Dangers
The Hawaiian eruptions of shield volcanoes do not pose much threat to humans, as they emit large amounts of slow moving lava over long periods of time. However, they are hazardous to agriculture and infrastructure; the ongoing 1983 eruption of Kīlauea has destroyed over 200 structures and buried kilometers of highways
Sunday, November 7, 2010
Distribution
Main article: List of shield volcanoes
Shield volcanoes are distinctive products of hotspot volcanism, but can form at rift and subduction zones as well.[4]
The largest and most prominent shield volcano chain in the world are the Hawaiian Islands, a chain of hotspot volcanoes in Pacific Ocean. This chain includes the largest volcano on Earth, Mauna Loa. Mauna Kea stands 4,170 m (13,681 ft) above sea level. In addition, its submarine flanks reach a further 5 km (3 mi) below the waterline, and Mauna Loa's massive size depresses the sea floor on which it stands a further 8 km (5 mi), making the volcano's summit about 17 km (56,000 ft) above its base. The volcano is approximately 80,000 km3 (19,193 cu mi) in total volume.[6]
Kilauea is one of the most active volcanoes on Earth, with the current ongoing eruption having begun in January 1983.[3] The Hawaiian volcanoes are characterized by frequent rift eruptions, their large size (thousands of km3 in volume), and their rough, decentralized shape.[1]
Another major center of shield volcanic activity is Iceland. There, the volcanoes are small (~15 km3 (4 cu mi)), symmetrical, and are characterized by eruptions from summit calderas.[1]
Olympus Mons on Mars is also a shield volcano.[6]
Main article: List of shield volcanoes
Shield volcanoes are distinctive products of hotspot volcanism, but can form at rift and subduction zones as well.[4]
The largest and most prominent shield volcano chain in the world are the Hawaiian Islands, a chain of hotspot volcanoes in Pacific Ocean. This chain includes the largest volcano on Earth, Mauna Loa. Mauna Kea stands 4,170 m (13,681 ft) above sea level. In addition, its submarine flanks reach a further 5 km (3 mi) below the waterline, and Mauna Loa's massive size depresses the sea floor on which it stands a further 8 km (5 mi), making the volcano's summit about 17 km (56,000 ft) above its base. The volcano is approximately 80,000 km3 (19,193 cu mi) in total volume.[6]
Kilauea is one of the most active volcanoes on Earth, with the current ongoing eruption having begun in January 1983.[3] The Hawaiian volcanoes are characterized by frequent rift eruptions, their large size (thousands of km3 in volume), and their rough, decentralized shape.[1]
Another major center of shield volcanic activity is Iceland. There, the volcanoes are small (~15 km3 (4 cu mi)), symmetrical, and are characterized by eruptions from summit calderas.[1]
Olympus Mons on Mars is also a shield volcano.[6]
Structure
Dark profile of Hualālai, showing typical shape of a shield volcano.
Because of their gradual buildup and near continuous eruptive characteristics, shield volcanoes are the largest volcanoes on Earth,[4][n 1] usually being at least 3 to 4 mi (5 to 6 km) across and surpassing 1,500 to 2,000 ft (457 to 610 m) in height. The largest shield volcano (and the largest active volcano) in the world is Mauna Loa in Hawaiʻi, which projects 13,677 ft (4,169 m) above sea level.[3]
Shield volcanoes are composed almost exclusively of basalt.[4] Their lower slopes are generally gentle (~2 degrees), but steepen with elevation (reaching ~10 degrees) before flattening near the summit, giving the volcanoes a convex shape.[1]
Over the volcano's lifespan, collapse-driven calderas that form on shield volcanoes are often filled up, and new ones formed elsewhere, in an ongoing cycle of collapse and regeneration.[4]
Dark profile of Hualālai, showing typical shape of a shield volcano.
Because of their gradual buildup and near continuous eruptive characteristics, shield volcanoes are the largest volcanoes on Earth,[4][n 1] usually being at least 3 to 4 mi (5 to 6 km) across and surpassing 1,500 to 2,000 ft (457 to 610 m) in height. The largest shield volcano (and the largest active volcano) in the world is Mauna Loa in Hawaiʻi, which projects 13,677 ft (4,169 m) above sea level.[3]
Shield volcanoes are composed almost exclusively of basalt.[4] Their lower slopes are generally gentle (~2 degrees), but steepen with elevation (reaching ~10 degrees) before flattening near the summit, giving the volcanoes a convex shape.[1]
Over the volcano's lifespan, collapse-driven calderas that form on shield volcanoes are often filled up, and new ones formed elsewhere, in an ongoing cycle of collapse and regeneration.[4]
Geology
Eruptive mechanicsShield volcanoes are built almost entirely of highly fluid basaltic lava. They are distinct from the two other major volcanic types, stratovolcanoes, which are driven by the accumulation of more viscous lavas, and cinder cones, which are built up by the consolidation of tephra ejected by explosive eruptions.[1] The types of eruptions that occur at shield volcanoes have been named Hawaiian eruptions, after the Hawaiian chain where they are most prominent. Hawaiian eruptions are characterized by the effusive emission of fluid lavas.[2] The nature of these lavas allows them to travel a longer distance than flows from other volcanic types, resulting in a large, spread-out sheet of lava[3] just 1 m (3 ft) thick.[1] The gradual buildup of thousands of these flows slowly constructs a low-lying, broad, and gently sloping form of a mature shield volcano.[3]
Continuous shield volcanic activity is very common,[2] and will, over time, build up splatter cones at the eruptive sites, despite Hawaiian activity being 90% lava flows.[4] An example of this is Puʻu ʻŌʻō, a product of Kīlauea's continuous activity.[5]
A hallmark of shield volcanism are lava tubes,[6] cave-like volcanic straights that are formed by the hardening of overlaying lava. These structures further the propagation of lava, as the walls of the tube insulate the flows within.[7] They are an important eruptive element; for example, an estimated 58% of Kilauea is covered by lava tube lava.[6]
Interactions between water and lava at shield volcanoes can cause some eruptions to become hydrovolcanic, which are an explosive eruptive type drastically different from usual shield volcanic activity.[4] These eruptions are especially prevelent at the waterbound volcanoes of the Hawaiian Isles.[2]
Rift zones are another prevalent feature on shield volcanoes that is rare on other volcanic types. The large, decentralized shape of Hawaiian volcanoes versus their small, symmetrical Icelandian cousins can be attributed to these types of eruptions; fissure venting is common in Hawaiʻi, accounting for their asymettrical, non-centralized shapes, and rare in Iceland, where central eruptions from summit calderas dominate and thus the lava distribution is far more even.[1][3][5]
In some shield volcano eruptions, basaltic lava pours out of a long fissure instead of a central vent, and shrouds the countryside with a long band of volcanic material in the form of a broud plateau. Plateaus of this type exist in Iceland, Washington, Oregon, and Idaho; the most prominent ones are situated along the Snake River in Idaho and the Columbia River in Washington and Oregon, where they have been measured to be over a 1 mi (2 km) in thickness.[3] Many eruptions start as a so-called "curtain of fire"—a long eruptive chain along a fissure vent on the volcano. Eventually these eruptions die down and start to focus around a few points on the fissure, where activity is concentrated.[2]
Shield volcano
A shield volcano is a type of volcano built almost entirely of fluid lava flows. They are named so because of their large size and low profile, resembling a warrior's shield. This is caused by the highly fluid lava they erupt, which travel farther than those erupted from more explosive volcanoes. This results in the steady accumulation of broad sheets of lava, building up the shield volcano's distinctive form.
Lava
Main article: lava
Lava flows from stratovolcanoes are generally not a significant threat to people because the highly viscous lava moves slowly enough for people to move out of the path of flow. The lava flows are more of a property threat.
However, not all stratovolcanoes have viscous lava. Mount Nyiragongo is dangerous because its magma has an unusually low silica content, making it quite fluid (even when comparing to Hawaiian lava) and having lower viscosity. Compounded by the very steep slope of Nyiragongo gives the lava the ability to flow at up to about 100 km/h (60 mph).
Lava flows from stratovolcanoes are generally not a significant threat to people because the highly viscous lava moves slowly enough for people to move out of the path of flow. The lava flows are more of a property threat.
However, not all stratovolcanoes have viscous lava. Mount Nyiragongo is dangerous because its magma has an unusually low silica content, making it quite fluid (even when comparing to Hawaiian lava) and having lower viscosity. Compounded by the very steep slope of Nyiragongo gives the lava the ability to flow at up to about 100 km/h (60 mph).
Volcanic bombs
Main article: volcanic bomb
Volcanic bombs are extrusive igneous rocks that range from the size of a book to small automobile, that are explosively ejected from Stratovolcanoes during their peak eruptive phases. These bombs can travel over fifteen miles (20 km) away from the volcano and present a risk to buildings and people while traveling at very high speeds (hundreds of miles per hour or km/h) through the air. The bombs do not themselves explode on impact, but rather carry enough force so as to have destructive effects as if they exploded
Volcanic bombs are extrusive igneous rocks that range from the size of a book to small automobile, that are explosively ejected from Stratovolcanoes during their peak eruptive phases. These bombs can travel over fifteen miles (20 km) away from the volcano and present a risk to buildings and people while traveling at very high speeds (hundreds of miles per hour or km/h) through the air. The bombs do not themselves explode on impact, but rather carry enough force so as to have destructive effects as if they exploded
Mudflows
A mudflow from Mount St. Helens in March 1982.Since the year A.D. 1600, nearly 300,000 people have been killed by volcanic eruptions.[3] Most deaths were caused by pyroclastic flows and mudflows, deadly hazards which often accompany explosive eruptions of subduction-zone stratovolcanoes. Pyroclastic flows are fast-moving, avalanche-like, ground-hugging incandescent mixtures of hot volcanic debris, ash, and gases that can travel at speeds in excess of 100 miles per hour (160 km/h). Approximately 30,000 people were killed by pyroclastic flows during the 1902 eruption of Mont Pelée on the island of Martinique in the Caribbean.[3] In March–April 1982, three explosive eruptions of El Chichón Volcano in the State of Chiapas, southeastern Mexico, caused the worst volcanic disaster in that country's history. Villages within 8 km (5.0 mi) of the volcano were destroyed by pyroclastic flows, killing more than 2,000 people.[3]
Mudflows (also called debris flows or lahars, an Indonesian term for volcanic mudflows) are mixtures of volcanic debris and water. The water usually comes from two sources: rainfall or the melting of snow and ice by hot volcanic debris. Depending on the proportion of water to volcanic material, mudflows can range from soupy floods to thick flows that have the consistency of wet cement.[3] As mudflows sweep down the steep sides of composite volcanoes, they have the strength and speed to flatten or bury everything in their paths. Hot ash and pyroclastic flows from the 1985 eruption of the Nevado del Ruiz Volcano in Colombia, South America, melted snow and ice atop the 5,390-m-high Andean peak; the ensuing mudflows buried the city of Armero, killing 25,000 people.[3]
Mudflows (also called debris flows or lahars, an Indonesian term for volcanic mudflows) are mixtures of volcanic debris and water. The water usually comes from two sources: rainfall or the melting of snow and ice by hot volcanic debris. Depending on the proportion of water to volcanic material, mudflows can range from soupy floods to thick flows that have the consistency of wet cement.[3] As mudflows sweep down the steep sides of composite volcanoes, they have the strength and speed to flatten or bury everything in their paths. Hot ash and pyroclastic flows from the 1985 eruption of the Nevado del Ruiz Volcano in Colombia, South America, melted snow and ice atop the 5,390-m-high Andean peak; the ensuing mudflows buried the city of Armero, killing 25,000 people.[3]
Ash
Apart from possibly affecting climate, volcanic clouds from explosive eruptions also pose a hazard to aviation safety.[3] For example, during the 1982 eruption of Galunggung in Java; British Airways Flight 9 flew into the ash cloud, suffering temporary engine failure and structural damage. During the past two decades, more than 60 airplanes, mostly commercial jetliners, have been damaged by in-flight encounters with volcanic ash. Some of these encounters have resulted in the power loss of all engines, necessitating emergency landings. Luckily, to date no crashes have happened because of jet aircraft flying into volcanic ash.[3] Ashfall is a threat to health when inhaled, and is also a threat to property with high enough accumulation. Greater than 30 cm (12 in) of accumulation is sufficient to collapse most buildings.
Climatic effects
As per the above examples, while the Unzen eruptions have caused deaths and considerable local damage in the historic past, the impact of the June 1991 eruption of Mount Pinatubo was global. Slightly cooler-than-usual temperatures were recorded worldwide and brilliant sunsets and sunrises were attributed to the particulates this eruption lofted high into the stratosphere. The aerosol that formed from the sulfur dioxide (SO2) and other gasses dispersed around the world. The SO2 mass in this cloud—about 22 million tons—combined with water (both of volcanic and stratospheric origin) formed droplets of sulfuric acid, blocking a portion of the sunlight from reaching the troposphere and ground. The cooling in some regions is thought to have been as much as 0.5 °C.[3] An eruption the size of Mount Pinatubo tends to affect the weather for a few years; the material injected into the stratosphere gradually drops into the troposphere where it is washed away by rain and cloud precipitation.
A similar, but extraordinarily more powerful phenomenon occurred in the cataclysmic April 1815 eruption of Mount Tambora on Sumbawa Island in Indonesia. The Mt. Tambora eruption is recognized as the most powerful eruption in recorded history. Its volcanic cloud lowered global temperatures by as much as 3.5 °C.[3] In the year following the eruption, most of the northern hemisphere experienced sharply cooler temperatures during the summer months. In parts of Europe and in North America, 1816 was known as "The Year Without a Summer", which caused a brief but bitter famine.
A similar, but extraordinarily more powerful phenomenon occurred in the cataclysmic April 1815 eruption of Mount Tambora on Sumbawa Island in Indonesia. The Mt. Tambora eruption is recognized as the most powerful eruption in recorded history. Its volcanic cloud lowered global temperatures by as much as 3.5 °C.[3] In the year following the eruption, most of the northern hemisphere experienced sharply cooler temperatures during the summer months. In parts of Europe and in North America, 1816 was known as "The Year Without a Summer", which caused a brief but bitter famine.
Hazards
In recorded history, explosive eruptions at subduction zone (convergent-boundary) volcanoes have posed the greatest hazard to civilizations.[3] Subduction-zone stratovolcanoes, like Mount St. Helens and Mount Pinatubo, typically erupt with explosive force: the magma is too stiff to allow easy escape of volcanic gases. As a consequence the tremendous internal pressures of the trapped volcanic gases remain in the pasty magma. Following the breaching of the magma chamber, the magma degasses explosively. Such an explosive process can be likened to shaking a bottle of carbonated water vigorously, and then quickly removing the cap. The shaking action nucleates the dissolution of CO2 from the liquid as bubbles, increasing the internal volume. The gases and water gush out with speed and force.[3]
Two Decade Volcanoes erupted in 1991 provide examples of stratovolcano hazards. On June 15, Mount Pinatubo spewed ash 40 kilometres (25 mi) into the air and produced huge pyroclastic flows and mudflows that devastated a large area around the volcano. Pinatubo, located 90 km (56 mi) from Manila, had been dormant for 600 years before the 1991 eruption, which ranks as one of the largest eruptions in the 20th Century.[3] Also in 1991, Japan's Unzen Volcano, located on the island of Kyushu about 40 km (25 mi) east of Nagasaki, awakened from its 200-year slumber to produce a new lava dome at its summit. Beginning in June, repeated collapse of this erupting dome generated ash flows that swept down the mountain's slopes at speeds as high as 200 km/h (120 mph). Unzen is one of more than 75 active volcanoes in Japan; an eruption in 1792 killed more than 15,000 people — the worst volcanic disaster in the country's history.[3]
The 79 CE Plinian eruption of Mount Vesuvius, a stratovolcano looming adjacent to Naples, completely covered the cities of Pompeii and Herculaneum with pyroclastic surge deposits. The death toll ranged between 10,000 and 25,000. Mount Vesuvius is recognized as one of the most dangerous volcanoes, jointly because of its potential for powerful explosive eruptions and the high population density of the area (around 3 million people) around its perimeter.
Two Decade Volcanoes erupted in 1991 provide examples of stratovolcano hazards. On June 15, Mount Pinatubo spewed ash 40 kilometres (25 mi) into the air and produced huge pyroclastic flows and mudflows that devastated a large area around the volcano. Pinatubo, located 90 km (56 mi) from Manila, had been dormant for 600 years before the 1991 eruption, which ranks as one of the largest eruptions in the 20th Century.[3] Also in 1991, Japan's Unzen Volcano, located on the island of Kyushu about 40 km (25 mi) east of Nagasaki, awakened from its 200-year slumber to produce a new lava dome at its summit. Beginning in June, repeated collapse of this erupting dome generated ash flows that swept down the mountain's slopes at speeds as high as 200 km/h (120 mph). Unzen is one of more than 75 active volcanoes in Japan; an eruption in 1792 killed more than 15,000 people — the worst volcanic disaster in the country's history.[3]
The 79 CE Plinian eruption of Mount Vesuvius, a stratovolcano looming adjacent to Naples, completely covered the cities of Pompeii and Herculaneum with pyroclastic surge deposits. The death toll ranged between 10,000 and 25,000. Mount Vesuvius is recognized as one of the most dangerous volcanoes, jointly because of its potential for powerful explosive eruptions and the high population density of the area (around 3 million people) around its perimeter.
Thursday, November 4, 2010
The creation of Stratovolcanoes
Stratovolcanoes are common in subduction zones,forming chains along plate tectonic boundaries where oceanic crust is drawn under continetal crust(Continental ARc Volcanism, e.g. Cascade Range,central Andes) or another oceanic plate (Island arc Volcanism, e.g. Japan,Aleutian Islands).The magma that forms stratovolcanoes rises when water trapped both in hydrated minerals and in the porous basalt rock of the upper oceanic crust,is released inyo mantle rock of the asthenosphere above the sinking oceanic slab. The realised of water from hydrated minerals is termed "dewatering, '' and occurs at specific pressures and tempreratures for each mineral,as occurs at specific pressures and temperture for each mineral,as the plate descends to greater depths.The water freed from the rock lowers the melting point of the overlying mantle rock, which undergoes partial melting and rises due to its lighter density relative to the surroundings mantle rock, and pools temporarily at the base of the lithosphere. The ,magma then rises through the crust, incorporating silica-rich crustal rock, leading to a final intermediate composition (see Classification of igneous rock).When the magma near the top surface,it pools in a magma chamber under or within the volcano.There,the relatively low presure allows water and other other volatiles(mainly CO2 ,SO2, CI2and H2O) dissolved in the magma to escape from solution, as occurs when a bottle of carbonated water is open ,releasing CO2.Once a cirtical volume of magma and gas accumulates, the abstacle (mass blockage) of the volcanic cone is overcome , leading to a sudden explosive eruption.
Infomation about stratovolcano
A stratovolcano, also known as a composite volcano,is a tall,conical volcano bulit up by many layers(strata) of hardened larva ,tephra,pumice,and volcanic ash .Unlike shield volcanoes,stratovolcanoes are characterized by a steep profile and periodic,explosive erupitons.The larva that flows from stratovolcanoes typically cools and hardens before spreading far due to high viscosity.The magma forming this larva is often felsic,having high-to-intermediate levels of silica (as in rhyolite,dacite,or andeste0,with lesser amounts of less-viscous mafic magma.Extensive felsic larve flows are uncommon, but have travelled as far as 15(9.3 mi).
Stratovolcanoes are sometimes called "composite volcanoes" because of their composite layered structure built up from sequential outpourings of eruptive materials. They are among the most common types of volcanoes,in contrast to the less common sheild volcanoes.Two famous stratovolcanoes are Krakatoa,best known for its catastrophic ib1883 and Vesuvius,famous for its destructian of the towns Pompeii and Herculaneum in AD79.
Stratovolcanoes are sometimes called "composite volcanoes" because of their composite layered structure built up from sequential outpourings of eruptive materials. They are among the most common types of volcanoes,in contrast to the less common sheild volcanoes.Two famous stratovolcanoes are Krakatoa,best known for its catastrophic ib1883 and Vesuvius,famous for its destructian of the towns Pompeii and Herculaneum in AD79.
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