You know what a volcano is

Do You know what  a volcano is ?

  Volcanoes are created when hot material from below rise and leak into the crust. This hot material, called magma, can come from a melt of subducted *crustal material which is light and very active after melting, or can come from deeper in the interior of the planet and is light and very active because it's very hot

* subduction refers to the movement of one tectonic plate under another one)

  Magma, rising from lower reaches, gathers in a reservoir, in a weak portion of the overlying rock called the magma chamber. Eventually, but not always, the magma erupt into the surface. Strong earthquakes accompany rising magma, and the volcanic cone may swell in appearance just before an eruption, it's illustrated in a picture you can see HERE. The picture show the volcano getting bigger as magma rising inside. Scientist often monitor the changing shape of a volcano, especially prior to an eruption. The different reason why a volcano forms are : 
   *via plumes or hot spots in the lithosphere ( a plume is the name for the volcano column of air with gas or ash emmites when there's an eruption)
   *as the result of subduction of the nearby lithosphere, that is to say, the outer shell of the planet.

For more information you can visit these websites :

    *   http://mail.colonial.net/~hkaiter/volcanoes.html

    *   http://geology.com/volcanoes/chaiten/