gold-bearing rocks, for example, that the wealth of the Inca
empire became an attraction for invaders from Spain. They
conquered the Inca and plundered the mineral wealth of their
empire. In similar fashion, plutonic formations in South Africa
have yielded large quantities of diamonds, with consequent
effects on the South African economy. Not all rich mineral
deposits associated with volcanic activity are so exotic; cop-
per and other minerals have played an equally important, if
not greater, role in the wealth of the countries in which they
have been mined. Volcanism may create new land that may
be settled profitably, and the tephra (ash and other ejecta)
from an eruption may itself have considerable economic value,
in applications ranging from abrasives to building material.
A minor but colorful economic benefit of volcanism in some
places is tourism. Certain volcanoes in the United States ben-
efit especially from tourism: notably Oregon’s Crater Lake;
the spectacular water-filled caldera left by an ancient erup-
tion; Mount Saint Helens, whose explosive eruption in 1980
made the then little-known volcano famous throughout the
world; and the volcanoes of Hawaii, whose relatively safe but
dramatic eruptions draw many visitors.
Although volcanism has been studied intensively for
thousands of years, the science of volcanology operates under
certain limitations. Except in a few locations such as Hawaii,
eruptions are only intermittent and cannot be predicted far in
advance. Many eruptions occur in isolated areas that are dif-
ficult to reach with the personnel and equipment required for
on-site studies. Despite these inconveniences, however, the sci-
ence of volcanology has advanced rapidly in the 20th century,
thanks to a variety of new sensing techniques and computer
analysis, which together have made possible the collection and
processing of great quantities of data on eruptions. The 1980
eruption of Mount Saint Helens alone expanded the scientific
knowledge of eruptions tremendously by giving volcanolo-
gists an active volcano to study within easy reach of numerous
well-equipped laboratories and other research facilities.
See also “year without a summer.”
Volcán Nuevo See Nevados de Chilian.
volcano, active See active volcano.
volcano A volcano is defined, in a broad sense, as an open-
ing in Earth’s crust through which magma escapes to the sur-
face where it is transformed into lava. More specifically, the
word volcano refers to mountains produced by volcanic activ-
ity, known as volcanism or vulcanism. (The words volcano,
VOLCANISM, and vulcanism all are derived from the Latin
Volasnus, or Vulcan, the god of fire in Roman mythology.)
In a volcanic eruption, magma or just the hot gases from
magma escape from an underground reservoir to the surface
through a relatively narrow vent, or conduit. Eruptions differ
greatly in character from one volcano to another and some-
times within the history of the same volcano. Some eruptions
are extremely violent and involve great outbursts of ash, gas,
and lava. These eruptions produce cinder cones and compos-
ite volcano.
A cinder cone is made up of fragments of rock ejected
from the vent. Paricutín in Mexico is one example of a cin-
der cone. These fragments tend to be low-density rock that
are formed when dissolved gases in the magma bubble out of
solution as the rock solidifies, producing pumice that appears
to be “rock foam,” full of small cavities that give the rock a
“frothy” texture. Rock fragments between about a half-inch
(1 cm) and two inches (5 cm) in diameter are called cinders
and constitute most of the cinder cone. Cinders are distinct
from ash, finer material that winds may carry for great dis-
tances away from the volcano. Though there is a significant
component of ash to every cinder cone. Mixed in with the
cinders, in most cases, are volcanic bombs that are formed as
large masses of lava are ejected intact. Cinder cones may grow
rapidly, rising hundreds of feet in the first few days or weeks
of their existence, but seldom reach heights of more than
1,000 feet (305 m). Cinder cones are typically steep-sided; the
slope of a newly formed cinder cone can be 28°. The crater
tends to be large and to have a rim higher on one side than the
other because of prevailing winds that carry the volcano’s out-
put in a given direction. Cinder cones may occur virtually any-
where the appropriate kind of magma rich in dissolved gases
can reach the surface. Clusters of cinder cones are common-
place. Eruptions of cinder cones may include lava flows.
A composite volcano is more complex than a single
cinder cone. Composite volcanoes are made up of layers of
cinder and ash alternating with lava. Because of these alter-
nating strata, composite volcanoes are known as strato-
volcanoes. A stratovolcano has steeply sloping sides, as
cinder cones have, but has greater structural strength due
to the rigid lava layers inside it. Stratovolcanoes may reach
thousands of feet in height. Examples of stratovolcanoes are
Vesuvius in Italy, Mount Fuji in Japan, and Mount Saint
Helens in the United States. Most stratovolcanoes are con-
centrated in two parts of the world: in the “Ring of Fire,”
a belt of intense volcanic and earthquake activity encircling
the Pacific Ocean basin; and in the Mediterranean Sea.
Eruptions of stratovolcanoes involve release of hot gases, ash,
cinders, bombs, and lava.
Eruptions may occur with such violence that they demol-
ish part of the mountain, as happened in the 1980 eruption of
Mount Saint Helens. An even more violent eruption, that of
the volcanic island Krakatoa in 1883, destroyed most of the
island and generated a tsunami that inundated shorelines in
the vicinity and killed 36,000 people. Eruptions of stratovol-
canoes sometimes produce calderas, when the entire central
portion of the volcano is blasted away or, alternatively, col-
lapses as magma within it is drained away. Young stratovol-
canoes are characterized by their conical shape and symmetry.
Older stratovolcanoes are less symmetrical because water and
ice have eroded their flanks. Erosion produces a character-
istic radial drainage pattern down the sides of the volcanoes.
Another, separate drainage pattern occurs in some cases within
the crater, as water flows downward from the rim to the floor
of the crater. As erosion continues, the mountain is worn down
gradually until only the volcanic neck, the solidified mass of
magma where the vent once extended upward to the summit,
remains. Ship Rock in New Mexico is a famous example of a
volcanic neck, from which dikes radiate. Because of their great
resistance to erosion, lava flows from stratovolcanoes may
endure as mesas rising above the surrounding terrain.
278 Volcán Nuevo