examples
include Mt. Saint Helens in the Pacific northwest and
Mt. Pinatubo in the Phillipines.
If the magma does not contain much dissolved gas, then
it oozes out the sides or top of the volcano and forms
lava flows. This is the type of eruption that commonly
occurs in Hawaii.
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Update: June 2012