CHAPTER 4 Volcanoes and Volcanic Hazards
100
parasitic cone (parasitus one who eats at
the table of another). Italy’ s Mount Etna, for
example, has more than 200 secondary
vents, some of which have built parasitic
cones. Many of these vents, however, emit
only gases and are appropriately called
fumaroles (fumus smoke).
Shield Volcanoes
Shield volcanoes are produced by the accumulation of fluid basaltic lavas and exhibit
the shape of a broad, slightly domed structure that resembles a warrior’ s shield
(FIGURE 4.13). Most shield volcanoes begin
on the ocean floor as seamounts, a few of
which grow large enough to form volcanic
islands. In fact, with the exception of the
volcanic islands that form above subduction
zones, most other oceanic islands are either
single shield volcanoes, or more often the
coalescence of two or more shields built
upon massive amounts of pillow lavas.
Examples include the Canary Islands, the
Hawaiian Islands, the Galapagos, and Easter
=
=
Island. In addition, some shield volcanoes
form on continental crust. Included in this
group are several volcanic structures located
in East Africa.
MAUNA LOA: A CLASSIC SHIELD VOLCANO. Extensive study of the Hawaiian
Islands confirms that they are constructed
of a myriad of thin basaltic lava flows averaging a few meters thick intermixed with
relatively minor amounts of pyroclastic
ejecta. Mauna Loa is one of five overlapping shield volcanoes that together comprise the Big Island of Hawaii (see Figure
4.13). From its base on the floor of the
Pacific Ocean to its summit, Mauna Loa is
over 9 kilometers (6 miles) high, exceeding
the height of Mount Everest. This massive
pile of basaltic rock has a volume of
80,000 cubic kilometers that was extruded
over a span of about 1 million years. The
volume of material composing Mauna Loa
is roughly 200 times greater than the
amount composing a large composite cone
such as Mount Rainier (FIGURE 4.14).
Although the shield volcanoes that
comprise islands are often quite large,
some are more modest in size. In addition,
an estimated 1 million basaltic submarine
volcanoes (seamounts) of various sizes dot
the ocean floor.
The flanks of Mauna Loa have gentle
slopes of only a few degrees. The low angle
results because very hot, fluid lava travels
“fast and far” from the vent. In addition,
most of the lava (perhaps 80 percent) flows
through a well-developed system of lava
tubes (see Figure 4.7). This greatly
increases the distance lava can travel before
it solidifies. Thus, lava emitted near the
summit often reaches the sea, thereby
adding to the width of the cone at the
expense of its height.
Another feature common to many
active shield volcanoes is a large, steepwalled caldera that occupies the summit
(see Figure 4.26). Calderas on large shield
volcanoes form when the roof above the
magma chamber collapses. This usually
occurs as the magma reservoir empties
following a large eruption or as magma
migrates to the flank of a volcano to feed a
fissure eruption.
In the final stage of growth, shield volcanoes are more sporadic, and pyroclastic
Crater
Mount
Vesuvius,
Italy
A.
B.
Kilometers
0
1
2
3
4
5
Caldera
Mount
Tambora,
Indonesia
Kilometers
0
1
2
3
4
5
FIGURE 4.12 Crater versus
caldera. A. The crater of
Mount Vesuvius, Italy, is
about 0.5 kilometer in
diameter. The city of Naples
is located northwest of
Vesuvius, whereas Pompeii, the Roman town that was buried by an eruption in AD 79, is located
southeast of the volcano. B. The huge caldera—6 kilometers in diameter—formed when Tambora’s
peak was removed during an explosive eruption in 1815. (Photos courtesy of NASA)
Located at the summit of most volcanoes is a somewhat funnel-shaped depression, called a crater (crater a bowl).
Volcanoes that are built primarily of pyroclastic materials typically have craters that
form by gradual accumulation of volcanic
debris on the surrounding rim. Other
craters form during explosive eruptions as
the rapidly ejected particles erode the crater
walls. Craters also form when the summit
area of a volcano collapses following an
eruption (FIGURE 4.12). Some volcanoes
have very large circular depressions called
calderas that have diameters greater than
one kilometer and in rare cases can exceed
50 kilometers. We will consider the formation of various types of calderas later in this
chapter.
During early stages of growth most volcanic discharges come from a central summit vent. As a volcano matures, material
also tends to be emitted from fissures that
develop along the flanks or at the base of
the volcano. Continued activity from a
flank eruption may produce a small
=
100
parasitic cone (parasitus one who eats at
the table of another). Italy’ s Mount Etna, for
example, has more than 200 secondary
vents, some of which have built parasitic
cones. Many of these vents, however, emit
only gases and are appropriately called
fumaroles (fumus smoke).
Shield Volcanoes
Shield volcanoes are produced by the accumulation of fluid basaltic lavas and exhibit
the shape of a broad, slightly domed structure that resembles a warrior’ s shield
(FIGURE 4.13). Most shield volcanoes begin
on the ocean floor as seamounts, a few of
which grow large enough to form volcanic
islands. In fact, with the exception of the
volcanic islands that form above subduction
zones, most other oceanic islands are either
single shield volcanoes, or more often the
coalescence of two or more shields built
upon massive amounts of pillow lavas.
Examples include the Canary Islands, the
Hawaiian Islands, the Galapagos, and Easter
=
=
Island. In addition, some shield volcanoes
form on continental crust. Included in this
group are several volcanic structures located
in East Africa.
MAUNA LOA: A CLASSIC SHIELD VOLCANO. Extensive study of the Hawaiian
Islands confirms that they are constructed
of a myriad of thin basaltic lava flows averaging a few meters thick intermixed with
relatively minor amounts of pyroclastic
ejecta. Mauna Loa is one of five overlapping shield volcanoes that together comprise the Big Island of Hawaii (see Figure
4.13). From its base on the floor of the
Pacific Ocean to its summit, Mauna Loa is
over 9 kilometers (6 miles) high, exceeding
the height of Mount Everest. This massive
pile of basaltic rock has a volume of
80,000 cubic kilometers that was extruded
over a span of about 1 million years. The
volume of material composing Mauna Loa
is roughly 200 times greater than the
amount composing a large composite cone
such as Mount Rainier (FIGURE 4.14).
Although the shield volcanoes that
comprise islands are often quite large,
some are more modest in size. In addition,
an estimated 1 million basaltic submarine
volcanoes (seamounts) of various sizes dot
the ocean floor.
The flanks of Mauna Loa have gentle
slopes of only a few degrees. The low angle
results because very hot, fluid lava travels
“fast and far” from the vent. In addition,
most of the lava (perhaps 80 percent) flows
through a well-developed system of lava
tubes (see Figure 4.7). This greatly
increases the distance lava can travel before
it solidifies. Thus, lava emitted near the
summit often reaches the sea, thereby
adding to the width of the cone at the
expense of its height.
Another feature common to many
active shield volcanoes is a large, steepwalled caldera that occupies the summit
(see Figure 4.26). Calderas on large shield
volcanoes form when the roof above the
magma chamber collapses. This usually
occurs as the magma reservoir empties
following a large eruption or as magma
migrates to the flank of a volcano to feed a
fissure eruption.
In the final stage of growth, shield volcanoes are more sporadic, and pyroclastic
Crater
Mount
Vesuvius,
Italy
A.
B.
Kilometers
0
1
2
3
4
5
Caldera
Mount
Tambora,
Indonesia
Kilometers
0
1
2
3
4
5
FIGURE 4.12 Crater versus
caldera. A. The crater of
Mount Vesuvius, Italy, is
about 0.5 kilometer in
diameter. The city of Naples
is located northwest of
Vesuvius, whereas Pompeii, the Roman town that was buried by an eruption in AD 79, is located
southeast of the volcano. B. The huge caldera—6 kilometers in diameter—formed when Tambora’s
peak was removed during an explosive eruption in 1815. (Photos courtesy of NASA)
Located at the summit of most volcanoes is a somewhat funnel-shaped depression, called a crater (crater a bowl).
Volcanoes that are built primarily of pyroclastic materials typically have craters that
form by gradual accumulation of volcanic
debris on the surrounding rim. Other
craters form during explosive eruptions as
the rapidly ejected particles erode the crater
walls. Craters also form when the summit
area of a volcano collapses following an
eruption (FIGURE 4.12). Some volcanoes
have very large circular depressions called
calderas that have diameters greater than
one kilometer and in rare cases can exceed
50 kilometers. We will consider the formation of various types of calderas later in this
chapter.
During early stages of growth most volcanic discharges come from a central summit vent. As a volcano matures, material
also tends to be emitted from fissures that
develop along the flanks or at the base of
the volcano. Continued activity from a
flank eruption may produce a small
=