Mechanics
Deep inside the Earth, between the molten iron core and the thin crust on the surface, there is a solid body of rock called the mantle, still hot from the formation of our planet about 4.6 billion years ago. Since rocks are good insulators, the heat has been slow to dissipate.

When rock from the mantle melts, it becomes magma, which moves to the surface through the Earth’s outer crust and releases pent-up gases. When the pressure is too much, volcanoes erupt. Pressure builds if the supply of magma to the volcano from the Earth's mantle is high.

Alternatively, pressure can increase within the magma chamber inside the volcano. This is because, as the magma in the chamber starts to cool, it releases gases that expand, thereby increasing the pressure. When the pressure gets too high, the rocks that make up the volcano break and the magma escapes to the surface to erupt. In some volcanoes the supply from within the earth is relatively constant, so eruptions are frequent; in others the magma rises in blobs every 100 or even 1000 years, and eruptions are infrequent.

Volcano Types
The moment magma is released, it becomes lava. There is virtually no difference between the two; one denotes the molten rock when it is underground, the other refers to it once it erupts. In broad terms there are two types of volcanic eruptions: · effusive - the gentle outpouring of lava · explosive - the violent ejection of ash, older rock, or globs of lava (pyroclastic material).

The four most common categories of volcano are:

· Cinder cones – simple volcanoes with a bowl-shaped crater at the summit and which only grow to about 1,000 feet. They are built from particles and blobs of lava ejected from a single vent.

 · Composite/Stratovolcanoes – these steep-sided volcanoes erupt cool, thick magma and lots of ash. Some are constantly erupting at a relatively low rate, while others erupt infrequently but very explosively.

 · Shield volcanoes – large, gentle-sloped volcanoes that erupt hot fluid lava and are rarely explosive.

 · Lava domes – these are formed by relatively small, bulbous masses of lava which are too thick to flow any distance, and which form a distinctive dome shape over and around their vents.