One way of classifying volcanoes is by the type of material erupted, which affects the shape of the volcano. If the erupting magma contains a high percentage (65%) of silica the lava is called felsic or acidic and tends to be highly viscous (not very fluid) and is pushed up in a blob that will solidify relatively quickly. Lassen Peak in California is an example. This type of volcano has a tendency to explode because it retains the volatiles or gases and easily plugs. Mount Pelée on the island of Martinique is another example.

If, on the other hand, the magma contains a relatively low percentage of silica, the lava is called mafic or basic and will be very fluid as it erupts, capable of flowing for long distances. Due to the low viscosity the volatiles are able to escape. A good example of a mafic lava flow is the Great flow produced by an eruptive fissure almost in the geographical center of Iceland roughly 8,000 years ago; it flowed to the sea, a distance of 130 kilometers, and covered an area of 800 square km.

===Explosivity=== The behaviour of volcanoes range from rare collossally explosive events to common cases of long term, gradual and gentle flow of magma. The Volcanic Explosivity Index is an attempt to categorise these into clear types, with low VEI values corresponding to gentle flows and high VEIs indicating a cataclysmic event with severe global consequences.