The process of a volcano erupting begins with the generation of magma, which then migrates towards the surface until the pressure and temperature can equalize. Find out why sometimes there is merely a cooling and crystallization below the surface of a volcano with help from the chair of a department of environmental studies in this free video on volcano eruptions.
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As far as what is going on, first of all you have got to generate the magma. And again, if you look there's certain areas where you are going to get magma being generated. Typically at plate boundaries where you have one plate rubbing against another, and what you are going to get there is just frictional heating. And a lot of times that frictional heating is just like rubbing your hands together, you can feel it warming up. That frictional heating is enough to melt the rock. Other places you can get it is any areas down deep below the surface where you may have a concentration of radioactive materials. Particularly things like uranium, because as uranium breaks down it gives off heat. And if there is enough of it down there it will give off enough heat to melt the rock or a combination of the two. If you are on a plate boundary with radioactive materials you have got enough of a heat source to melt the rock. Once you get the rock melted well then it starts migrating towards the surface, because again it is just a simple density thing. Hotter objects are less dense than cooler objects so they are going to start rising towards the surface. They will melt their way through the rock. They will percolate along cracks and crannies and nooks, and anything else. Any way they can make their way up, and eventually they will come towards the surface. It is the path of least resistance just like water moving, and they will move towards the surface until they get to a place where the pressure and the temperature can equalize. Now sometimes that is below the surface, and you never have an eruption. You will just have cooling and crystallization below the surface. And other times it will continue to move on until it hits the earths surface.