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 encircle the Pacific Ocean as a “Ring of
Fire.”
[Illustration: The distribution of some of the Earth’s 500 active
volcanoes.]
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,
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 channelways 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 granitelike 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.
[Illustration: Mount Sinabung, Sumatra (By J. Baylor Roberts (c)
National Geographic Society).]
[Illustration: Island-arc environment]
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.
[Illustration: Mauna Kea Volcano, Hawaii.]
[Illustration: Oceanic environment]
In the typical “continental” environment, volcanoes are located in
unstable, mountainous belts that have thick roots of granite or
granitelike rock. Magmas, generated near the base of the mountain root,
rise slowly or intermittently along fractures in the crust. During
passage through the granitic layer, magmas are commonly modified or
changed in composition and erupt on the surface to form volcanoes
constructed of nonbasaltic rocks.
[Illustration: Mount Adams, Washington.]
[Illustration: Continental environment]