Geology Reference
In-Depth Information
Geologists have devised the volcanic explosivity index
as a measure of an eruption's size.
Most volcanoes are located in belts at or near diver-
gent and convergent plate boundaries.
Some volcanoes are carefully monitored to help geolo-
gists anticipate eruptions.
ridges. Oceanic islands such as the Hawaiian Islands, Ice-
land, and the Azores owe their existence to volcanism, and
weathering of lava fl ows, pyroclastic materials, and volcanic
mudfl ows in tropical areas such as Indonesia converts them
to productive soils.
People who live in Hawaii, southern Alaska, the Philippines,
Japan, and Iceland are well aware of volcanic eruptions; however,
eruptions in the continental United States have occurred only
three times since 1914, all in the Cascade Range, which stretches
from northern California through Oregon and Washington and
into southern British Columbia, Canada. Canada has had no
eruptions during historic time. Ancient and ongoing volcanism
in the western United States has yielded interesting features,
several of which are featured in this chapter.
One very good reason to study volcanic eruptions is
that they illustrate the complex interactions among Earth's
systems. Volcanism, especially the emission of gases and
pyroclastic materials, has an immediate and profound
impact on the atmosphere, hydrosphere, and biosphere, at least
in the vicinity of an eruption. And in some cases, the effects are
worldwide, as they were following the eruptions of Tambora in
1815, Krakatau in 1883, and Pinatubo in 1991. Furthermore,
the fact that lava fl ows and explosive eruptions cause property
damage, injuries, fatalities (Table 5.1), and at least short-term
atmospheric changes indicates that volcanic eruptions are cat-
astrophic events, at least from the human perspective.
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No other geologic phenomenon has captured the public
imagination more than erupting volcanoes, especially lava
flows issuing forth as fiery streams or particulate matter
blasted into the atmosphere in sensational pyrotechnic dis-
plays. What better subject for a disaster movie? Several such
movies of varying quality and scientifi c accuracy have been
released in recent years, but one of the best was
Dante's Peak
in 1997. Certainly the writers and the director exaggerated
some aspects of volcanism, but the movie rather accurately
depicted the phenomenal power of an explosive eruption. In-
cidentally, the volcano called Dante's Peak was a 10-m-high
model built of wood and steel.
Incandescent streams of molten rock are often portrayed
in movies as a great danger to humans, and, in fact, on a
few occasions lava fl ows have caused fatalities. In 2002 lava
fl ows in Goma, Zarie (Democratic Republic of the Congo),
caused gasoline storage tanks to explode and kill 147 people.
Furthermore, lava fl ows destroy buildings and cover other-
wise productive land, but actually they are the least danger-
ous manifestation of volcanic eruptions. Explosive eruptions,
in contrast, are quite dangerous, especially if they occur near
populated areas.
One of the best-known volcanic catastrophes ever
recorded was the
A
.
D
. 79 eruption of Mount Vesuvius that
destroyed the thriving Roman communities of Pompeii,
Herculaneum, and Stabiae in what is now Italy (
What do we mean by the terms
volcanism
and
volcano
? The
latter is a landform—that is, a feature on Earth's surface—
whereas
volcanism
is the process by which magma rises
through Earth's crust and issues forth at the surface as lava
fl ows and/or pyroclastic materials and gases. Volcanism is
responsible for all extrusive igneous rocks, such as basalt,
tuff, and obsidian.
Volcanic eruptions are common; about 550 volcanoes
are
active
, that is, they have erupted during historic time,
but only about a dozen are erupting at any one time. Most
eruptions are minor and go unreported in the popular press
unless they occur near populated areas or have tragic conse-
quences. However, large eruptions that cause extensive prop-
erty damage, injuries, and fatalities are not uncommon, so a
great amount of effort is devoted to understanding and more
effectively anticipating large eruptions.
In addition to active volcanoes, Earth has numerous
dor-
mant
volcanoes that have not erupted during historic time
but may do so in the future. Prior to its eruption in
A
.
D
. 79,
Mount Vesuvius had not been active in human memory. The
largest volcanic outburst in the last 50 years was when Mount
Pinatubo in the Philippines erupted in 1991 after lying dor-
mant for 600 years. Some volcanoes have not erupted in his-
toric time and show no signs of doing so again; thousands of
these
extinct
or
inactive
volcanoes are known.
All terrestrial planets and Earth's Moon were volcani-
cally active during their early histories, but now volcanoes
are known only on Earth and on one or two other bodies in
Figure 5.1).
Fortunately for us, Pliny the Younger recorded the event in
detail; his uncle, Pliny the Elder, died while trying to investi-
gate the eruption.
Pompeii, a city of approximately 20,000 people only 9 km
downwind from the volcano, was buried in nearly 3 m of
pyroclastic materials that covered all but the tallest buildings
(Figure 5.1). About 2000 victims have been discovered in the
city, but certainly far more were killed. Pompeii was covered
by volcanic debris rather gradually, but surges of incandes-
cent volcanic materials in glowing avalanches swept through
Herculaneum, quickly burying the town to a depth of about
20 m. Since
A
.
D
. 79, Mount Vesuvius has erupted 80 times,
most violently in 1631 and 1906; it last erupted in 1944.
Ongoing volcanic and seismic activity in this area poses a
continuing threat to the many cities and towns along the
shores of the Bay of Naples (Figure 5.1).
Ironically, when considered in the context of Earth
history, volcanism is actually a constructive process. The
atmosphere and surface waters most likely resulted from the
emission of gases during Earth's early history, and oceanic
crust is continuously produced by volcanism at spreading
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