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Volcanoes are generally found where two to three tectonic plates diverge or converge. The mid-oceanic ridges, like the Mid-Atlantic Ridge, are typical examples of divergent tectonic plates where volcanoes are formed, whereas the Pacific Ring of Fire is a typical example of volcanic activity on convergent tectonic plates. Where two tectonic plates slide past one another (like the San Andreas fault) volcanic activity is generally not found.

Volcanic activity can also occur from mantle plumes, the so-called hotspots, which occur at locations far from plate boundaries; hotspot volcanoes are also found elsewhere in the solar system, especially on its rocky planets and moons. In July 2006, a new type of volcano was discovered,[1] which was called a petitspot, to accentuate the difference with volcanoes formed by mantle plumes.

Divergent plate boundaries
At the mid-oceanic ridges, two tectonic plates diverge from one another. New oceanic crust is being formed by hot molten rock slowly cooling down and solidifying. In these places, the crust is very thin and eruptions occur frequently because of the pull by the tectonic plates. The main part of the mid-oceanic ridges are at the bottom of the ocean, and most volcanic activity is submarine. Black smokers are a typical example of this kind of volcanic activity. Where the mid-oceanic ridge comes above sea-level, volcanoes like the Hekla on Iceland are formed.

Convergent plate boundaries
In places where one tectonic plate submerges beneath another, the crust melts and becomes magma. This surplus amount of magma generated in one location causes the formation of the volcano. Typical examples for this kind of volcano are the volcanoes in the Pacific Ring of Fire, and also Mount Etna and Mount Vesuvius.

Hotspots
Hotspots are not located on the ridges of tectonic plates, but on top of mantle plumes, where the convection of Earth's mantle creates a column of hot material that rises until it reaches the crust. The temperature of the plume causes the crust to melt and form pipes, which can vent magma. Because the tectonic plates move whereas the mantle plume remains in the same place, each volcano becomes extinct after a while and a new volcano is then being formed as the plate shifts over the hotspot. The Hawaiian Islands are thought to be formed in such a manner, as well as the Snake River Plain, with the Yellowstone Caldera being the current part of the North American plate over the hotspot.

Petitspots
In July 2006, volcanoes were discovered that did not fit in any of the above-mentioned categories, since they are located far from the plate boundary, but are too small to be the result of a mantle plume. A new theory suggests that submergence of tectonic plates causes stress all over the plate, which causes the plate to crack in some places. However, other scientists believe the mantle plume theory to be incorrect, and consider this discovery a confirmation of their ideas.[2]

Shape
The most common perception of a volcano is of a conical mountain, spewing lava and poisonous gases from a crater in its top. This describes just one of many types of volcano and the features of volcanoes are much more complicated. The structure and behaviour of volcanoes depends on a number of factors. Some volcanoes have rugged peaks formed by lava domes rather than a summit crater, whereas others present landscape features such as massive plateaus. Vents that issue volcanic material (lava, which is what magma is called once it has broken the surface, and ash) and gases (mainly steam and magmatic gases) can be located anywhere on the landform. Many of these vents give rise to smaller cones such as Puʻu ʻŌʻō on a flank of Hawaiʻi's Kīlauea.

When hot magma rises from a vent,
the cooled lava hardens and forms a volcano.
Ash,rock fragments,gases,and lava are emitted violently during an eruption.
Volcanic ash is the material forming most of the landmass of a volcano.

Active volcano:
This is a volcano that has erupted within the last 500 years and still shows signs of activity.
e.g.St. Helen(USA)
most recent eruption:1990-1991

Dormant volcano:
This is a volcano that has not erupted within the last 500 years but shows signs of activity such as hot springs.
e.g.Mt. Fuji(Japan)
most recent eruption:1707

Extinct volcano:
This is a volcano that has not erupted within the last 500 years and shows no signs of activity.
e.g.Kohala volcano(Hawaii)
no eruption records in human history

火山= vol-ca-no
volcanos a mountain with a large opening called a crater at the top through which melting rock called lava