Shield volcano
A shield volcano is a large volcano and is formed from the layers of successive fluid basaltic eruptions, for this reason they have a spongy slope. It is a term similar to that of volcanic caldera.
Along with composite volcanoes – known as composite cones – shield volcanoes have recurring eruptions that can last for millions of years.
Etymology
The name comes from a translation of Skjaldbreiður, a shield volcano in Iceland whose name means “Broad Shield”, since it is reminiscent of a warrior's shield.
Training
Shield volcanoes are formed by lava flows of low viscosity – that is, lava that flows easily. A volcanic mountain that has a wide profile -conical profile with a base much greater than its height- is formed over time by floods of relatively fluid basaltic lava that comes from fissures -chimneys- on the surface of the volcano. Many of the largest volcanoes on earth are of this type. The largest is Mauna Loa in Hawaii; all the volcanoes on those islands are shield volcanoes.
Volcanoes of this type can be so large that they are sometimes mistaken for mountain ranges, such as the Ilgachuz Range and the Rainbow Range, both located in Canada. These volcanoes formed when the North American Plate passed over a hot spot similar to the one that feeds the volcanoes in the Hawaiian Islands, called the Anahim hot spot.
There are also shield volcanoes, for example, in Washington, Oregon, and the Galapagos Islands. Pico del Horno, on Reunion Island, is one of the most active shield volcanoes in the world, with an average eruption of one a year.
Features
The viscosity of magma depends on its temperature and composition. The magma of volcanoes in the Hawaiian Islands spews out at 1,200°C, while most mainland volcanoes spew lava at 850°C, usually composed of acid lava. Because of the fluidity of the lava in shield volcanoes, large, highly explosive eruptions do not occur. The strongest explosions occur when water enters through a chimney. There are also explosions due to gas expansion, which can produce spectacular low-viscosity lava projections.
Shield volcanoes are known to form on other planets as well. The largest known mountain in the solar system, Mount Olympus on Mars, is a shield volcano believed to be extinct. Shield volcanoes on Mars are taller and more massive than those on Earth, which is probably due to the absence of plate tectonics on Mars.
On earth, due to the existence of tectonic plates and their displacement, the "fixed" hot spots present inside the earth's crust produce volcanic mountain ranges, so that the individual volcanoes that form are smaller than what they might be if there were no plate displacements.
Shield volcanoes occur along tectonic plate boundaries or on top of hot spots. However, the many large shield volcanoes in the Cascades, northern California, and Oregon arise from more complex causes.