Stratovolcano

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Mount Vesuvius, near the city of Naples in Italy, violently erupted in 79 AD. The last eruption of this stratovolcano occurred in March 1944. Vesuvius from Naples at sunset.jpg
Mount Vesuvius, near the city of Naples in Italy, violently erupted in 79 AD. The last eruption of this stratovolcano occurred in March 1944.

A stratovolcano, also known as a composite volcano, is a conical volcano built up by many layers (strata) of hardened lava, tephra, pumice and ash. [1] Unlike shield volcanoes, stratovolcanoes are characterized by a steep profile with a summit crater and periodic intervals of explosive eruptions and effusive eruptions, although some have collapsed summit craters called calderas. The lava flowing from stratovolcanoes typically cools and hardens before spreading far, due to high viscosity. The magma forming this lava is often felsic, having high-to-intermediate levels of silica (as in rhyolite, dacite, or andesite), with lesser amounts of less-viscous mafic magma. [2] Extensive felsic lava flows are uncommon, but have travelled as far as 15 km (9.3 mi). [3]

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Stratovolcanoes are sometimes called "composite volcanoes" because of their composite stratified structure built up from sequential outpourings of erupted materials. They are among the most common types of volcanoes, in contrast to the less common shield volcanoes. Two famous examples of stratovolcanoes are Krakatoa in Indonesia, known for its catastrophic eruption in 1883, and Vesuvius in Italy, whose catastrophic eruption in AD 79 ruined the Roman cities of Pompeii and Herculaneum. Both eruptions claimed thousands of lives. In modern times, Mount St. Helens and Mount Pinatubo have erupted catastrophically, with fewer deaths.

The possible existence of stratovolcanoes on other terrestrial bodies of the Solar System has not been conclusively demonstrated. [4] The one feasible exception is the existence of some isolated massifs on Mars, for example the Zephyria Tholus. [5]

Creation

Cross-section of subduction zone and associated stratovolcanoes Subduction-en.svg
Cross-section of subduction zone and associated stratovolcanoes

Stratovolcanoes are common at subduction zones, forming chains and clusters along plate tectonic boundaries where oceanic crust is drawn under continental crust (continental arc volcanism, e.g. Cascade Range, Andes, Campania) or another oceanic plate (island arc volcanism, e.g. Japan, Philippines, Aleutian Islands). The magma forming stratovolcanoes rises when water trapped both in hydrated minerals and in the porous basalt rock of the upper oceanic crust is released into mantle rock of the asthenosphere above the sinking oceanic slab. The release of water from hydrated minerals is termed "dewatering", and occurs at specific pressures and temperatures 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 then undergoes partial melting and rises due to its lighter density relative to the surrounding 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. When the magma nears the top surface, it pools in a magma chamber within the crust below the stratovolcano.

There, the relatively low pressure allows water and other volatiles (mainly CO2, SO2, Cl2, and H2O) dissolved in the magma to escape from solution, as occurs when a bottle of carbonated water is opened, releasing CO2. Once a critical volume of magma and gas accumulates, the plug (solidified blockage) of the volcanic vent is broken, leading to a sudden explosive eruption.[ citation needed ]

Hazards

Mount Etna on the island of Sicily, in southern Italy Etna from 2900m.jpg
Mount Etna on the island of Sicily, in southern Italy
Mt.Fuji from misaka pass 2.jpg
Fugendake 04.JPG
Mount Fuji on Honshu (top) and Mount Unzen on Kyushu (bottom), two of Japan's stratovolcanoes.

In recorded history, explosive eruptions at subduction zone (convergent-boundary) volcanoes have posed the greatest hazard to civilizations. [6] Subduction-zone stratovolcanoes, such as Mount St. Helens, Mount Etna 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 and intermingle in the pasty magma. Following the breaching of the vent and the opening of the crater, the magma degasses explosively. The magma and gases blast out with high speed and full force. [6]

Since 1600 CE, nearly 300,000 people have been killed by volcanic eruptions. [6] Most deaths were caused by pyroclastic flows and lahars, deadly hazards that often accompany explosive eruptions of subduction-zone stratovolcanoes. Pyroclastic flows are swift, avalanche-like, ground-sweeping, incandescent mixtures of hot volcanic debris, fine ash, fragmented lava and superheated gases that can travel at speeds in excess of 160 km/h (100 mph). Around 30,000 people were killed by pyroclastic flows during the 1902 eruption of Mount Pelée on the island of Martinique in the Caribbean. [6] In March to April 1982, three explosive eruptions of El Chichón in the State of Chiapas in southeastern Mexico, caused the worst volcanic disaster in that country's history. Villages within 8 km (5 mi) of the volcano were destroyed by pyroclastic flows, killing more than 2,000 people. [6]

Two Decade Volcanoes that erupted in 1991 provide examples of stratovolcano hazards. On June 15, Mount Pinatubo spewed an ash cloud 40 km (25 mi) into the air and produced huge pyroclastic surges and lahar floods that devastated a large area around the volcano. Pinatubo, located in Central Luzon just 90 km (56 mi) west-northwest from Manila, had been dormant for 6 centuries before the 1991 eruption, which ranks as one of the largest eruptions in the 20th century. [6] 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 nation's history. [6]

The eruption of Mount Vesuvius in 79 completely smothered the nearby ancient cities of Pompeii and Herculaneum with thick deposits of pyroclastic surges and lava flows. Although death toll is estimated between 13,000 and 26,000 remains, the exact number is still unclear. Vesuvius is recognized as one of the most dangerous volcanoes, due to its capacity for powerful explosive eruptions combined with the high population density of the surrounding Metropolitan Naples area (totaling about 3.6 million inhabitants).

Ash

Snow-like blanket of Mount Pinatubo's ashfall deposits in a parking lot on Clark Air Base (June 15, 1991) Ashfall from Pinatubo, 1991.jpg
Snow-like blanket of Mount Pinatubo's ashfall deposits in a parking lot on Clark Air Base (June 15, 1991)

Apart from possibly affecting the climate, volcanic clouds from explosive eruptions also pose a serious hazard to aviation safety. [6] 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 airliners, 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. [6] Ashfalls are a threat to health when inhaled and is also a threat to property with enough accumulation. An accumulation of 30 cm (12 in) is sufficient to cause most buildings to collapse.[ citation needed ] Dense clouds of hot volcanic ash, caused by the collapse of an eruptive column or by being laterally expelled from the partial collapse of a volcanic edifice or lava dome during explosive eruptions, can generate devastating pyroclastic flows or surges, which can sweep off everything in their paths.

Lava

Mayon Volcano extruding lava flows during its eruption on December 29, 2009 Mayon 0052.jpg
Mayon Volcano extruding lava flows during its eruption on December 29, 2009

Lava flows from stratovolcanoes are generally not a significant threat to humans and animals because the highly viscous lava moves slowly enough for everyone to flee out of the path of flow. The lava flows are more of a threat to property. However, not all stratovolcanoes erupt viscous and sticky lava. Nyiragongo is very dangerous because its magma has an unusually low silica content, making it quite fluid. Fluid lavas are typically associated with the formation of broad shield volcanoes such as those of Hawaii, but Nyiragongo has very steep slopes down which lava can flow at up to 100 km/h (60 mph). Lava flows could melt down ice and glaciers that accumulated on the volcano's crater and upper slopes, generating massive lahar flows. Rarely, generally fluid lava could also generate massive lava fountains, while lava of thicker viscosity can solidify within the vent, creating a block which can result in highly explosive eruptions.

Volcanic bombs

Volcanic bombs are extrusive igneous rocks ranging from the size of books to small cars, that are explosively ejected from stratovolcanoes during their climactic eruptive phases. These "bombs" can travel over 20 km (12 mi) away from the volcano, and present a risk to buildings and living beings while shooting at very high speeds (hundreds of kilometers/miles per hour) through the air. Most bombs do not themselves explode on impact, but rather carry enough force so as to have destructive effects as if they exploded.

Lahar

Lahars (from a Javanese term for volcanic mudflows) are mixtures of volcanic debris and water. Lahars usually come from two sources: rainfall or the melting of snow and ice by hot volcanic elements, such as lava. Depending on the proportion and temperature of water to volcanic material, lahars can range from thick, gooey flows that have the consistency of wet concrete to fast-flowing, soupy floods. [6] As lahars flood down the steep sides of stratovolcanoes, they have the strength and speed to flatten or drown everything in their paths. Hot ash clouds, lava flows and pyroclastic surges ejected during 1985 eruption of Nevado del Ruiz in Colombia melted snow and ice atop the 5,321 m (17,457 ft) high Andean volcano. The ensuing lahar flooded the city of Armero and nearby settlements, killing 25,000 people. [6]

Effects on climate and atmosphere

Paluweh eruption as seen from space Paluweh2013labeled.jpg
Paluweh eruption as seen from space

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, with brilliant sunsets and intense sunrises attributed to the particulates; this eruption lofted particles high into the stratosphere. The aerosols that formed from the sulfur dioxide (SO2), carbon dioxide (CO2), and other gases dispersed around the world. The SO2 mass in this cloud—about 22 million tons—combined with water (both of volcanic and atmospheric 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 (0.9 °F). [6] 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 Mount Tambora eruption is recognized as the most powerful eruption in recorded history. Its eruption cloud lowered global temperatures by as much as 3.5 °C (6.3 °F). [6] In the year following the eruption, most of the Northern Hemisphere experienced sharply cooler temperatures during the summer. In parts of Europe, Asia, Africa and North America, 1816 was known as the "Year Without a Summer", which caused considerable agricultural crisis and a brief but bitter famine, which generated a series of distresses across much of the affected continents.

List

See also

Related Research Articles

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Mount Vesuvius Active stratovolcano in the Gulf of Naples, Italy

Mount Vesuvius is a somma-stratovolcano located on the Gulf of Naples in Campania, Italy, about 9 km (5.6 mi) east of Naples and a short distance from the shore. It is one of several volcanoes which form the Campanian volcanic arc. Vesuvius consists of a large cone partially encircled by the steep rim of a summit caldera caused by the collapse of an earlier and originally much higher structure.

Ring of Fire Area of high earthquake and volcanic activity, also the circum-Pacific belt

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Nevado del Ruiz Volcanic mountain in Colombia

The Nevado del Ruiz, also known as La Mesa de Herveo is a volcano located on the border of the departments of Caldas and Tolima in Colombia, about 129 kilometers (80 mi) west of the capital city Bogotá. It is a stratovolcano composed of many layers of lava alternating with hardened volcanic ash and other pyroclastic rocks. Volcanic activity at Nevado del Ruiz began about two million years ago, since the Early Pleistocene or Late Pliocene, with three major eruptive periods. The current volcanic cone formed during the present eruptive period, which began 150 thousand years ago.

Geology of the Lassen volcanic area Geology of a national park in California

The geology of the Lassen volcanic area presents a record of sedimentation and volcanic activity in the area in and around Lassen Volcanic National Park in Northern California, U.S. The park is located in the southernmost part of the Cascade Mountain Range in the Pacific Northwest region of the United States. Pacific Oceanic tectonic plates have plunged below the North American Plate in this part of North America for hundreds of millions of years. Heat from these subducting plates has fed scores of volcanoes in California, Oregon, Washington and British Columbia over at least the past 30 million years and is also responsible for activities in the Lassen volcanic area.

Mount Pinatubo

Mount Pinatubo is an active stratovolcano in the Zambales Mountains, located on the tripoint boundary of the Philippine provinces of Zambales, Tarlac and Pampanga, all in Central Luzon on the northern island of Luzon. Its eruptive history was unknown to most before the pre-eruption volcanic activities of 1991, just before June. Pinatubo was heavily eroded, inconspicuous and obscured from view. It was covered with dense forests which supported a population of several thousand indigenous Aetas.

Mount Garibaldi

Mount Garibaldi is a potentially active stratovolcano in the Sea to Sky Country of British Columbia, 80 km (50 mi) north of Vancouver, British Columbia, Canada. Located in the southernmost Coast Mountains, it is one of the most recognized peaks in the South Coast region, as well as British Columbia's best known volcano. It lies within the Garibaldi Ranges of the Pacific Ranges.

Mount Unzen Mountain in Nagasaki Prefecture, Japan

Mount Unzen is an active volcanic group of several overlapping stratovolcanoes, near the city of Shimabara, Nagasaki on the island of Kyushu, Japan's southernmost main island.

Plinian eruption

Plinian eruptions or Vesuvian eruptions are volcanic eruptions marked by their similarity to the eruption of Mount Vesuvius in 79 AD, which destroyed the ancient Roman cities of Herculaneum and Pompeii. The eruption was described in a letter written by Pliny the Younger, after the death of his uncle Pliny the Elder.

Cerro Azul (Chile volcano) Mountain in Curicó Province, Chile

Cerro Azul, sometimes referred to as Quizapu, is an active stratovolcano in the Maule Region of central Chile, immediately south of Descabezado Grande. Part of the South Volcanic Zone of the Andes, its summit is 3,788 meters (12,428 ft) above sea level, and is capped by a summit crater that is 500 meters (1,600 ft) wide and opens to the north. Beneath the summit, the volcano features numerous scoria cones and flank vents.

Villarrica (volcano)

Villarrica is one of Chile's most active volcanoes, rising above the lake and town of the same name, 750 km (470 mi) south of Santiago. It is also known as Rucapillán, a Mapuche word meaning "great spirit's house". It is the westernmost of three large stratovolcanoes that trend NW-SW obliquely perpendicular to the Andean chain along the Mocha-Villarrica Fault Zone, along with Quetrupillán and the Chilean portion of Lanín, are protected within Villarrica National Park. Guided ascents are popular during summer months.

Explosive eruption Type of volcanic eruption in which lava is violently expelled

In volcanology, an explosive eruption is a volcanic eruption of the most violent type. A notable example is the 1980 eruption of Mount St. Helens. Such eruptions result when sufficient gas has dissolved under pressure within a viscous magma such that expelled lava violently froths into volcanic ash when pressure is suddenly lowered at the vent. Sometimes a lava plug will block the conduit to the summit, and when this occurs, eruptions are more violent. Explosive eruptions can send rocks, dust, gas and pyroclastic material up to 20 km (12 mi) into the atmosphere at a rate of up to 100,000 tonnes per second, traveling at several hundred meters per second. This cloud may then collapse, creating a fast-moving pyroclastic flow of hot volcanic matter.

Prediction of volcanic activity

Prediction of volcanic eruption, or volcanic eruption forecasting, is an interdisciplinary monitoring and research effort to predict the time and severity of a volcano's eruption. Of particular importance is the prediction of hazardous eruptions that could lead to catastrophic loss of life, property, and disruption of human activities.

Cascade Volcanoes Chain of stratovolcanoes in western North America

The Cascade Volcanoes are a number of volcanoes in a volcanic arc in western North America, extending from southwestern British Columbia through Washington and Oregon to Northern California, a distance of well over 700 miles (1,100 km). The arc formed due to subduction along the Cascadia subduction zone. Although taking its name from the Cascade Range, this term is a geologic grouping rather than a geographic one, and the Cascade Volcanoes extend north into the Coast Mountains, past the Fraser River which is the northward limit of the Cascade Range proper.

Types of volcanic eruptions Basic mechanisms of eruption and variations

Several types of volcanic eruptions—during which lava, tephra, and assorted gases are expelled from a volcanic vent or fissure—have been distinguished by volcanologists. These are often named after famous volcanoes where that type of behavior has been observed. Some volcanoes may exhibit only one characteristic type of eruption during a period of activity, while others may display an entire sequence of types all in one eruptive series.

Phreatomagmatic eruption Volcanic eruption involving both steam and magma

Phreatomagmatic eruptions are volcanic eruptions resulting from interaction between magma and water. They differ from exclusively magmatic eruptions and phreatic eruptions. Unlike phreatic eruptions, the products of phreatomagmatic eruptions contain juvenile (magmatic) clasts. It is common for a large explosive eruption to have magmatic and phreatomagmatic components.

Tequila Volcano

Standing at a height of 2,920 meters above sea level, the Tequila Volcano, or Volcán de Tequila is a Stratovolcano located near Tequila, Jalisco, in Mexico. Stratovolcanoes, also referred to as Composite Volcanoes, are the "iconically" conical-shaped volcanoes, found most commonly along subduction zones. Stratovolcanoes are composed of steeply dipping layers of lava, hardened ash, and other material that erupted from the main vent such as tephra and pumice. Commonly higher than 2500 meters above sea-level, Stratovolcanoes have gentle lower slopes which gradually become steeper the higher you get with a relatively small summit crater. Due to their eruptions, Stratovolcanoes have several distinct variations giving some a specific feature such as calderas and amphitheaters.

Lava Molten rock expelled by a volcano during an eruption

Lava is molten rock (magma) that has been expelled from the interior of some planets and some of their moons. Magma is generated by the internal heat of the planet or moon and it is erupted as lava at volcanoes or through fractures in the crust, usually at temperatures from 700 to 1,200 °C. The solid rock resulting from subsequent cooling is also often described as lava.

Mount Cayley massif

The Mount Cayley massif is a group of mountains in the Pacific Ranges of southwestern British Columbia, Canada. Located 45 km (28 mi) north of Squamish and 24 km (15 mi) west of Whistler, the massif resides on the edge of the Powder Mountain Icefield. It consists of an eroded but potentially active stratovolcano that towers over the Cheakamus and Squamish river valleys. All major summits have elevations greater than 2,000 m (6,600 ft), Mount Cayley being the highest at 2,385 m (7,825 ft). The surrounding area has been inhabited by indigenous peoples for more than 7,000 years while geothermal exploration has taken place there for the last four decades.

References

  1. PD-icon.svg This article incorporates  public domain material from the United States Geological Survey document: "Principal Types of Volcanoes" . Retrieved 2009-01-19.
  2. Carracedo, Juan Carlos; Troll, Valentin R., eds. (2013). Teide Volcano: Geology and Eruptions of a Highly Differentiated Oceanic Stratovolcano. Active Volcanoes of the World. Berlin Heidelberg: Springer-Verlag. ISBN   978-3-642-25892-3.
  3. "Garibaldi volcanic belt: Garibaldi Lake volcanic field". Catalogue of Canadian volcanoes. Geological Survey of Canada. 2009-04-01. Archived from the original on June 26, 2009. Retrieved 2010-06-27.CS1 maint: unfit url (link)
  4. Barlow, Nadine (2008). Mars : an introduction to its interior, surface and atmosphere. Cambridge, UK: Cambridge University Press. ISBN   9780521852265.
  5. Stewart, Emily M.; Head, James W. (1 August 2001). "Ancient Martian volcanoes in the Aeolis region: New evidence from MOLA data". Journal of Geophysical Research. 106 (E8): 17505. Bibcode:2001JGR...10617505S. doi:10.1029/2000JE001322.
  6. 1 2 3 4 5 6 7 8 9 10 11 12 13 PD-icon.svg This article incorporates  public domain material from the United States Geological Survey document: Kious, W. Jacquelyne; Tilling, Robert I. "Plate tectonics and people".