105
Living in the Shadow of a Composite Cone
activity, and molten lava has been observed
in their summit craters for decades. Stromboli is so well known for eruptions that
eject incandescent blobs of lava that it has
been referred to as the “Lighthouse of the
Mediterranean.” Mount Etna, on the other
hand, has erupted, on average, once every
2 years since 1979.
Just as shield volcanoes owe their
shape to fluid basaltic lavas, composite
cones reflect the viscous nature of the material from which they are made. In general,
composite cones are the product of gas-rich
magma having an andesitic composition.
However, many composite cones also emit
various amounts of fluid basaltic lava and
occasionally pyroclastic material having
rhyolitic composition. Relative to shields,
the silica-rich magmas typical of composite
cones generate thick viscous lavas that
travel less than a few kilometers. In addition, composite cones are noted for generating explosive eruptions that eject huge
quantities of pyroclastic material.
A conical shape, with a steep summit
area and more gradually sloping flanks, is
typical of many large composite cones. This
classic profile, which adorns calendars and
postcards, is partially a consequence of the
way viscous lavas and pyroclastic ejecta
contribute to the growth of the cone.
Coarse fragments ejected from the summit
crater tend to accumulate near their
source. Because of their high
angle of repose, coarse materials contribute
to the steep slopes of the summit area.
Finer ejecta, on the other hand, are
deposited as a thin layer over a large area.
This acts to flatten the flank of the cone. In
addition, during the early stages of growth,
lavas tend to be more abundant and flow
greater distances from the vent than lavas
do later in the volcano’ s history. This contributes to the cone’ s broad base. As the
volcano matures, the shorter flows that
come from the central vent serve to armor
and strengthen the summit area. Consequently, steep slopes exceeding 40 degrees
are sometimes possible. Two of the most
perfect cones—Mount Mayon in the Philippines and Fujiyama in Japan—exhibit the
classic form we expect of a composite cone,
with its steep summit and gently sloping
flanks (FIGURE 4.19).
Despite the symmetrical forms of many
composite cones, most have complex histories. Huge mounds of volcanic debris surrounding these structures provide evidence
that large sections of these volcanoes slid
downslope as massive landslides. Others
develop horseshoe-shaped depressions at
their summits as a result of explosive lateral
eruptions—as occurred during the 1980
eruption of Mount St. Helens. Often, so
much rebuilding has occurred since these
eruptions that no trace of the amphitheatershaped scars remain.
Many composite cones have numerous
small, parasitic cones on their flanks, while
others, such as Crater Lake, have been
truncated by the collapse of their summit
(see FIGURE 4.25). Still others have a lake in
their crater that may be hot and muddy.
Such lakes are often highly acidic because
of the influx of sulfur and chlorine gases
that react with water to produce sulfuric
(H 2 SO 4 ) and hydrochloric acid (HCl).
C O N C E P T C H E C K 4 . 4
Compare a volcanic crater to a caldera.
Compare and contrast the three main
types of volcanoes (consider size, composition, shape, and eruptive style).
Name a prominent volcano for each of the
three types of volcanoes.
Briefly compare the eruptions of Kilauea
and Parícutin.
4
3
2
1
Living in the Shadow
of a Composite Cone
More than 50 volcanoes have erupted in
the United States in the past 200 years
(FIGURE 4.20). Fortunately, the most explosive of these eruptions occurred in sparsely
inhabited regions of Alaska. On a global
scale many destructive eruptions have
occurred during the past few thousand
years, a few of which may have influenced
the course of human civilization.
Nuée Ardente: A Deadly
Pyroclastic Flow
One of the most destructive forces of nature
is the pyroclastic flow, which consists of hot
gases infused with incandescent ash and
larger lava fragments. Also referred to as
nuée ardentes (glowing avalanches), these
Mount
Baker
Mount
Jefferson
Three
Sisters
Crater Lake
Mount
Shasta
Mount
Rainier
Mount
St. Helens
Mount
Adams
Mount
Hood
Lassen
Peak
Medicine
Lake Volcano
Newberry
Volcano
Glacier Peak
CA
OR
WA
FIGURE 4.20 Of the 13 potentially active
volcanoes in the Cascade Range, 11 have erupted
in the past 4000 years and 7 in just the past 200
years. More than 100 eruptions, most of which
were explosive, have occurred in the past 4000
years. Mount St. Helens is the most active volcano
in the Cascades. Its eruptions have ranged from
relatively quiet outflows of lava to explosive
events much larger than that of May 18, 1980.
Living in the Shadow of a Composite Cone
activity, and molten lava has been observed
in their summit craters for decades. Stromboli is so well known for eruptions that
eject incandescent blobs of lava that it has
been referred to as the “Lighthouse of the
Mediterranean.” Mount Etna, on the other
hand, has erupted, on average, once every
2 years since 1979.
Just as shield volcanoes owe their
shape to fluid basaltic lavas, composite
cones reflect the viscous nature of the material from which they are made. In general,
composite cones are the product of gas-rich
magma having an andesitic composition.
However, many composite cones also emit
various amounts of fluid basaltic lava and
occasionally pyroclastic material having
rhyolitic composition. Relative to shields,
the silica-rich magmas typical of composite
cones generate thick viscous lavas that
travel less than a few kilometers. In addition, composite cones are noted for generating explosive eruptions that eject huge
quantities of pyroclastic material.
A conical shape, with a steep summit
area and more gradually sloping flanks, is
typical of many large composite cones. This
classic profile, which adorns calendars and
postcards, is partially a consequence of the
way viscous lavas and pyroclastic ejecta
contribute to the growth of the cone.
Coarse fragments ejected from the summit
crater tend to accumulate near their
source. Because of their high
angle of repose, coarse materials contribute
to the steep slopes of the summit area.
Finer ejecta, on the other hand, are
deposited as a thin layer over a large area.
This acts to flatten the flank of the cone. In
addition, during the early stages of growth,
lavas tend to be more abundant and flow
greater distances from the vent than lavas
do later in the volcano’ s history. This contributes to the cone’ s broad base. As the
volcano matures, the shorter flows that
come from the central vent serve to armor
and strengthen the summit area. Consequently, steep slopes exceeding 40 degrees
are sometimes possible. Two of the most
perfect cones—Mount Mayon in the Philippines and Fujiyama in Japan—exhibit the
classic form we expect of a composite cone,
with its steep summit and gently sloping
flanks (FIGURE 4.19).
Despite the symmetrical forms of many
composite cones, most have complex histories. Huge mounds of volcanic debris surrounding these structures provide evidence
that large sections of these volcanoes slid
downslope as massive landslides. Others
develop horseshoe-shaped depressions at
their summits as a result of explosive lateral
eruptions—as occurred during the 1980
eruption of Mount St. Helens. Often, so
much rebuilding has occurred since these
eruptions that no trace of the amphitheatershaped scars remain.
Many composite cones have numerous
small, parasitic cones on their flanks, while
others, such as Crater Lake, have been
truncated by the collapse of their summit
(see FIGURE 4.25). Still others have a lake in
their crater that may be hot and muddy.
Such lakes are often highly acidic because
of the influx of sulfur and chlorine gases
that react with water to produce sulfuric
(H 2 SO 4 ) and hydrochloric acid (HCl).
C O N C E P T C H E C K 4 . 4
Compare a volcanic crater to a caldera.
Compare and contrast the three main
types of volcanoes (consider size, composition, shape, and eruptive style).
Name a prominent volcano for each of the
three types of volcanoes.
Briefly compare the eruptions of Kilauea
and Parícutin.
4
3
2
1
Living in the Shadow
of a Composite Cone
More than 50 volcanoes have erupted in
the United States in the past 200 years
(FIGURE 4.20). Fortunately, the most explosive of these eruptions occurred in sparsely
inhabited regions of Alaska. On a global
scale many destructive eruptions have
occurred during the past few thousand
years, a few of which may have influenced
the course of human civilization.
Nuée Ardente: A Deadly
Pyroclastic Flow
One of the most destructive forces of nature
is the pyroclastic flow, which consists of hot
gases infused with incandescent ash and
larger lava fragments. Also referred to as
nuée ardentes (glowing avalanches), these
Mount
Baker
Mount
Jefferson
Three
Sisters
Crater Lake
Mount
Shasta
Mount
Rainier
Mount
St. Helens
Mount
Adams
Mount
Hood
Lassen
Peak
Medicine
Lake Volcano
Newberry
Volcano
Glacier Peak
CA
OR
WA
FIGURE 4.20 Of the 13 potentially active
volcanoes in the Cascade Range, 11 have erupted
in the past 4000 years and 7 in just the past 200
years. More than 100 eruptions, most of which
were explosive, have occurred in the past 4000
years. Mount St. Helens is the most active volcano
in the Cascades. Its eruptions have ranged from
relatively quiet outflows of lava to explosive
events much larger than that of May 18, 1980.