USGS/Cascades Volcano Observatory, Vancouver, Washington
- Shield Volcanoes
- Plate Tectonics and Shield Volcanoes
- Central Oregon Shield Volcanoes
- Hawaiian Shield Volcanoes
- Medicine Lake Shield Volcano, California
Belknap Shield Volcano, Oregon.
USGS Photograph taken on October 1, 1984, by Lyn Topinka.
[medium size] ...
[large size] ...
[TIF Format, 25 M] ...
Tilling, 1985, Volcanoes, USGS General Interest Publication
Shield volcanoes ... are built almost entirely of
fluid lava flows.
Flow after flow pours out in all directions from a central summit vent, or group
of vents, building a broad, gently sloping cone of flat, domical shape, with a
profile much like that a a warrior's shield. They are built up slowly by the
accretion of thousands of flows of highly fluid basaltic (from basalt, a
hard, dense dark volcanic rock) lava that spread widely over great distances,
and then cool as thin, gently dipping sheets. Lavas also commonly erupt from
vents along fractures (rift zones) that develop on the flanks of the cone. Some
of the largest volcanoes in the world are shield volcanoes. In northern
California and Oregon, many shield volcanoes have diameters of 3 or 4 miles and
heights of 1,500 to 2,000 feet. The
are composed of linear chains of these volcanoes including
Mauna Loa on the
island of Hawaii -- two of the world's most active volcanoes. The floor of the
ocean is more than 15,000 feet deep at the bases of the islands.
As Mauna Loa,
the largest of the shield volcanoes (and also the world's largest active
volcano), projects 13,677 feet above sea level, its top is over 28,000 feet
above the deep ocean floor.
In some shield-volcano eruptions, basaltic lava pours out quietly from
long fissures instead of central vents and floods the surrounding countryside with
lava flow upon lava flow, forming broad plateaus.
of this type can be seen in Iceland, southeastern Washington, eastern Oregon, and southern
Idaho. Along the
Snake River in Idaho, and the
Columbia River in Washington and Oregon,
these lava flows are beautifully exposed and measure more than a mile in
Plate Tectonics and Shield Volcanoes
Kious and Tilling, 1996,
This Dynamic Earth: The Story of Plate Tectonics:
USGS Special Interest Publication
As with earthquakes, volcanic activity is linked to
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),
and typically erupt with explosive force, because the magma is too stiff to
allow easy escape of
volcanic gases. ...
Eruptions of Hawaiian and most other mid-plate volcanoes differ greatly from
those of composite cones. Mauna Loa and
Kilauea, on the island of Hawaii, are known as
because they resemble the wide, rounded shape of an
ancient warrior's shield. Shield volcanoes tend to erupt non-explosively,
mainly pouring out huge volumes of fluid lava.
Hawaiian-type eruptions are rarely life threatening because the lava advances
slowly enough to allow safe evacuation of
people, but large lava flows can cause considerable economic loss by destroying
property and agricultural lands. For
example, lava from the ongoing eruption of Kilauea, which began in J
anuary 1983, has destroyed more than 200
structures, buried kilometers of highways, and disrupted the daily
lives of local residents. Because Hawaiian volcanoes
erupt frequently and pose little danger to humans, they provide an
ideal natural laboratory to safely study volcanic
phenomena at close range. The
USGS Hawaiian Volcano Observatory,
on the rim of Kilauea, was among the world's
first modern volcano observatories, established early in this century.
Plate Tectonics Menu
Central Oregon Shield Volcanoes
Mount Bachelor as seen from South Sister.
USGS Photograph taken in February 9, 2005, by Gene Iwatsubo.
[medium size] ...
Hoblitt, Miller, and Scott, 1987,
Volcanic Hazards with Regard to Siting Nuclear-Power Plants
in the Pacific Northwest:
USGS Open-File Report 87-297
Another type of basaltic activity is characterized by the concentration
of many tephra and lava-flow eruptions at a central
vent and several flank vents. This type of activity has built
shield volcanoes typically 5-15 kilometers in diameter and several
hundred meters to more than 1000 meters high. Many have summit
Belknap in central Oregon
is the youngest
such shield volcano in the Cascades and has lava flows as young as 1400 years.
Several large basaltic shield volcanoes along the range have
steep-sided summit cones, such as
Mount Washington and
Mount Bailey, and
Mount Thielsen, and
A few of these volcanoes contain rocks as silicic
as andesite and may have been constructed during
several eruptive episodes. These peaks rival the major composite cones
in size but contrast with them in origin and
structure. Most are composed of central scoria and tuff cones intruded
by numerous dikes and one or more plugs. Thin lava
flows intertongue with the scoria and mantle the central cone, and
more voluminous lava flows typically extend beyond
the base of the central cone. No evidence suggests that these volcanoes
formed during highly explosive eruptions. Most
lava flows and thick tephra-fall deposits are restricted within a
few kilometers of vents, and scoriaceous tephras are
typically not traceable farther than 20 kilometers from vents.
which is between 11,000 and 15,000 years old, is
the youngest of these volcanoes in the Cascades.
Central Oregon High Cascades Menu
Hawaiian Shield Volcanoes
Tilling, Heliker, and Wright, 1987, Eruptions of Hawaiian Volcanoes:
Past, Present, and Future:
USGS General Interest Publication
exemplify the common type of volcano called a shield volcano,
built by countless outpourings of fluid lava flows that advance
great distances from a central summit vent or group of vents. The successive
piling up these flows results in a broad, gently sloping, convex-upward
landform, whose profile resembles that of a Roman warrior's shield.
The Hawaiian shield volcanoes are the largest mountains on Earth.
Mauna Kea Volcano
rises 13,796 feet above sea level but extends about 19,700 feet below
sea level to meet the deep ocean floor. Its total height is nearly 33,500 feet,
considerable higher than the height of the tallest mountain on land, Mount
Everest in the Himalaya (29,028 feet above sea level).
Mauna Loa stands no
quite as high as Mauna Kea but is much larger in volume. The profile of the
Mauna Loa shield appears smooth, whereas the shield profile of Mauna Kea has a
more uneven appearance, reflecting the growth of numerous small cinder cones on
its upper slopes after shield formation. In size
are dwarfed by the Hawaiian shield volcanoes.
Hawaiian and other shield volcanoes characteristically have a broad summit,
indented with a
a term commonly used for a large depression of
volcanic origin. ...
Link to: Hawaiian Volcano Observatory Website for MORE Information
Medicine Lake Shield Volcano, California
Dzurisin, et.al., 1991,
Crustal Subsidence, Seismicity, and Structure Near Medicine Lake Volcano,
California: Journal of Geophysical Research, v.96, no.B10
Medicine Lake volcano
is a Pleistocene-Holocene shield volcano ...
Lavas from Medicine lake volcano cover nearly 2000 square kilometers, and their
volume is estimated to be at least 600 cubic kilometers, making it the largest
volcano by volume in the Cascade Range.
Medicine Lake Menu
[Shield Volcano Menu] ...
[Lava Plateaus and Flood Basalts Menu] ...
[Volcano Types Menu] ...
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12/28/05, Lyn Topinka