Shield volcano

Shield volcano

A shield volcano is a volcano with shallow-sloping sides. Shield volcanoes are formed from fluid lava that can travel long distances across slight inclines, resulting in their relatively flat, broad profile. In contrast, steeply sloped stratovolcanoes better match the popular stereotype of a volcano.

Some of the largest volcanoes on Earth are shield volcanoes.

Characteristics

The name derives from a translation of "Skjaldbreiður", an Icelandic shield volcano whose name means "broad shield", from its resemblance to a warrior's shield.

Shield volcanoes are formed by lava flows of low viscosity — lava that flows easily. Consequently, a volcanic mountain having a broad profile is built up over time by flow after flow of relatively fluid basaltic lava issuing from vents or fissures on the surface of the volcano. The largest in terms of area covered is Mauna Loa of Hawaii; the tallest measured from its base under the ocean, however, is Mauna Kea of Hawai'i. All the volcanoes in the Hawaiian Islands are shield volcanoes. Many of the largest volcanoes on Earth are shield volcanoes. Many shield volcanoes have a collapsed caldera.

Geographical extent

Shield volcanoes can be so large that they are sometimes considered to be a mountain range, such as the Ilgachuz Range and the Rainbow Range, both of which are located in Canada. These shield volcanoes formed when the North American Plate moved over a hotspot similar to the one feeding the Hawaiian Islands, called the Anahim hotspot. Shield volcanoes can be found in many places around the world, including Australia, Ethiopia, and the Galapagos Islands. The Piton de la Fournaise, on Reunion Island, is one of the most active shield volcanoes on earth, with one eruption per year on average.

Magma

The viscosity of magma as it approaches the surface is dependent on its temperature and composition. Shield volcanoes in the Hawai'ian Islands erupt magma as hot as 1,200 °C (2,200 °F), compared with 850 °C (1,560 °F) for most continental volcanoes, which are usually composed of acidic lava. Because of the fluidity of the lava, major explosive eruptions do not occur. The most severe explosions occur if water enters a vent, although expanding gases in the magma can produce spectacular fountaining of the low viscosity lava.

Examples

Skjaldbreiður, eponymous for a shield volcano

Dark profile of Hualālai, showing typical shape of a shield volcano

• Mount Edziza volcanic complex (British Columbia, Canada)
• Mauna Kea (Hawai'i, US)
• Mauna Loa (Hawai'i, US)
• Ilgachuz Range (British Columbia, Canada)
• Rainbow Range (British Columbia, Canada)
• Itcha Range (British Columbia, Canada)
• Kilauea (Hawai'i, US)
• Menengai (Menengai, Kenya)
• Medicine Lake Volcano (California, US)
• Lunar dome (Moon)
• Mount Moulton (Marie Byrd Land, Antarctica)
• Mount Andrus (Marie Byrd Land, Antarctica)
• Mount Okmok (Umnak Island, Alaska, US)
• Indian Heaven (Washington, US)
• Mount Warning (or the Tweed Volcano), (New South Wales, Australia)
• Verkhovoy (Kamchatka, Russia)
• Olympus Mons (Mars)
• Level Mountain Range (British Columbia, Canada)
• Lord Howe Island (Pacific Ocean, Australia)
• Mount Sanford (Alaska, US)
• Mount Elgon (Uganda/Kenya)
• Three Fingered Jack (Oregon, US)
• Kohala (Hawaii, US)
• Mount Berlin (Antarctica)
• Queen Mary's Peak (Atlantic ocean)
• Erta Ale (Afar Region, Ethiopia)
• Mount Terror (Ross Island, Antarctica)

Pyroclastic shields

Rarer pyroclastic shield volcanoes are similar to normal mafic shields in shape. But rather than being formed entirely by basalt lavas, pyroclastic shields are mainly formed from explosive eruptions of ignimbrite.

References

Wikimedia Commons has media related to: Shield volcanoes

• List of shield volcanoes
• Photo glossary of volcano terms - USGS
• Shield volcanoes - USGS Cascades Volcano Observatory
• Shield Volcanoes - Volcano World