A vent, hole, crack, or opening in the earth's crust through which magma (molten rock), associated gases, matter, and ash erupt and/or the hill, mountain or structure, usually conical, produced by the accumulation of the ejected material.
As the magma and ash cool, they harden to form rock.
Volcanoes occur mainly near plate tectonic boundaries and are especially common around the Pacific basin, called the Pacific Ring of Fire. Generally, volcanoes erupt when the Earth's plates move apart or when one plate is forced beneath another.
Volcanic eruptions may be violent, even catastrophic, or relatively mild. The most explosive eruptions are essentially blasts of steam that create spectacular displays. Humanity has long been awed by this powerful force of nature.
The Romans attributed volcanic events to Vulcan, the god of fire and metalworking. In A.D. 79 the eruption of Mount Vesuvius destroyed the Roman cities of Pompeii and Herculaneum. Polynesians believe volcanoes to be ruled by the fire goddess Pele.
One of the most spectacular volcanic eruptions in recorded history occurred in 1883 with the explosion of Krakatoa, an island in the Sunda Strait near Java.
A more recent example is the dramatic 1980 eruption of Mount St. Helens in the Cascade Range in Washington State. Quieter fissure eruptions occur when molten rock pushes through long cracks in the Earth's crust and floods the surrounding landscape. Such repeated outpourings of lava can fill surrounding valleys and bury low hills, creating thick lava sequences that eventually become plateaus.
Throughout the world there are 500 to 800 active volcanoes. However, during a year only 30 might erupt in land. An inactive, or dormant, volcano is still capable of erupting. An extinct volcano is dead and cannot erupt. Undersea volcanoes can form islands such as Iceland and Hawaii.
Mt. Mayon, One of the Most Dangerous Volcanoes in the World, Above Rice Paddys, Albay, Philippines, by John Pennock — Buy this art print at AllPosters.
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