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Shield volcano

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Diagram of a shield volcano, showing the gradual buildup of lava in many layers that eventually constructs them.

A Shield volcano is a type of volcano built almost entirely of fluid lava flows. They are named so because of their large size and low profile, resembling a warrior's shield. This is caused by the highly fluid lava they erupt, which travel farther than those erupted from more explosive volcanoes. This results in the steady accumulation of broad sheets of lava, building up the shield volcano's distinctive form.

Geology

Eruptive mechanics

Skjaldbreiður, eponymous for a shield volcano

Shield volcanoes are built almost entirely of highly fluid basaltic lava. They are distinct from the two other major volcanic types, stratovolcanoes, which are driven by the accumulation of more viscous lavas, and cinder cones, which are built up by the consolidation of tephra ejected by explosive eruptions.[1] The types of eruptions that occur at shield volcanoes have been named Hawaiian eruptions, after the Hawaiian chain where they are most prominent. Hawaiian eruptions are characterized by the effusive emission of fluid lavas.[2] The nature of these lavas allows them to travel a longer distance than flows from other volcanic types, resulting in a large, spread-out sheet of lava[3] just 1 m (3 ft) thick.[1] The gradual buildup of thousands of these flows slowly constructs a low-lying, broad, and gently sloping form of a mature shield volcano.[3]

Continuous shield volcanic activity is very common,[2] and will, over time, build up splatter cones at the eruptive sites, despite Hawaiian activity being 90% lava flows.[4] An example of this is Puʻu ʻŌʻō, a product of Kīlauea's continuous activity.[5]

A hallmark of shield volcanism are lava tubes,[6] cave-like volcanic straights that are formed by the hardening of overlaying lava. These structures further the propagation of lava, as the walls of the tube insulate the flows within.[7] They are an important eruptive element; for example, an estimated 58% of Kilauea is covered by lava tube lava.[6]

Interactions between water and lava at shield volcanoes can cause some eruptions to become hydrovolcanic, which are an explosive eruptive type drastically different from usual shield volcanic activity.[4] These eruptions are especially prevelent at the waterbound volcanoes of the Hawaiian Isles.[2]

Rift zones are another prevalent feature on shield volcanoes that is rare on other volcanic types. The large, decentralized shape of Hawaiian volcanoes versus their small, symmetrical Icelandian cousins can be attributed to these types of eruptions; fissure venting is common in Hawaiʻi, accounting for their asymettrical, non-centralized shapes, and rare in Iceland, where central eruptions from summit calderas dominate and thus the lava distribution is far more even.[1][3][5]

In some shield volcano eruptions, basaltic lava pours out of a long fissure instead of a central vent, and shrouds the countryside with a long band of volcanic material in the form of a broud plateau. Plateaus of this type exist in Iceland, Washington, Oregon, and Idaho; the most prominent ones are situated along the Snake River in Idaho and the Columbia River in Washington and Oregon, where they have been measured to be over a 1 mi (2 km) in thickness.[3] Many eruptions start as a so-called "curtain of fire"—a long eruptive chain along a fissure vent on the volcano. Eventually these eruptions die down and start to focus around a few points on the fissure, where activity is concentrated.[2]

Structure

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

Because of their gradual buildup and near continuous eruptive characteristics, shield volcanoes are the largest volcanoes on Earth,[4][n 1] usually being at least 3 to 4 mi (5 to 6 km) across and surpassing 1,500 to 2,000 ft (457 to 610 m) in height. The largest shield volcano (and the largest active volcano) in the world is Mauna Loa in Hawaiʻi, which projects 13,677 ft (4,169 m) above sea level.[3]

Shield volcanoes are composed almost exclusively of basalt.[4] Their lower slopes are generally gentle (~2 degrees), but steepen with elevation (reaching ~10 degrees) before flattening near the summit, giving the volcanoes a convex shape.[1]

Over the volcano's lifespan, collapse-driven calderas that form on shield volcanoes are often filled up, and new ones formed elsewhere, in an ongoing cycle of collapse and regeneration.[4]

Distribution

Main article: List of shield volcanoes

Shield volcanoes are distinctive products of hotspot volcanism, but can form at rift and subduction zones as well.[4]

The largest and most prominent shield volcano chain in the world are the Hawaiian Islands, a chain of hotspot volcanoes in Pacific Ocean. This chain includes the largest volcano on Earth, Mauna Loa. Mauna Kea stands 4,170 m (13,681 ft) above sea level. In addition, its submarine flanks reach a further 5 km (3 mi) below the waterline, and Mauna Loa's massive size depresses the sea floor on which it stands a further 8 km (5 mi), making the volcano's summit about 17 km (56,000 ft) above its base. The volcano is approximately 80,000 km3 (19,193 cu mi) in total volume.[6]

Kilauea is one of the most active volcanoes on Earth, with the current ongoing eruption having begun in January 1983.[3] The Hawaiian volcanoes are characterized by frequent rift eruptions, their large size (thousands of km3 in volume), and their rough, decentralized shape.[1]

Another major center of shield volcanic activity is Iceland. There, the volcanoes are small (~15 km3 (4 cu mi)), symmetrical, and are characterized by eruptions from summit calderas.[1]

Olympus Mons on Mars is also a shield volcano.[6]

Dangers

The Hawaiian eruptions of shield volcanoes do not pose much threat to humans, as they emit large amounts of slow moving lava over long periods of time. However, they are hazardous to agriculture and infrastructure; the ongoing 1983 eruption of Kīlauea has destroyed over 200 structures and buried kilometers of highways.

Pyroclastic shields

Main article: Pyroclastic shield

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.

Notes

  1. ^ This excludes flood basalts, large sheets of volcanic material that do not actually resemble volcanoes.

See also

References

  1. ^ a b c d e f "How Volcanoes Work: Shield Volcanoes". San Diego State University. Retrieved 22 August 2010.
  2. ^ a b c d "How Volcanoes Work: Hawaiian eruptions". San Diego State University. Retrieved 22 August 2010.
  3. ^ a b c d e f Topinka, Lyn (28 December 2005). "Description: Shield Volcano". USGS. Retrieved 21 August 2010.
  4. ^ a b c d e f "Shield Volcanoes". University of North Dakota. Retrieved 22 August 2010.
  5. ^ a b World Book: U  · V  · 20. Chicago, Illinois: Scott Fetzer. 2009. pp. 438–443. ISBN 978-0-7166-0109-8. Retrieved 22 August 2010.
  6. ^ a b c d "VHP Photo Glossary: Shield volcano". USGS. 17 July 2009. Retrieved 23 August 2010.
  7. ^ Topinka, Lyn (18 April 2002). "Description: Lava Tubes and Lava Tube Caves". USGS. Retrieved 23 August 2010.

Media related to Shield volcanoes at Wikimedia Commons

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