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 Stratovolcanoes, such as Mount Shasta, are tall conical mountains composed of alternating layers of lava flows and ejected material.

 Lava domes, such as Lassen Peak, are built up by slow eruptions of highly viscous lavas.

 Cinder cones result from the eruption of small pieces of scoria or pyroclastics, which resemble cinders. These are often relatively short-lived eruptions and build up cones of between 100ft and 1000ft high. Most cinder cones erupt only once and often form as flank vents on the sides of larger volcanoes.

 Shield volcanoes are formed by the eruption of low viscosity lavas which can flow a great distance from the vent. They don’t usually explode catastrophically or form volcanic cones but they can produce massive lava fields.

There is no agreed distinction between an active and a dormant volcano but the Smithsonian Institute defines a volcano as active if it has erupted within the last 10,000 years and many of the volcanoes in the Pacific North-West fall into this category. Volcanoes can be dormant for thousands of years and become eroded and worn down but magma remains close to the surface, which means they could erupt again. Often these eruptions will be violent, as pressure will have built up beneath the plug that has prevented lava escaping. Volcanoes are only considered extinct when they no longer have a lava supply.