This article was contributed by Charles W. Finkl, Jr., Professor of Geology, Florida Atlantic University; Director, Coastal Education and Research Foundation; Editor, Journal of Coastal Research.
Volcanoes are usually classified by shape and size. These are determined by such factors as the volume and type of volcanic material ejected, the sequence and variety of eruptions, and the environment. Among the most common types are shield volcanoes, stratovolcanoes, and cinder cones.
Shield volcanoes have a low, broad profile created by highly fluid basalt flows that spread over wide areas. The fluid basalt cannot build up a cone with sides much steeper than 7 degrees. Over thousands of years, however, these cones can reach massive size. The Hawaiian Islands are composed of shield volcanoes that have built up from the sea floor to the surface some 3 miles (5 kilometers) above. Peaks such as Mauna Loa and Mauna Kea rise to more than 13,600 feet (4,145 meters) above sea level. Hawaii is the largest lava structure in the world, while Mauna Loa, if measured from the sea floor, is the world's largest mountain in terms of both height and volume.
Stratovolcanoes are the most common volcanic form. They are composed of alternating layers of lava and pyroclastic material. When a quiet lava flow ends, it creates a seal of solidified lava within the conduit of the volcano. Pressure gradually builds up below, setting the stage for a violent blast of pyroclastic material. These alternating cycles repeat themselves, giving stratovolcanoes a violent reputation.
A cinder cone is a conical hill of mostly cinder-sized pyroclastics. The profile of the cone is determined by the angle of repose, that is, the steepest angle at which debris remains stable and does not slide downhill. Larger cinder fragments, which fall near the summit, can form slopes exceeding 30 degrees. Finer particles are carried farther from the vent and form gentle slopes of about 10 degrees at the base of the cone. These volcanoes tend to be explosive but may also extrude some lava. Cinder cones are numerous, occur in all sizes, and tend to rise steeply above the surrounding area. Those occurring on the flanks of larger volcanoes are called parasitic cones.
Volcanic activity typically alternates between short active periods and much longer dormant periods. An extinct volcano is one that is not erupting and is not likely to erupt in the future. A dormant volcano, while currently inactive, has erupted within historic times and is likely to do so in the future. An inactive volcano is one that has not been known to erupt within historic times. Such classification is arbitrary, however, since almost any volcano is capable of erupting again.
In the late stages of volcanic activity, magma can heat circulating groundwater, producing hot springs and geysers (see Geyser and Fumarole). A geyser is a hot-water fountain that spouts intermittently with great force. One of the best-known examples is Old Faithful in Yellowstone National Park. Fumaroles are vents that emit gas fumes or steam.
Volcanoes occur along belts of tension, where continental plates diverge, and along belts of compression, where the plates converge. Styles of eruption and types of lava are associated with different kinds of plate boundaries. Most lavas that issue from vents in oceanic divergence zones and from midoceanic volcanoes are basaltic. Where ocean plates collide, the rock types basalt and andesite predominate. Near the zone where an ocean plate and continental margin converge, consolidated ash flows are found.
Nearly 1,900 volcanoes are active today or known to have been active in historical times. Of these, almost 90 percent are situated in the Pacific Ring of Fire. This belt partly coincides with the young mountain ranges of western North and South America, and the volcanic island arcs fringing the north and western sides of the Pacific basin. The Mediterranean-Asian orogenic belt has few volcanoes, except for Indonesia and the Mediterranean where they are more numerous. Oceanic volcanoes are strung along the world's oceanic ridges, while the remaining active volcanoes are associated with the African rift valleys.
A cinder cone is a conical hill of mostly cinder-sized pyroclastics. The profile of the cone is determined by the angle of repose, that is, the steepest angle at which debris remains stable and does not slide downhill. Larger cinder fragments, which fall near the summit, can form slopes exceeding 30 degrees. Finer particles are carried farther from the vent and form gentle slopes of about 10 degrees at the base of the cone. These volcanoes tend to be explosive but may also extrude some lava. Cinder cones are numerous, occur in all sizes, and tend to rise steeply above the surrounding area. Those occurring on the flanks of larger volcanoes are called parasitic cones.
Volcanic activity typically alternates between short active periods and much longer dormant periods. An extinct volcano is one that is not erupting and is not likely to erupt in the future. A dormant volcano, while currently inactive, has erupted within historic times and is likely to do so in the future. An inactive volcano is one that has not been known to erupt within historic times. Such classification is arbitrary, however, since almost any volcano is capable of erupting again.
In the late stages of volcanic activity, magma can heat circulating groundwater, producing hot springs and geysers (see Geyser and Fumarole). A geyser is a hot-water fountain that spouts intermittently with great force. One of the best-known examples is Old Faithful in Yellowstone National Park. Fumaroles are vents that emit gas fumes or steam.
Volcanoes occur along belts of tension, where continental plates diverge, and along belts of compression, where the plates converge. Styles of eruption and types of lava are associated with different kinds of plate boundaries. Most lavas that issue from vents in oceanic divergence zones and from midoceanic volcanoes are basaltic. Where ocean plates collide, the rock types basalt and andesite predominate. Near the zone where an ocean plate and continental margin converge, consolidated ash flows are found.
Nearly 1,900 volcanoes are active today or known to have been active in historical times. Of these, almost 90 percent are situated in the Pacific Ring of Fire. This belt partly coincides with the young mountain ranges of western North and South America, and the volcanic island arcs fringing the north and western sides of the Pacific basin. The Mediterranean-Asian orogenic belt has few volcanoes, except for Indonesia and the Mediterranean where they are more numerous. Oceanic volcanoes are strung along the world's oceanic ridges, while the remaining active volcanoes are associated with the African rift valleys.
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