Volcanoes, they are fascinating forces in nature that contains more power and energy than all the bombs and power plants man ever built combined, and in just one mountain! They are amazing to watch and wonder about, and one of the things people frequently wonder about is what causes a volcanic eruption! The answer is amazingly simple, heat and pressure!
What is happening is basic physics. The Earth's crust produces a tremendous amount of force and energy and is also shifting along various plates and places. When the shifting occurs, you get friction and energy is released in the form of heat, which results in either the melting or vaporization of any substances in the crust. The effected material starts to expand and rise, pushing its way along fault or fracture lines in the mantle of the Earth. As it rises, it will cool some, but also expand as pressure lessens. When it reaches the surface, you have a volcano!
This means that volcanoes are the areas where the pressure and heat of the Earth are released. That pressure can be released in several ways and thus there are different types of volcanoes. A geyser is a type of volcanic eruption, water is the compound that is being effected by the heat and shooting to the surface. Most geologists will tell you the Old Faithful in Yellowstone National Part is a volcano that erupts on a regular basis.
Montserrat in the Caribbean is a shield volcano that has been erupting violently since 1995. It is producing primarily gas and volcanic ash due to the mineral content found in the area. Now with Montserrat, there is dome formations, which are basic bubble forming under the surface of the earth which can rise. Sometimes they start to cool and the dome instead of exploding, collapses. This can send up dust, in some cases over 50,000 feet, but is not technically an eruption!
Lava is when the molten rock reaches the surface and is comprised mainly of iron, silica and numerous minerals. It is very destructive, but often easier to predict than pyroclastic flow with ash and clouds of invisible gas!
Volcanoes, an amazing natural phenomenon involving tremendous forces and power, but the forces and causes are basic physics. The causes are simply heat and pressure!