Volcanoes erupt due to pressure that the weight of the rocks puts on the magma and forces it to the surface out any available vent or exit provided to it. The magma is in a reservoir like area under the surface and the pressure from the rocks causes the less dense magma to move into any available crack or fissure that it can reach to try to release in some fashion. The magma can even melt its way to the surface if no exit is available for it at the time.
As the magma rises, the pressure is reduced, allowing the magma gas to rise as well. This equals out the pressure between the lower and upper chambers. This rise of gas pressure is an indicator that an eruption is about to occur. Any volcano that has the possibility of erupting, but is currently showing no signs is called a dormant volcano. There are some volcanoes that are no longer dormant; these are called extinct because they have been cut off from the source of their magma. For an extinct volcano to become active again a new source of magma would have to break through to the same vent or exit point. Not all eruptions are giant or horribly destructive. A number of the volcanoes in Hawaii erupt constantly and put out long slow moving trails of lava that move only a few feet a year.