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Volcanois a vent in the earth through which hot gases and molten rock rise to the surface. A coneshaped mountain of erupted material around such a vent is also called a volcano.
There are about 2,500 known active volcanoes in the world. Nearly all of them are located in chains stretching across the earth, often for many thousands of miles.
A volcano that is erupting or has erupted during historic times is called active. A Volcano in which there is no evidence of any activity is considered extinct. Volcanoes that appear recent but have no recorded activity other than the quiet escape of gas are called dormant.
Some volcanoes are much more active than others. Stromboli, in the Lipari Islands near Sicily, has been constantly active since ancient times. Many active volcanoes are found in a belt, called the Ring of Fire, that circles the Pacific Ocean. Such volcanoes as Mount Vesuvius, continue in a state of activity for longer or shorter periods and then become dormant.
The eruption that follows long dormancy is violent. This was shown in the 1980 eruption of Mount Saint Helens in the state of Washington. It had been dormant for a period of 123 years of being in a dormant state. Active volcanoes are dangerous to life. Mudflows are also troublesome. A mudflow triggered by an eruption that melted ice and snow on Colombia's Nevado del Ruiz volcano in 1985 killed more than 25,000 people.