The world is a big, spinning, ball of molten rock, with large rocky tectonic plates floating on top, and a big blob of solid iron in the middle. Although there is some argument as to what it is that causes these vast tectonic plates to move against each other, it is generally argued that there are convection currents in the earth's mantle (the layer just below the crust), creating a similar effect to what happens when you put milk in a cup of coffee. These swirling currents beneath the crust cause the plates to move against each other, and this huge build up of friction and heat on the plate boundaries is what causes volcanoes and earthquakes to appear.
There are a number of different types of "margins" which describe the different ways plates move against each other, and the different margins create different volcanic activity.
1. At a destructive margin, one plate is pushed under another, and the immense heat from the friction destroys the plate's edge (hence the name). This creates a large mass of magma (molten rock) under the plate. The pressure from the plate being driven down, and also the force from the earth spinning drives the magma up through cracks in the plate to near the surface, where it collects in a magma chamber. When the pressure from below becomes too much, or the volcano is dislodged by a landslide or earthquake, the magma blasts the top off the volcano, or blasts forth from a crack. The violence of these eruptions creates a steep-sided, conical volcano, as can be seen in the "Pacific ring of fire" - a ring of volcanoes along the boundaries of the Pacific plate.
2. At a constructive margin, eruptions are less exciting. The plates are being pulled away from each other, and the magma bubbles up through the gap, rather than being blasted into the air. It then forms a relatively flat volcano, known as a shield volcano, becoming wider and wider as the plates diverge. This can be seen in the Mid-Atlantic ridge, which is formed more or less entirely from magma.
3. The final type of volcano can be where a plate is stretched thin, known as a "hot-spot". This means that it is more likely that magma will find its way to the surface through cracks, and by melting pipes in the rock, and then can burst forth even where there is no plate boundary nearby. The Hawaiian islands are thought to have been created this way.
Plate tectonics as described above explains the formation of volcanoes but do not explain fully why they erupt. There are three predominant theories as to why this occurs. In reality it is probably a combination of all three that leads to an eruption:
1. The first theory argues that it is the gas that is found dissolved in magma in the earth's mantle that forces magma upwards through cracks in the crust until it builds up huge pressure below the surface. The pressure builds until the top of the volcano gets blasted clear and there is an eruption.
2. The second theory emphasises the importance of the dissolved substances in the magma. As pressure builds, water separates from the magma but other substances remain dissolved, causing a violent creation of pyroclasts, combinations of molten and solid elements, which again blasts the top off the volcano.
3. The final theory simply focuses on the fact that as more magma forms in the magma chamber below the volcano, it must push the magma that was already there upwards, and this will also cause pressure to build up below the top of the volcano, leading to an eruption.