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Volcanoes are classified on various bases. On the bases of activity volcanoes are of three types:
(1) Active Volcanoes. The volcanoes which eject lava, gas etc., are called active volcanoes. Some of them emit lava after every 15 or 20 minutes. Stromboli is most well known among them. Active volcanoes number about 300 in the world.
(2) Dormant Volcanoes. These are the volcanoes, which have stopped ejecting lava, but they can became active any time. Vesuvius is such a dormant volcano.
(3) Extinct Volcanoes. These are the volcanoes, which are quiet now. Though the volcanoes had exploded in the past yet there is no hope of any explosion now. There are thousands of extinct volcanoes, which are undergoing erosion by running water and wind and therefore look really extinct.
Classification Based upon Exploded and Ejected Material.
The explosion of volcano has different styles. The ejected material is also not the same in all the volcanic explosions.
(1) Hawaiian Type. The main ejected material from these types of volcanoes is lava but it does not come out at the time of explosion. Lava continues coming later from main crater or secondary craters. When wind blows the lava spreads in the form of red threads.
(2) Vesuvian Type. These types of volcanoes explode after a long time but the explosion is very devastating. The lava that comes out of it has a large quantity of gas in it. The erupted material balances itself in air in the form of a cauliflower. Lava, ash, etc., are rained down from it. The magma of this type of volcano is more viscous than that of Vesuvius.
(3) Pelean Type. The magma from this type of volcanoes is very dense. This dense magma forms a plug and seals its crater. When the upsurge of magma takes place the great pressure exerted by the upcoming magma breaks the seal with a devastating explosion. Some magma also comes out of the cracks situated in the weaker cracks around the volcano. Another volcano of Pelean type is named Hibok-Hibok and is situated in Philippines. It exploded in 1951.
In brief, very hot lava is emitted out of volcanoes. It does not rise vertically upward but is inclined to the horizontal.
The magma from volcanoes starts flowing fast on the slope.
(4) Mud Volcano. These are not the type of volcanoes as have been described earlier. In areas where petroleum or gas is found, sand and clay accompanies the gas emitting out in those areas. This sand and clay is deposited at the vent and form a cone. If water also accompanies the gas, sand and clay form mud. This mud is deposited on the mound and gets hardened. It also stops the emission of gases. When the pressure of the gas increases, the top of the cone is exploded away.