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The roots of Mount St. Helens are 100 to 330 kilometers (70 to 200 miles)
below the Earth's surface.
Here in the Earth's mantle temperatures are hot enough to melt rock and
form a thick, flowing substance called magma. Lighter than the solid
rock that surrounds it, magma is buoyant much like a cork in water; being
buoyant, it rises.
As the magma rises, some of it collects in large reservoirs, or
magma chambers that fuel volcanoes. As the rising magma nears the
Earth's surface, pressure decreases, which causes the gases in the magma to expand.
This expansion
propels the magma through openings in the Earth's surface: a volcanic eruption occurs.
Once magma is
erupted, it is called lava.
-- Excerpts from:
Volcanoes! -- U.S. Department of the Interior,
U.S. Geological Survey, Teaching Packet
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