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Shield Volcano

Shield Volcanoes

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Belknap Shield Volcano, Oregon.
USGS Photograph taken on October 1, 1984, by Lyn Topinka.
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From: 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 Hawaiian Islands are composed of linear chains of these volcanoes including Kilauea and 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. Lava 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 total thickness.

Plate Tectonics and Shield Volcanoes

From: Kious and Tilling, 1996, This Dynamic Earth: The Story of Plate Tectonics: USGS Special 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 Pacific Rise.

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. ...

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 shield volcanoes, 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.

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Central Oregon Shield Volcanoes

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Mount Bachelor as seen from South Sister.
USGS Photograph taken in February 9, 2005, by Gene Iwatsubo.
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From: 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 cinder cones. 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 Three-Fingered Jack, Mount Washington and North Sister, Mount Bachelor, Diamond Peak, Mount Bailey, and Mount Thielsen, and Mount McLoughlin. 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. Mount Bachelor, which is between 11,000 and 15,000 years old, is the youngest of these volcanoes in the Cascades.

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Hawaiian Shield Volcanoes

From: Tilling, Heliker, and Wright, 1987, Eruptions of Hawaiian Volcanoes: Past, Present, and Future: USGS General Interest Publication
Hawaiian volcanoes 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 composite volcanoes are dwarfed by the Hawaiian shield volcanoes.

Hawaiian and other shield volcanoes characteristically have a broad summit, indented with a caldera, a term commonly used for a large depression of volcanic origin. ...

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Medicine Lake Shield Volcano, California

From: Dzurisin,, 1991, Crustal Subsidence, Seismicity, and Structure Near Medicine Lake Volcano, California: Journal of Geophysical Research, v.96, no.B10
Medicine Lake volcano (California) 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.

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12/28/05, Lyn Topinka