Stratovolcano
A stratovolcano, also known as a composite volcano, is a
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.
The existence of stratovolcanoes on other bodies of the Solar System has not been conclusively demonstrated.[5] One possible exception is the existence of some isolated massifs on Mars, for example the Zephyria Tholus.[6]
Creation
Stratovolcanoes are common at
The processes that trigger the final eruption remain a question for further research. Possible mechanisms include:[9][10]
- Magma differentiation, in which the lightest, most silica-rich magma and volatiles such as water, halogens, and sulfur dioxide accumulate in the uppermost part of the magma chamber. This can dramatically increase pressures.[11]
- Fractional crystallization of the magma. When anhydrous minerals such as feldspar crystallize out of the magma, this concentrates volatiles in the remaining liquid, which can lead to second boiling that causes a gas phase (carbon dioxide or water) to separate from the liquid magma and raise magma chamber pressures.[12]
- Injection of fresh magma into the magma chamber, which mixes and heats the cooler magma already present. This could force volatiles out of solution and lower the density of the cooler magma, both of which increase pressure. There is considerable evidence for magma mixing just before many eruptions, including magnesium-rich olivine crystals in freshly erupted silicic lava that show no reaction rim. This is possible only if the lava erupted immediately after mixing since olivine rapidly reacts with silicic magma to form a rim of pyroxene.[13]
- Progressive melting of the surrounding country rock.[14]
These internal triggers may be modified by external triggers such as sector collapse, earthquakes, or interactions with groundwater. Some of these triggers operate only under limited conditions. For example, sector collapse (where part of the flank of a volcano collapses in a massive landslide) can trigger eruption only of a very shallow magma chamber. Magma differentiation and thermal expansion also are ineffective as triggers for eruptions from deep magma chambers.[14]
Hazards
In recorded history, explosive eruptions at subduction zone (convergent-boundary) volcanoes have posed the greatest hazard to civilizations.[15] Subduction-zone stratovolcanoes, such as Mount St. Helens, Mount Etna and Mount Pinatubo, typically erupt with explosive force because the magma is too viscous 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.[15]
Since 1600 CE, nearly 300,000 people have been killed by volcanic eruptions.[15] 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 over 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.[15] During March and 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.[15]
Two
The
Ash
In addition to potentially affecting the climate, volcanic clouds from explosive eruptions pose a serious hazard to aviation.
Lava
Lava flows from stratovolcanoes are generally not a significant threat to humans or animals because the highly
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 to have destructive effects as if they exploded.[citation needed]
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.[15] 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.[15]
Effects on climate and atmosphere
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
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).[15] 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 a considerable agricultural crisis and a brief but bitter famine, which generated a series of distresses across much of the affected continents.[citation needed]
List
See also
- Cinder cone – Steep hill of pyroclastic fragments around a volcanic vent
- Mountain formation – Geological processes that underlie the formation of mountains
- Orogeny – The formation of mountain ranges
- Pyroclastic shield – Shield volcano formed mostly of pyroclastic and highly explosive eruptions
References
- ^ This article incorporates public domain material from Principal Types of Volcanoes. United States Geological Survey. Retrieved 19 January 2009.
- ISBN 978-3-642-25892-3.
- ^ "Garibaldi volcanic belt: Garibaldi Lake volcanic field". Catalogue of Canadian volcanoes. Geological Survey of Canada. 1 April 2009. Archived from the original on 26 June 2009. Retrieved 27 June 2010.
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: CS1 maint: unfit URL (link) - ISBN 9783540436508.
- ISBN 9780521852265.
- .
- ^ a b Schmincke 2003, pp. 113–126.
- Bibcode:2001AGUFM.T41C0871S.
- ^ Schmincke 2003, pp. 51–56.
- .
- ^ Schmincke 2003, p. 52.
- S2CID 218648557.
- ^ Schmincke 2003, p. 54.
- ^ a b Cañón-Tapia 2014.
- ^ a b c d e f g h i j k l m This article incorporates public domain material from Kious, W. Jacquelyne; Tilling, Robert I. Plate tectonics and people. United States Geological Survey.