The surface on which we live is a series of continental size landmasses floating on a sea of molten rock called magma that is under tremendous pressure and at a temperature millions of degrees in magnitude. This seemingly solid surface is in fact constantly in motion with each continental size floating body constantly grinding up against and sometimes even sliding underneath its neighbour. The frequent result of this is earthquakes and volcanic activity where weakened sections of the upper crust are breached. In other words a volcano is literally the result of a crack in the hardened surface of the earth where magma and hot gas escape.
Volcanoes exist on every continent of the planet however most are either dormant or extinct. Many active volcanoes in the world today are located around the edges of the Pacific Ocean with some in Iceland, East Africa, Italy and Hawaii. Dormant volcanoes may cycle through periods of violent activity over as little as a few hundred years between events while others may sit quietly for eons before suddenly becoming active. Mount St Helens in the United States is one such example of volcanic activity cycling through relatively short periods of dormancy. Early settlers reported eruptions in the 19th century while the latest and most destructive occurred in 1980.
The cause of eruptions may be from a combination of events. Large continental landmasses are in motion because of convection currents and magma currents circulating in the planet's molten core. Where two landmasses meet there are two possibilities. They will either buckle in an upward direction the result being impressive mountain ranges, or one may begin to slowly slide beneath the other. This process is slow and occurs over periods of many thousands of years until the pressure builds enough to fracture the hard crust. This in turn allows the release of magma and hot gas once the pressure is sufficient to breach the hard crust. Hot lava, gasses and fine particles then discharge over a wide area. Gasses and dust billow upwards to the stratosphere and the lava flows can cover hundreds of square miles.
Sometimes it is a crack or geological weakness in the middle of a continental mass that becomes volcanically active. This is true of recent discoveries in Iceland where molten rock and gas overwhelm a relatively weak point in the middle of the continental landmass. Irrespective of where a volcano exists, an eruption will usually occur as the result of some geological shift and pressure imbalance that can only release through weak points in the earth's surface.
Where volcanoes are active, they can be unpredictable and quite dangerous. Where they are dormant or extinct - they are only mountains.