Lava composition

Another way of classifying volcanoes is by the composition of material erupted, since this affects the shape of the volcano. Lava can be generally classified into 4 special compositions:

If the erupted molten rock contains a high percentage of silica, the lava is called felsic.
Felsic lavas tend to be extremely viscous and are erupted as domes or short, broad flows. Viscous lavas tend to form stratovolcanoes or lava domes. Lassen Peak in California is an example of a volcano created from felsic lava and is actually a large lava ground.
Because siliceous magmas are so sticky, they tend to catch volatiles that are present, which cause the magma to erupt catastrophically, eventually forming stratovolcanoes. Pyroclastic flows are extremely hazardous products of such volcanoes, since they are composed of molten volcanic ash too heavy to go up into the environment, so they hug the volcano's slopes and travel far from their vents during large eruptions. Temperatures as high as 1,200 °C are known to happen in pyroclastic flows, which will incinerate everything combustible in their path and thick layers of hot pyroclastic flow deposits can be laid down, often up to many meters thick.