Volcanoes are created by earthquakes, and plates in the earth shifting. Therefore, most volcanoes are on plate boundraries, however there are some volcanoes that are in the middle of a plate and are said to be upon hot spots, or places where thermal energy form the middle of the earth rises up in plumes. An example of this is the Ring Of Fire which is found in the pacific.

The shifting of the plates creates friction, and heat. This heat and friction melts the rocks around it into magma. Magma is from the upper part of the mantle, not the crust. The magma gathers together in magma chambers by the plates, and begins to rise. Eventually it will be near the top inside a volcano. The reason it would most likely be in a volcano, is because volcanoes are tubes to the surface, the easiest way to get out.

So, the magma gets into the volcano. The volcanoes start to gather up immence pressure from the building magma, and eventually, the top bursts and the volcanoe blows up. No one really knows what determines the time a volcano erupts, but when it does, it really blows up. Sometimes the volcano blows up and destroies its crater, and it becomes a caldera. After this has happened, the volcano usually does not erupt, aside the small eruptions on the surface of the caldera.

When it blows up, it can either drip the lava down its side, or explode everything outwards. Examples of each type can be found here.

More information about what a volcano is can be found [here]

Information about major past volcanoes can be found [here]

Volcano links can be found [here]

More volcano pictures can be found [here]