There are more than 500 active
volcanoes (those that have erupted at least once within recorded history) in the world--50
of which are in the United States (Hawaii, Alaska, Washington, Oregon, and California) -
although many more are hidden under the seas.Most active volcanoes are strung like
beads along, or near, the margins of the continents, and more than half en circle the
Pacific Ocean as a "Ring of Fire."
Many volcanoes are in and around the Mediterranean Sea.
Mount Etna in Sicily is the largest and highest of these mountains. Italy's Vesuvius is
the only active volcano on the European mainland. Near the island of Vulcano, the volcano
Stromboli has been in a state of nearly continuous and mild eruption since early Roman
times.
At night, sailors in the Mediterranean can see the glow from the fiery molten material
that is hurled into the air.
Very appropriately, Stromboli has been called "the lighthouse of the
Mediterranean.
Some volcanoes crown island areas lying near the continents, and others form chains of
islands in the deep ocean basins.
Volcanoes tend to cluster along narrow mountainous belts where folding and fracturing
of the rocks provide channels to the surface for the escape of magma.
Significantly, major earthquakes also occur along these belts, indicating that
volcanism and seismic activity are often closely related, responding to the same dynamic
Earth forces.
In a typical "island-arc" environment, volcanoes lie along the crest of an
arcuate, crustal ridge bounded on its convex side by a deep oceanic trench.
The granite or granite-like layer of the continental crust extends beneath the ridge to
the vicinity of the trench. Basaltic magmas, generated in the mantle beneath the ridge,
rise along fractures through the granitic layer.
These magmas commonly will be modified or changed in composition during passage through
the granitic layer and erupt on the surface to form volcanoes built largely of
non-basaltic rocks.
In a typical "oceanic" environment, volcanoes are aligned along the crest of
a broad ridge that marks an active fracture system in the oceanic crust.
Basaltic magmas, generated in the upper mantle beneath the ridge, rise along fractures
through the basaltic layer.
Because the granitic crustal layer is absent, the magmas are not appreciably modified
or changed in composition and they erupt on the surface to form basaltic volcanoes.
In the typical "continental" environment, volcanoes are located unstable,
mountainous belts that have thick roots of granite or granite-like rock.
