Big bombs can measure more than 4 feet(1.2 meters) across and weigh several tons. Large pieces of tephra are called volcanic bombs. They are also steeper than shield volcanoes, with slopes of 30–35° compared to slopes of generally 5–10°, and their loose tephra are material for dangerous lahars. The combination is deadly, promoting explosive eruptions that produce great quantities of ash, as well as pyroclastic surges like the one that destroyed the city of Saint-Pierre in Martinique in 1902. High-silica lavas also tend to contain more dissolved gas. The lavas of stratovolcanoes are higher in silica, and therefore much more viscous, than lavas from shield volcanoes. Classic examples include Mount Fuji in Japan, Mayon Volcano in the Philippines, and Mount Vesuvius and Stromboli in Italy.Īsh produced by the explosive eruption of stratovolcanoes has historically posed the greatest volcanic hazard to civilizations. They are also known as composite volcanoes because they are created from multiple structures during different kinds of eruptions. Stratovolcanoes (composite volcanoes) are tall conical mountains composed of lava flows and tephra in alternate layers, the strata that gives rise to the name. Active mud volcanoes tend to involve temperatures much lower than those of igneous volcanoes except when the mud volcano is actually a vent of an igneous volcano. Other types of volcano include cryovolcanoes (or ice volcanoes), particularly on some moons of Jupiter, Saturn, and Neptune and mud volcanoes, which are formations often not associated with known magmatic activity. Vents that issue volcanic material (including lava and ash) and gases (mainly steam and magmatic gases) can develop anywhere on the landform and may give rise to smaller cones such as Puʻu ʻŌʻō on a flank of Hawaii's Kīlauea. Some volcanoes have rugged peaks formed by lava domes rather than a summit crater while others have landscape features such as massive plateaus. The features of volcanoes are much more complicated and their structure and behavior depends on a number of factors. The most common perception of a volcano is of a conical mountain, spewing lava and poisonous gases from a crater at its summit however, this describes just one of the many types of volcano. Skjaldbreiður, a shield volcano whose name means "broad shield" The study of volcanoes is called volcanology, sometimes spelled vulcanology. The word volcano is derived from the name of Vulcano, a volcanic island in the Aeolian Islands of Italy whose name in turn comes from Vulcan, the god of fire in Roman mythology.
This results in hotspot volcanism, of which the Hawaiian hotspot is an example. Volcanism away from plate boundaries has been postulated to arise from upwelling diapirs from the core–mantle boundary, 3,000 kilometers (1,900 mi) deep in the Earth. Volcanoes can also form where there is stretching and thinning of the crust's plates, such as in the East African Rift and the Wells Gray-Clearwater volcanic field and Rio Grande Rift in North America.
For example, a mid-ocean ridge, such as the Mid-Atlantic Ridge, has volcanoes caused by divergent tectonic plates whereas the Pacific Ring of Fire has volcanoes caused by convergent tectonic plates. On Earth, volcanoes are most often found where tectonic plates are diverging or converging, and most are found underwater. For broader coverage of this topic, see Volcanism.Ī volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface.