In recent decades, scientists
have learned that the earth's crust is divided into many large "plates"
that shift and slide against each other.
In recent decades, scientists
have learned that the earth's crust is divided into many large "plates"
that shift and slide against each other.
The rigid plates are an average of
50 miles thick. They slide past, spread apart from, or collide with each
other in slow motion. These plates move on top of the Earth's hot pliable
interior.
Mount St. Helens is one of the
volcanoes of the Pacific Northwest. These volcanoes lie on the land side
of the boundary between the Juan de Fuca and North American plates.
As you can see, the Juan de Fuca
plate is thrust beneath the North American continental plate. It moves at
the rate of about 1 inch per year. That's about the same rate as your
fingernails grow. As the Juan de Fuca plate is thrust beneath the
continent, it bends downward. As the plate descends, rock in the
overlying plate is partially melted.
As some of this newly-formed
magma rises toward the Earth's surface, it forms the volcanoes of the
Pacific Northwest.
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