Volcanic activity, or volcanism, is directly related to plate tectonics, and most active volcanoes are located near plate boundaries. As spreading or sinking lithospheric plates interact with other Earth materials, magma, or molten rock, including a small component of dissolved gases, mostly water vapour and carbon dioxide, is produced. Magma that has emerged from a volcano onto Earth's surface is called lava. Approximately two-thirds of all active volcanoes on Earth are located in the ring of fire that circumscribes the Pacific Ocean, an area corresponding to subduction zones on the border of the Pacific plate.
VOLCANO TYPES
Each type of volcano has a characteristic style of activity that is partly a result of the viscosity of the magma. Viscosity describes a liquids resistance to flow, with a high viscosity indicating a high resistance to flow. For example, honey is more viscous than water. Magma viscosity is determined by both its silica content, which can vary from about 50 to 70 percent, and its temperature. The higher the silica content of a magma, the more viscous it is. Also, the hotter the magma is, the less viscous it is. The effect of temperature may make sense to you if you think about honey again. If you keep honey in the refrigerator, it becomes very difficult to pour or squeeze out of its container. However, if it is stored in a cabinet, or heated slightly before serving, it flows much more readily. Highly viscous magmas often erupt explosively, as opposed to less viscous magmas, which tend to flow.
Shield Volcanoes
Shield volcanoes are by far the largest volcanoes. They are common in the Hawaiian Islands and are also found in Iceland and some islands in the Indian Ocean. Shield volcanoes are shaped like a gentle arch, or shield. They are among the tallest mountains on Earth when measured from their base, often located on the ocean floor. Shield volcanoes are characterized by generally non-explosive eruptions, which result from the relatively low silica content (about 50 percent) of the magma. When a shield volcano erupts, lava tends to flow down the sides of the volcano rather than explode violently into the atmosphere. The common rock type formed by the magma of shield volcanoes is basalt, composed mostly of feldspar and ferromagnesian minerals. Shield volcanoes are built up almost entirely from numerous lava flows, but they can also produce a lot of tephra. Tephra, also referred to as pyroclastic debris, includes all types of volcanic debris that are explosively ejected from a volcano. Debris particles range from ash, less than 2 mm (0.08 in.) in diameter, to cinders, 4 to 32 mm (0.16 to 1.26 in.) in diameter, to block sand bombs greater than 64 mm (2.52 in.) in diameter. Accumulation of tephra forms pyroclastic deposits (Greek pyro, fire, and klastos, broken ). Pyroclastic deposits may be consolidated to form pyroclastic rocks. The slope of a shield volcano is very gentle near the top, but it increases on the flanks. This change is due to the viscosity of the flowing lava. When magma comes out of vents, or openings, at the top of the volcano, it is quite hot and flows easily. As it flows down the sides of the volcano, it cools and becomes more viscous, so a steeper slope is needed for it to travel farther down slope. In addition to flowing down the sides of a volcano, lava can move away from a vent in a number of ways. Magma often moves for many kilometres underground in lava tubes. These tubes are often very close to the surface, and they insulate the magma, keeping it hot and fluid. After the lava cools and crystallizes to rock, the lava tubes may be left behind as long, sinuous cavern systems. They form natural conduits for movement of groundwater and may cause engineering problems when they are encountered during construction projects. Shield volcanoes may have a summit caldera, which is a steep, walled basin, often 10 km (6.2 mi) or more in diameter. A summit caldera is formed by the collapse of the summit of the volcano, in which a lava lake may form and from which lava may flow during an eruption. Eruptions of lava from shield volcanoes also commonly occur along linear fractures known as rift zones on the flank of a volcano. Rift eruptions from the Hawaiian shield volcano Kilauea on the big island of Hawaii continue to add new land to the island.
Composite Volcanoes
Composite volcanoes are known for their beautiful cone shape. Examples in the United States include Mount St. Helens and Mt. Rainier, both in Washington State. Composite volcanoes are characterized by magma with an intermediate silica content (about 60 percent), which is more viscous than the lower-silica magma of shield volcanoes. The common rock type formed by the magma of composite volcanoes is andesite, composed mostly of soda and lime-rich feldspar and ferromagnesian minerals, with small amounts of quartz. Composite volcanoes are distinguished by a mixture of explosive activity and lava flows. As a result, these volcanoes, also called stratovolcanoes, are composed of alternating layers of pyroclastic deposits and lava flows. Their steep flanks are due to the angle of repose, or the maximum slope angle for loose material, which, for many pyroclastic deposits, is approximately 30 percent to 35 percent. Do not let the beauty of these mountains fool you. Because of their explosive activity and relatively common occurrence, composite volcanoes are responsible for most of the volcanic hazards that have caused death and destruction throughout history. As the 1980 eruption of Mount St. Helens demonstrated, they can produce gigantic horizontal blasts, similar in form to the blast of a shotgun. We should consider such volcanoes armed and dangerous.
Volcanic Domes
Volcanic domes are characterized by viscous magma with a relatively high silica content (about 70 percent). The common rock type produced by this magma is rhyolite, composed mostly of potassium- and sodium-rich feldspar, quartz, and minor amounts of ferromagnesian minerals. The activity of volcanic domes is mostly explosive, making these volcanoes very dangerous. Mt. Lassen in north-eastern California is a good example of a volcanic dome. Mt. Lassens last series of eruptions, from 1914 to 1917, included a tremendous lateral blast that affected a large area.
Cinder Cones
Cinder cones are relatively small volcanoes formed from tephra, mostly volcanic ash and larger particles, including volcanic bombs. Bombs are formed from blobs of ejected lava that spin in the air to take on a rounded shape with tapered ends. Cinder cones grow from the accumulation of tephra near a volcanic vent. They are often found on the flanks of larger volcanoes or along normal faults and long cracks or fissures. The Paricutín cinder cone in the Itzicuaro Valley of central Mexico, west of Mexico City, offered a rare opportunity to observe the birth and rapid growth of a volcano at a location where none had existed before. On February 20, 1943, after several weeks of earthquakes and sounds like thunder coming from beneath the surface of Earth, an astounding event occurred. As Dionisio Pulido was preparing his cornfield for planting, he noticed that a hole in the ground he had been trying to fill for years had opened wider. As Señor Pulido watched, the surrounding ground swelled, rising over 2 m (6.5 ft), while sulphurous smoke and ash began billowing from the hole. By that night, the hole was ejecting glowing red rock fragments high into the air. By the next day, the cinder cone had grown as high as a three-story building, as rocks and ash continued to be blown into the sky by the eruption. After only five days, the cinder cone had grown to the height of a 30-story building. In June 1944, a fissure that had opened in the base of the now 400 m (1312 ft) high cone erupted a thick lava flow that overran the nearby village of San Juan Parangaricutiro, leaving little but the church steeple exposed. No one was killed by the ash or lava flow, and, within a decade, the Paricutín cinder cone became a dormant volcano. Nevertheless, the years of eruption from Dionisios cornfield had significant local impacts. Crops failed they were sometimes buried by ash faster than they could grow and livestock became sick and died. Some villages relocated to other areas, and some villagers moved back to the area. Locating property boundaries was difficult because boundary markers were often covered by ash and lava, resulting in property disputes.
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