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A shield volcano is a type of volcano usually composed almost entirely of fluid lava flows. It is named for its low profile, resembling a warrior's shield lying on the ground. This is caused by the highly fluid (low viscosity) lava erupted, which travels farther than lava erupted from a stratovolcano, and results in the steady accumulation of broad sheets of lava, building up the shield volcano's distinctive form.

Shield volcanoes are found worldwide. They can form over hotspots (points where magma from below the surface wells up), such as the hotspots (points where magma from below the surface wells up), such as the Hawaiian–Emperor seamount chain and the Galápagos Islands, or over more conventional rift zones, such as the Icelandic shields and the shield volcanoes of East Africa.

Although shield volcanoes are not usually associated with subduction, they can occur over subduction zones. Many examples are found in California and Oregon, including Prospect Peak in Lassen Volcanic National Park, as well as Pelican Butte and Belknap Crater in Oregon.

Many shield volcanoes are found in ocean basins, such as Although shield volcanoes are not usually associated with subduction, they can occur over subduction zones. Many examples are found in California and Oregon, including Prospect Peak in Lassen Volcanic National Park, as well as Pelican Butte and Belknap Crater in Oregon.

Many shield volcanoes are found in ocean basins, such as Tamu Massif, the world's largest, although they can be found inland as well—East Africa being one example of this.[25]

The largest and most prominent shield volcano chain in the world is the Hawaiian Islands, a chain of hotspot volcanoes in the Pacific Ocean. The Hawaiian volcanoes are characterized by frequent rift eruptions, their large size (thousands of km3 in volume), and their rough, decentralized shape. Rift zones are a prominent feature on these volcanoes, and account for their seemingly random volcanic structure.[4] They are fueled by the movement of the Pacific Plate over the Hawaii hotspot, and form a long chain of volcanoes, atolls, and seamounts 2,600 km (1,616 mi) long with a total volume of over 750,000 km3 (179,935 cu mi). The chain contains at least 43 major volcanoes, and Meiji Seamount at its terminus near the Kuril–Kamchatka Trench is 85 million years old.[26][27] The volcanoes follow a distinct evolutionary pattern of growth and death.[28]

The chain includes the second largest volcano on Earth, Mauna Loa, which stands 4,170 m (13,680 ft) above sea level and reaches a further 13 km (8 mi) below the waterline and into the crust, approximately 80,000 km3 (19,000 cu mi) o

The chain includes the second largest volcano on Earth, Mauna Loa, which stands 4,170 m (13,680 ft) above sea level and reaches a further 13 km (8 mi) below the waterline and into the crust, approximately 80,000 km3 (19,000 cu mi) of rock.[23] Kīlauea, meanwhile, is one of the most active volcanoes on Earth, with it's most recent eruption having lasted from January 1983 to 2018.[9]

The Galápagos Islands are an isolated set of volcanoes, consisting of shield volcanoes and lava plateaus, about 1,100 km (680 mi) west of Ecuador. They are driven by the Galápagos hotspot, and are between approximately 4.2 million and 700,000 years of age.[22] The largest island, Isabela Island, consists of six coalesced shield volcanoes, each delineated by a large summit caldera. Española, the oldest island, and Fernandina, the youngest, are also shield volcanoes, as are most of the other islands in the chain.[29][30][31] The Galápagos Islands are perched on a large lava plateau known as the Galápagos Platform. This platform creates a shallow water depth of 360 to 900 m (1,181 to 2,953 ft) at the base of the islands, which stretch over a 174 mi (280 km)-long diameter.[32] Since Charles Darwin's visit to the islands in 1835 during the Second voyage of HMS Beagle, there have been over 60 recorded eruptions in the islands, from six different shield volcanoes.[29][31] Of the 21 emergent volcanoes, 13 are considered active.[22]

Cerro Azul is a shield volcano on the south western part of Isabela Island in the Galápagos Islands and is one of the most activ

Cerro Azul is a shield volcano on the south western part of Isabela Island in the Galápagos Islands and is one of the most active in the Galapagos, with the last eruption between May and June 2008. The Geophysics Institute at the National Polytechnic School in Quito houses an international team of seismologists and volcanologists[33] whose responsibility is to monitor Ecuador's numerous active volcanoes in the Andean Volcanic Belt and the Galapagos Islands. La Cumbre is an active shield volcano on Fernandina Island in the Galapagos that has been erupting since April 11, 2009.[34]

The Galápagos islands are geologically young for such a big chain, and the pattern of their rift zones follows one of two trends, one north-northwest, and one east–west. The composition of the lavas of the Galápagos shields are strikingly similar to those of the Hawaiian volcanoes. Curiously, they do not form the same volcanic "line" associated with most hotspots. They are not alone in this regard; the Cobb–Eickelberg Seamount chain in the North Pacific is another example of such a delineated chain. In addition, there is no clear pattern of age between the volcanoes, suggesting a complicated, irregular pattern of creation. How exactly the islands were formed remains a geological mystery, although several theories have been proposed.[35]

Another major center of shield volcanic activity is Iceland. Located over the Mid-Atlantic Ridge, a divergent tectonic plate in the middle of the Atlantic Ocean, Iceland is the site of about 130 volcanoes of various types.[16] Icelandic shield volcanoes are generally of Holocene age, between 5,000 and 10,000 years old, except for the island of Surtsey, a Surtseyan shield. The volcanoes are also very narrow in distribution, occurring in two bands in the West and North Volcanic Zones. Like Hawaiian volcanoes, their formation initially begins with several eruptive centers before centralizing and concentrating at a single point. The main shield then forms, burying the smaller ones formed by the early eruptions with its lava.[32]

Icelandic shields are mostly small (~15 km3 (4 cu mi)), symmetrical (although this can affected by surface topography), and characterized by eruptions from summit calderas.[32] They are composed of either tholeiitic olivine or picritic basalt. The tholeiitic shields tend to be wider and shallower than the picritic shields.[36] They do not follow the pattern of caldera growth and destruction that other shield volcanoes do; caldera may form, but they generally do not disappear.[4][32]

East Africa

East Africa is the site of volcanic activity generated by the development of the East African Rift, a developing plate boundary in Africa, and from nearby hotspots.

Icelandic shields are mostly small (~15 km3 (4 cu mi)), symmetrical (although this can affected by surface topography), and characterized by eruptions from summit calderas.[32] They are composed of either tholeiitic olivine or picritic basalt. The tholeiitic shields tend to be wider and shallower than the picritic shields.[36] They do not follow the pattern of caldera growth and destruction that other shield volcanoes do; caldera may form, but they generally do not disappear.[4][32]

East Africa is the site of volcanic activity generated by the development of the East African Rift, a developing plate boundary in Africa, and from nearby hotspots. Some volcanoes interact with both. Shield volcanoes are found near the rift and off the coast of Africa, although stratovolcanoes are more common. Although sparsely studied, the fact that all of its volcanoes are of Holocene age reflects how young the volcanic center is. One interesting characteristic of East African volcanism is a penchant for the formation of lava lakes; these semi-permanent lava bodies, extremely rare elsewhere, form in about nine percent of African eruptions.[37]

The most active shield volcano in Africa is Nyamuragira. Eruptions at the shield volcano are generally centered within the large summit caldera or on the numerous fissures and cinder cones on the volcano's flanks. Lava flows from the most recent century exten

The most active shield volcano in Africa is Nyamuragira. Eruptions at the shield volcano are generally centered within the large summit caldera or on the numerous fissures and cinder cones on the volcano's flanks. Lava flows from the most recent century extend down the flanks more than 30 km (19 mi) from the summit, reaching as far as Lake Kivu. Erta Ale in Ethiopia is another active shield volcano, and one of the few places in the world with a permanent lava lake, which has been active since at least 1967, and possibly since 1906.[37] Other volcanic centers include Menengai, a massive shield caldera,[38] and Mount Marsabit, near the town of Marsabit.

Volcanoes are not limited to Earth; they can exist on any rocky planet or moon large or active enough to have a molten core, and since probes were first launched in the 1960s, volcanoes have been found across the solar system. Shield volcanoes and volcanic vents have been found on Mars, Venus, and Io; cryovolcanoes on Triton; and subsurface hotspots on Europa.[39]

The volcanoes of Mars are very similar to the shield volcanoes on Earth. Both have gently sloping flanks, collapse craters along their central structure, and are built of highly fluid lavas. Volcanic features on Mars were observed long before they were first studied in detail during the 1976–1979 Viking mission. The principal difference between the volcanoes of Mars and those on Earth is in terms of size; Martian volcanoes range in size up to 14 mi (23 km) high and 370 mi (595 km) in diameter, far larger than the 6 mi (10 km) high, 74 mi (119 km) wide Hawaiian shields.[40][41][42] The highest of these, Olympus Mons, is the tallest known mountain on any planet in the solar system.

Venus also has over 150 shield volcanoes which are much flatter, with a larger surface area than those found on Earth, some having a diameter of more than 700 km (430 mi).[43] Although the majority of these are long extinct it has been suggested, from observations by the Venus Express spacecraft, that many may still be active.The volcanoes of Mars are very similar to the shield volcanoes on Earth. Both have gently sloping flanks, collapse craters along their central structure, and are built of highly fluid lavas. Volcanic features on Mars were observed long before they were first studied in detail during the 1976–1979 Viking mission. The principal difference between the volcanoes of Mars and those on Earth is in terms of size; Martian volcanoes range in size up to 14 mi (23 km) high and 370 mi (595 km) in diameter, far larger than the 6 mi (10 km) high, 74 mi (119 km) wide Hawaiian shields.[40][41][42] The highest of these, Olympus Mons, is the tallest known mountain on any planet in the solar system.

Venus also has over 150 shield volcanoes which are much flatter, with a larger surface area than those found on Earth, some having a diameter of more than 700 km (430 mi).[43] Although the majority of these are long extinct it has been suggested, from observations by the Venus Express spacecraft, that many may still be active.[44]