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|Types of Volcanoes
Volcanologists divide volcanoes into three main groups, each of which is based on the shape of the volcanoes and the type of material they are built of.
Shield Volcanoes are formed when a large amount of free-flowing lava spills from a vent and spreads widely. The lava gradually builds up a low, broad,dome-shaped mountain around the vent. All the Hawaiian Islands are composed of shield volcanoes that have built above the sea surface from the ocean bottom. The more lava making up a volcano, the greater will be its slopes and the broader its base. If the lava erupts from very long cracks in the earth, like it does in Iceland, it spreads out in all directions to form extensive lava plains that can reach heights of 5,000 feet or more.
If a volcano is built of nothing but solid fragments, it is called a Cinder Cone. Such a volcano is formed when large amounts of exploding gas break up the lava into a bubbling froth. The froth is ejected as a fine spray which solidifies into solid particles, with little or no lava flowing as a liquid. These volcanoes have much smaller bases than shield volcanoes, but often reach much greater heights. In addition, shield volcanoes typically have gradual slopes, while cinder cones have steep, often almost vertical, slopes. Parícutin in Mexico is one of the most famous cinder cone volcanoes.
Most volcanoes are neither purely cinver cones nor purely shields. Sometimes they erupt quiety, sometimes explosively, and build a Composite Cone of fragmental material interbedded with lava. Most of the world's best-known volcanoes are compsoite types, including Japan's Mount Fuji and Italy's Mount Vesuvius. The eruptions that built them began with explosions of solid material from a central crater or vent. Later on, lava flowed through fissures in the sides of the mountain, or out of secondary vents near the summit. As such eruptions continue, the cones grow larger, many reaching many thousands of feet in height.
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This page was last updated on 05/07/2017.