Shield volcano

A shield volcano is a type of volcano named for its low profile, resembling a shield lying on the ground. It is formed by the eruption of highly fluid (low viscosity) lava, which travels farther and forms thinner flows than the more viscous lava erupted from a stratovolcano. Repeated eruptions result in the steady accumulation of broad sheets of lava, building up the shield volcano's distinctive form.

Shield volcanoes are found wherever fluid, low-silica lava reaches the surface of a rocky planet. However, they are most characteristic of ocean island volcanism associated with hot spots or with continental rift volcanism. They include the largest active volcanoes on Earth, such as Mauna Loa. Giant shield volcanoes are found on other planets of the Solar System, including Olympus Mons on Mars and Sapas Mons on Venus.

The term 'shield volcano' is taken from the German term Schildvulkan, coined by the Austrian geologist Eduard Suess in 1888 and which had been calqued into English by 1910.

Shield volcanoes are distinguished from the three other major volcanic types—stratovolcanoes, lava domes, and cinder cones—by their structural form, a consequence of their particular magmatic composition. Of these four forms, shield volcanoes erupt the least viscous lavas. Whereas stratovolcanoes and lava domes are the product of highly viscous flows, and cinder cones are constructed of explosively eruptive tephra, shield volcanoes are the product of gentle effusive eruptions of highly fluid lavas that produce, over time, a broad, gently sloped eponymous "shield". Although the term is generally applied to basaltic shields, it has also at times been applied to rarer scutiform volcanoes of differing magmatic composition—principally pyroclastic shields, formed by the accumulation of fragmentary material from particularly powerful explosive eruptions, and rarer felsic lava shields formed by unusually fluid felsic magmas. Examples of pyroclastic shields include Billy Mitchell volcano in Papua New Guinea and the Purico complex in Chile; an example of a felsic shield is the Ilgachuz Range in British Columbia, Canada. Shield volcanoes are similar in origin to vast lava plateaus and flood basalts present in various parts of the world. These are eruptive features which occur along linear fissure vents and are distinguished from shield volcanoes by the lack of an identifiable primary eruptive center.

Active shield volcanoes experience near-continuous eruptive activity over extremely long periods of time, resulting in the gradual build-up of edifices that can reach extremely large dimensions. With the exclusion of flood basalts, mature shields are the largest volcanic features on Earth. The summit of the largest subaerial volcano in the world, Mauna Loa, lies 4,169 m (13,678 ft) above sea level, and the volcano, over 60 mi (100 km) wide at its base, is estimated to contain about 80,000 km³ (19,000 cu mi) of basalt. The mass of the volcano is so great that it has slumped the crust beneath it a further 8 km (5 mi). Accounting for this subsidence and for the height of the volcano above the sea floor, the "true" height of Mauna Loa from the start of its eruptive history is about 17,170 m (56,000 ft). Mount Everest, by comparison, is 8,848 m (29,029 ft) in height. In 2013, a team led by the University of Houston's William Sager announced the discovery of Tamu Massif, an enormous extinct submarine volcano, approximately 450 by 650 km (280 by 400 mi) in area, which dwarfs all previously known volcanoes on Earth. However, the extents of the volcano have not been confirmed. Although Tamu Massif was initially believed to be a shield volcano, Sanger and his colleagues acknowledged in 2019 that Tamu Massif is not a shield volcano.

Shield volcanoes feature a gentle (usually 2° to 3°) slope that gradually steepens with elevation (reaching approximately 10°) before flattening near the summit, forming an overall upwardly convex shape. These slope characteristics have a correlation with age of the forming lava, with in the case of the Hawaiian chain, steepness increasing with age, as later lavas tend to be more alkali so are more viscous, with thicker flows, that travel less distance from the summit vents. In height they are typically about one twentieth their width. Although the general form of a "typical" shield volcano varies little worldwide, there are regional differences in their size and morphological characteristics. Typical shield volcanoes found in California and Oregon measure 3 to 4 mi (5 to 6 km) in diameter and 1,500 to 2,000 ft (500 to 600 m) in height, while shield volcanoes in the central Mexican Michoacán–Guanajuato volcanic field average 340 m (1,100 ft) in height and 4,100 m (13,500 ft) in width, with an average slope angle of 9.4° and an average volume of 1.7 km³ (0.4 cu mi).

Rift zones are a prevalent feature on shield volcanoes that is rare on other volcanic types. The large, decentralized shape of Hawaiian volcanoes as compared to their smaller, symmetrical Icelandic cousins can be attributed to rift eruptions. Fissure venting is common in Hawaiʻi; most Hawaiian eruptions begin with a so-called "wall of fire" along a major fissure line before centralizing to a small number of points. This accounts for their asymmetrical shape, whereas Icelandic volcanoes follow a pattern of central eruptions dominated by summit calderas, causing the lava to be more evenly distributed or symmetrical.

Most of what is currently known about shield volcanic eruptive character has been gleaned from studies done on the volcanoes of Hawaiʻi Island, by far the most intensively studied of all shields because of their scientific accessibility; the island lends its name to the slow-moving, effusive eruptions typical of shield volcanism, known as Hawaiian eruptions. These eruptions, the least explosive of volcanic events, are characterized by the effusive emission of highly fluid basaltic lavas with low gaseous content. These lavas travel a far greater distance than those of other eruptive types before solidifying, forming extremely wide but relatively thin magmatic sheets often less than 1 m (3 ft) thick. Low volumes of such lavas layered over long periods of time are what slowly constructs the characteristically low, broad profile of a mature shield volcano.