USGS/Cascades Volcano Observatory, Vancouver, Washington
Composite Volcanoes and Stratovolcanoes,
- Stratovolcanoes - Composite Volcanoes
- Composite Volcanoes and Shield Volcanoes
- Composite Volcanoes and Monogenetic Volcanoes
- Subduction-Zone Volcanoes and Composite Cones
Stratovolcanoes - Composite Volcanoes
Mount Rainier, Washington, as seen from Paradise, Mount Rainier National Park.
USGS Photograph taken in 1975 by Lyn Topinka.
[medium size] ...
Tilling, 1985, Volcanoes:
USGS General Interest Publication
Some of the Earth's grandest mountains are composite volcanoes --
sometimes called stratovolcanoes. They are typically steep-sided,
symmetrical cones of large dimension built of alternating layers of
lava flows, volcanic ash, cinders, blocks, and bombs
and may rise as much as 8,000 feet above their bases. Some of the most
conspicuous and beautiful mountains in the world are composite volcanoes,
Mount Fuji in Japan,
Mount Cotopaxi in Ecuador,
Mount Shasta in California,
Mount Hood in Oregon,
Mount St. Helens and
Mount Rainier in Washington.
Most composite volcanoes have a crater at the summit which contains a central
vent or a clustered group of vents. Lavas either flow through breaks in the
crater wall or issue from fissures on the flanks of the cone. Lava, solidified
within the fissures, forms dikes
that act as ribs which greatly strengthen the cone.
The essential feature of a composite volcano is a conduit system through which
magma from a reservoir deep in the Earth's crust rises to the surface. The
volcano is built up by the accumulation of material erupted through the conduit
and increases in size as lava, cinders, ash, etc., are added to its slopes.
When a composite volcano becomes dormant, erosion begins to destroy the cone.
As the cone is stripped away, the hardened magma filling the conduit (the
volcanic plug) and fissures (the dikes) becomes exposed, and it too is slowly
reduced by erosion. Finally, all that remains is the plug and dike complex
projecting above the land surface -- a telltale remnant of the vanished volcano.
An interesting variation of a composite volcano can be seen at
in Oregon. From what geologists can interpret of its past, a high volcano --
Mount Mazama -- probably similar in appearance to present-day
was once located at this spot. Following a series of tremendous
explosions about 6,600 years ago, the volcano lost its top. Enormous volumes of
volcanic ash and dust were expelled and swept down the slopes as ash flows and
avalanches. These large-volume explosions rapidly drained the lava beneath the
mountain and weakened the upper part. The top then collapsed to form a large
depression, which later filled with water and is now completely occupied by
beautiful Crater Lake.
A last gasp of eruptions produced a small
which rises above the water surface as
in, and near the rim, of the lake.
Depressions such as Crater Lake, formed by collapse of volcanoes, are
They are usually large, steep-walled, basin-shaped depressions formed by the
collapse of a large area over, and around, a volcanic vent or vents. Calderas
range in form and size from roughly circular depressions 1 to 15 miles in
diameter to huge elongated depressions as much as 60 miles long.
Composite Volcanoes and Shield Volcanoes
Tilling, Topinka, and Swanson, 1990,
Eruptions of Mount St. Helens: Past, Present, and Future:
USGS General Interest Publication
Mount St. Helens
a composite volcano (or stratovolcano),
a term for steep-sided, often symmetrical cones
constructed of alternating layers of lava flows, ash, and other volcanic debris.
Composite volcanoes tend to erupt explosively and pose considerable
danger to nearby life and property.
In contrast, the gently sloping
those in Hawaii,
typically erupt nonexplosively, producing fluid lavas
that can flow great distances from the active vents. Although Hawaiian-type
eruptions may destroy property, they rarely cause death or injury.
Composite Volcanoes and Monogenetic Volcanoes
Walder, et.al., 1999,
Volcano Hazards in the Mount Jefferson Region, Oregon:
USGS Open-File Report 99-24
Two types of volcanoes are found in the Mount
Composite volcanoes erupt episodically over tens
to hundreds of thousand of years and can display a
wide range of eruption styles.
is a composite volcano that has been active
episodically for about 300,000 years.
typically erupt for only
brief time intervals -- weeks to perhaps
centuries -- and generally display a narrower
range in eruptive behavior. Most monogenetic
volcanoes are basaltic in composition, but just
north of Mount Jefferson a few are of andesite
and dacite composition -- that is, with a relatively
higher silica content. Over a
time span of hundreds of thousands of years,
these monogenetic volcanoes have built a broad
upland areas (hundreds to thousands of square
kilometers (miles)) of mostly basaltic lava flows
and small volcanoes. Prominent basaltic
volcanoes in the Mount Jefferson region include
Olallie Butte, Potato Butte, Sisi Butte, and North
and South Cinder Peaks. Fresh-looking basalt
lava flows can be seen along the Cabot Creek,
Jefferson Creek, and upper Puzzle Creek
drainages. Hundreds more basaltic volcanoes
form the High Cascades of central Oregon to the
south of Mount Jefferson, as far as Crater Lake,
180 kilometers (110 miles) away.
Subduction-Zone Volcanoes and Composite Cones
Kious and Tilling, 1996,
This Dynamic Earth: The Story of Plate Tectonics:
USGS General Interest Publication
As with earthquakes,
volcanic activity is linked to plate-tectonic processes.
Most of the world's active above-sea
volcanoes are located near convergent plate boundaries where subduction is occurring,
particularly around the Pacific
basin. However, much more volcanism -- producing about three quarters of all lava
erupted on Earth -- takes place
unseen beneath the ocean, mostly along the oceanic spreading centers,
such as the Mid-Atlantic Ridge and the East
Subduction-zone volcanoes like
Mount St. Helens
(in Washington State) and
Mount Pinatubo (Luzon, Philippines),
are called composite cones
and typically erupt with explosive force, because the magma is too stiff to
allow easy escape of
volcanic gases. As a consequence, tremendous internal pressures mount as the
trapped gases expand during ascent,
before the pent-up pressure is suddenly released in a violent eruption.
Such an explosive process can be compared to
putting your thumb over an opened bottle of a carbonated drink,
shaking it vigorously, and then quickly removing the
thumb. The shaking action separates the gases from the liquid to form bubbles,
increasing the internal pressure. Quick
release of the thumb allows the gases and liquid to gush out
with explosive speed and force.
In 1991, two volcanoes on the western edge of the
Philippine Plate produced major eruptions. On June 15,
spewed ash 40 kilometers into the air and produced huge ash flows
(also called pyroclastic flows) and mudflows that
devastated a large area around the volcano.
Pinatubo, located 90 kilometers from Manila, had been dormant for 600 years
before the 1991 eruption, which ranks as one of the largest eruptions in this century.
Also in 1991, Japan's
located on the Island of Kyushu about 40 kilometers east of Nagasaki,
awakened from its 200-year slumber to
produce a new lava dome at its summit. Beginning in June,
repeated collapses of this active dome generated destructive
ash flows that swept down its slopes at speeds as high as 200 kilometers per hour.
Unzen is one of more than 75 active
volcanoes in Japan; its eruption in 1792 killed more than
15,000 people -- the worst volcanic disaster in the country's
While the Unzen eruptions have caused deaths and considerable local damage,
the impact of the
June 1991 eruption of
Mount Pinatubo was global. Slightly cooler than usual temperatures
recorded worldwide and the brilliant sunsets and
sunrises have been attributed to this eruption that sent fine ash and
gases high into the stratosphere, forming a large
volcanic cloud that drifted around the world. The sulfur dioxide (SO2)
in this cloud -- about 22 million tons -- combined
with water to form droplets of sulfuric acid, blocking some of the
sunlight from reaching the Earth and thereby cooling
temperatures in some regions by as much as 0.5 °C. An eruption the
size of Mount Pinatubo could affect the weather for
a few years. A similar phenomenon occurred in April of 1815
with the cataclysmic eruption of
in Indonesia, the most powerful eruption in recorded history.
Tambora's volcanic cloud lowered global temperatures by as
much as 3 °C. Even a year after the eruption, most of the
northern hemisphere experienced sharply cooler temperatures
during the summer months. In part of Europe and in North America,
1816 was known as "the year without a summer."
Apart from possibly affecting climate, volcanic clouds from explosive eruptions
also pose a hazard to aviation safety.
During the past two decades, more than 60 airplanes, mostly commercial jetliners,
have been damaged by in-flight
encounters with volcanic ash. Some of these encounters have resulted in the power
loss of all engines, necessitating
emergency landings. Luckily, to date no crashes have happened because of
jet aircraft flying into volcanic ash.
Since the year A.D. 1600, nearly 300,000 people have been killed by volcanic eruptions.
Most deaths were caused by
pyroclastic flows and mudflows, deadly hazards which often accompany explosive
eruptions of subduction-zone volcanoes.
Pyroclastic flows, also called nuées ardentes ("glowing clouds" in French),
are fast-moving, avalanche-like,
ground-hugging incandescent mixtures of hot volcanic debris, ash, and gases that
can travel at speeds in excess of 150 kilometers
per hour. Approximately 30,000 people were killed by pyroclastic flows during the
1902 eruption of
on the Island of Martinique in the Caribbean. In March-April 1982,
three explosive eruptions of
El Chichon Volcano in the State
of Chiapas, southeastern Mexico, caused the worst volcanic disaster in that
country's history. Villages within 8 kilometers of the
volcano were destroyed by pyroclastic flows, killing more than 2,000 people.
Mudflows (also called debris flows or lahars, an Indonesian term for volcanic mudflows)
are mixtures of volcanic debris
and water. The water usually comes from two sources:
rainfall or the melting of snow and ice by hot volcanic debris.
Depending on the proportion of water to volcanic material,
mudflows can range from soupy floods to thick flows that
have the consistency of wet cement.
As mudflows sweep down the steep sides of composite volcanoes, they have the
strength and speed to flatten or bury everything in their paths.
Hot ash and pyroclastic flows from the eruption of the
Nevado del Ruiz Volcano
in Colombia, South America,
melted snow and ice atop the 5,390-meter-high Andean peak; the
ensuing mudflows buried the city of Armero, killing 25,000 people.
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05/10/05, Lyn Topinka