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No geological
phenomenon assails our senses quite like a volcanic eruption. Stay close
enough, and you can hear the explosion, see the fire fountaining, smell
the gases, feel the ground tremble, taste the ash in your mouth. "I think
that is why volcanoes are so cool to grade school kids," says Chris Nye,
a volcanologist at the Alaska Volcano Observatory in Anchorage. "And scientifically,
volcanoes are interesting because they bring you information about the
interior of the planet, down to 60 miles or more, and help you study the
evolution of the planets, on a human time scale. Mostly in geology you
think of processes taking thousands or millions of years."
All
volcanoes are born when hot magma rises to the surface, infiltrates a weak
spot or opening in the Earth's outer crust, and erupts through.
Most of the 600-plus active volcanoes on Earth are
associated with the boundaries of the tectonic plates, the seven great
plates that carry the oceans and continents. Volcanoes tend to cluster
along mountain belts. The folding and fracturing of plates that creates
mountains, breaks up solid rock and creates channels for magma.
Volcanoes
are especially common in mountainous regions of subduction zones, places
where one plate dips beneath another. As the plate dives into the mantle
-- the layer of hot, flexible rock on which the plates glide -- it gradually
is heated. That releases fluids which heat the overlying rock, producing
blobs of molten rock that rise to the surface. The molten rock -- or magma
-- collects in weak patches of crust, in structures called magma chambers.
If the pressure in the magma chamber builds high enough, the
magma will erupt. A volcano is born. Page
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