Shield volcano
Repeated eruptions result in the steady accumulation of broad sheets of lava, building up the shield volcano's distinctive form.
[4][5] 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.
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".
[10] Shield volcanoes are similar in origin to vast lava plateaus and flood basalts present in various parts of the world.
[17] 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.
[7] Although the general form of a "typical" shield volcano varies little worldwide, there are regional differences in their size and morphological characteristics.
The large, decentralized shape of Hawaiian volcanoes as compared to their smaller, symmetrical Icelandic cousins[7] can be attributed to rift eruptions.
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.
[23] 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.
Central-vent eruptions, meanwhile, often take the form of large lava fountains (both continuous and sporadic), which can reach heights of hundreds of meters or more.
The particles from lava fountains usually cool in the air before hitting the ground, resulting in the accumulation of cindery scoria fragments; however, when the air is especially thick with pyroclasts, they cannot cool off fast enough because of the surrounding heat, and hit the ground still hot, accumulating into spatter cones.
Some volcanoes, such as Mount Wrangell in Alaska and Cofre de Perote in Mexico, exhibit large enough swings in their historical magmatic eruptive characteristics to cast strict categorical assignment in doubt; one geological study of de Perote went so far as to suggest the term "compound shield-like volcano" instead.
The calderas are often filled up by progressive eruptions, or formed elsewhere, and this cycle of collapse and regeneration takes place throughout the volcano's lifespan.
These explosive eruptions are drastically different from the usual shield volcanic activity[11] and are especially prevalent at the waterbound volcanoes of the Hawaiian Isles.
[32] The youngest part of the chain is Hawaii, where the volcanoes are characterized by frequent rift eruptions, their large size (thousands of km3 in volume), and their rough, decentralized shape.
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) diameter.
[27] Cerro Azul is a shield volcano on the southwestern part of Isabela Island 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[38] whose responsibility is to monitor Ecuador's numerous active volcanoes in the Andean Volcanic Belt and the Galapagos Islands.
[39] 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.
[40] Located over the Mid-Atlantic Ridge, a divergent tectonic plate boundary in the middle of the Atlantic Ocean, Iceland is the site of about 130 volcanoes of various types.
[37] Icelandic shields are mostly small (~15 km3 (4 cu mi)), symmetrical (although this can be affected by surface topography), and characterized by eruptions from summit calderas.
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.
On both planets, they have gently sloping flanks, collapse craters along their central structure, and are built of highly fluid lavas.
Venus 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).
