A volcano is a vent at Earth's surface through which lava and gasses are erupted. Often, the material erupting from the vent piles up to form a mountain or hill.
Volcanoes occur in several types, depending on the geologic setting and the composition of magma forming the lava and releasing the gasses.
Volcanoes such as those on Hawaii are called shield volcanoes because their shape resembles a shield laid on the ground. Shield volcanoes are formed by the eruption of fluid-like lava that flows quickly.
Volcanoes such as Mount St. Helens in Washington are composite volcanoes and erupt explosively, seldom producing rapidly flowing liquid-like lava. They have steep flanks and are formed by aggregation of pyroclastic deposits.
The difference between volcano and eruptive styles is a consequence of the temperature and type of magma feeding the volcano. Magma rich in silica is more viscous. Viscosity is the measure of a material's resistance to flow. For example, water has a lower viscosity than maple syrup.
In a magma, abundant silica form chain-like molecules that increase viscosity. The lava erupted in Hawaii is basaltic in composition and thus has a relatively low viscosity. It also has little gaseous content that makes it less volatile. It normally erupts at a high temperature and flows quickly. In composite volcanoes such as the Cascades, the magma is rich in silica and volatile gases. The result is explosive potential.
The gases are held in the magma by the overlying rocks. Eventually the pressure builds and the top of the mountain can be blown away in a tremendous explosion that ejects ash clouds high into the atmosphere, creates pyroclastic flows down the mountain side, and can hurl rock bombs for miles.
The volcano type is directly related to the plate tectonic setting of a volcano. We will study the distribution of these features in later chapters.
Several other features that distinguish volcanic rocks and processes are: Cinder cones, formed by the ejection of basaltic lava; pumice is the rock formed by the fine-grained particles of ash; pillow basalts are formed when basaltic magma is erupted underwater and quickly quenched.