Editing Stratovolcano (section)

==Hazards==
{{Main|Volcanic hazard}}
[[File:Etna from 2900m.jpg|thumb|Mount Etna on the island of [[Sicily]], in southern Italy]]
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| footer            = [[Mount Fuji]] on [[Honshu]] (top) and [[Mount Unzen]] on [[Kyushu]] (bottom), two of [[Japan]]'s stratovolcanoes
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In [[recorded history]], explosive eruptions at [[Subduction|subduction zone]] ([[Convergent boundary|convergent-boundary]]) volcanoes have posed the greatest hazard to civilizations.<ref name="dynearth">{{USGS|title=Plate tectonics and people|last1=Kious|first1=W. Jacquelyne|last2=Tilling|first2=Robert I.|url=http://pubs.usgs.gov/gip/dynamic/tectonics.html}}</ref> [[Subduction|Subduction-zone]] stratovolcanoes, such as [[Mount St. Helens]], [[Mount Etna]] and [[Mount Pinatubo]], typically erupt with explosive force because the [[magma]] is too [[Viscosity|viscous]] to allow easy escape of [[volcanic gas]]es.<ref>{{Cite web|title=Types of volcano|url=https://www.bgs.ac.uk/discovering-geology/earth-hazards/volcanoes/how-volcanoes-form/#:~:text=NASA%20Earth%20Observatory.-,Stratovolcano,a%20volcano%20with%20steep%20sides.|access-date=24 October 2024|publisher=British Geological Survey}}</ref> As a consequence, the tremendous internal pressures of the trapped volcanic gases remain and intermingle in the pasty [[magma]].  Following the breaching of the [[Volcanic vent|vent]] and the opening of the crater, the [[magma]] degasses explosively. The magma and gases blast out with high speed and full force.<ref name="dynearth"/>

Since 1600 [[Common Era|CE]], nearly 300,000 people have been killed by [[volcanic eruptions]]. Most deaths were caused by [[pyroclastic flow]]s and [[lahar]]s, deadly hazards that often accompany explosive eruptions of [[Subduction|subduction-zone]] stratovolcanoes.<ref name="dynearth" /> Pyroclastic flows are swift, avalanche-like, ground-sweeping, incandescent mixtures of hot volcanic debris, fine [[Volcanic ash|ash]], fragmented [[lava]], and superheated gases that can travel at speeds over {{convert|150|km/h|sigfig=1|abbr=on}}.<ref name="dynearth" /> Around 30,000 people were killed by pyroclastic flows during the [[1902 eruption of Mount Pelée]] on the island of [[Martinique]] in the [[Caribbean]].<ref name="dynearth"/> During March and April 1982, [[El Chichón]] in the State of [[Chiapas]] in southeastern [[Mexico]], erupted 3 times, causing the worst volcanic disaster in that country's history and killied more than 2,000 people in pyroclastic flows.<ref name="dynearth"/>

Two [[Decade Volcanoes]] that erupted in 1991 provide examples of stratovolcano hazards. On 15 June, Mount Pinatubo erupted and caused an [[Volcanic ash|ash cloud]] to shoot 40&nbsp;km (25&nbsp;mi) into the air. It produced large [[pyroclastic surge]]s and [[lahar]] floods that caused a lot of damage to the surrounding area.<ref name="dynearth" /> Mount Pinatubo, located in [[Central Luzon]] just 90&nbsp;km (56&nbsp;mi) west-northwest of [[Manila]], had been dormant for six centuries before the 1991 eruption. This eruption was one of the 2nd largest in the 20th century.<ref name=":1">{{Cite web|title=The Cataclysmic 1991 Eruption of Mount Pinatubo, Philippines, Fact Sheet 113-97|url=https://pubs.usgs.gov/fs/1997/fs113-97/|access-date=24 October 2024|publisher=United States Geological Survey}}</ref> It produced a large [[volcanic ash]] cloud that affected global temperatures, lowering them in areas as much as .5&nbsp;°C.<ref name=":1" /> The [[volcanic ash]] cloud consisted of 22 million tons of [[Sulfur dioxide|SO<sub>2</sub>]] which combined with water droplets to create [[sulfuric acid]].<ref name="dynearth" /> In 1991 Japan's [[Mount Unzen]] also erupted, after 200 years of inactivity. It's located on the island of [[Kyushu]] about 40&nbsp;km (25&nbsp;mi) east of [[Nagasaki]].<ref name="dynearth" /> Beginning in June, a newly formed lava dome repeatedly collapsed. This generated a pyroclastic flow that flowed down the mountain's slopes at speeds as high as 200&nbsp;km/h (120&nbsp;mph).<ref name="dynearth" /> The 1991 eruption of Mount Unzen was one of the worst volcanic disasters in Japan's history, once killing more than 15,000 people in 1792.<ref>{{Cite web|title=Mount Unzen eruption of 1792 {{!}} Volcanic Disaster, Deceit & Death {{!}} Britannica|url=https://www.britannica.com/event/Mount-Unzen-eruption-of-1792|access-date=24 October 2024|website=Encyclopædia Britannica}}</ref>

The [[eruption of Mount Vesuvius in 79 AD]] is the most famous example of a hazardous stratovolcano eruption. It completely smothered the nearby ancient cities of [[Pompeii]] and [[Herculaneum]] with thick deposits of [[pyroclastic surge]]s and [[pumice]] ranging from 6–7 meters deep. Pompeii had 10,000–20,000 inhabitants at the time of eruption.<ref>{{Cite web|date=25 September 2024|title=Pompeii {{!}} History, Volcano, Map, Population, Ruins, & Facts {{!}} Britannica|url=https://www.britannica.com/place/Pompeii#ref5859|access-date=25 September 2024|website=Encyclopædia Britannica}}</ref> [[Mount Vesuvius]] is recognized as one of the most dangerous of the world's volcanoes, due to its capacity for [[Plinian eruption|powerful explosive eruptions]] coupled with the high population density of the surrounding [[Metropolitan City of Naples|Metropolitan Naples]] area (totaling about 3.6 million inhabitants).<ref>{{Cite journal|last=Rolandi|first=G.|date=January 2010|title=Volcanic hazard at Vesuvius: An analysis for the revision of the current emergency plan|url=https://linkinghub.elsevier.com/retrieve/pii/S0377027309003254|journal=Journal of Volcanology and Geothermal Research|volume=189|issue=3–4|pages=347–362|doi=10.1016/j.jvolgeores.2009.08.007|bibcode=2010JVGR..189..347R }}</ref>

===Ash===
{{main|Volcanic ash}}
[[File:Ashfall from Pinatubo, 1991.jpg|thumb|Snow-like blanket of [[Mount Pinatubo]]'s ashfall deposits in a parking lot on [[Clark Air Base]] (15 June 1991)]]

In addition to potentially affecting the climate, [[volcanic ash]] clouds from [[explosive eruption]]s pose a serious hazard to [[aviation]].<ref name="dynearth"/> [[Volcanic ash]] clouds consist of silt- or sand-sized pieces of rock, mineral, [[volcanic glass]]. Volcanic ash grains are jagged, abrasive, and don't dissolve in water.<ref name=":22">{{Cite web|title=Ash Fall—A "Hard Rain" of Abrasive Particles {{!}} USGS Volcano Fact Sheet|url=https://pubs.usgs.gov/fs/fs027-00/|access-date=16 October 2024|publisher=United States Geological Survey}}</ref> For example, during the 1982 eruption of [[Galunggung]] in [[Java]], [[British Airways Flight 9]] flew into the ash cloud, causing it to sustain temporary engine failure and structural damage.<ref>{{Cite web|title=Global Volcanism Program {{!}} Report on Galunggung (Indonesia) – June 1982|url=https://volcano.si.edu/showreport.cfm?doi=10.5479/si.GVP.SEAN198206-263140|access-date=24 October 2024|publisher=Global Volcanism Program|doi=10.5479/si.gvp.sean198206-263140}}</ref> Although no crashes have happened due to ash, more than 60, mostly [[commercial aircraft]], have been damaged. Some of these incidents resulted in emergency landings.<ref name=":05">{{Cite web|title=Plate tectonics and people [This Dynamic Earth, USGS]|url=https://pubs.usgs.gov/gip/dynamic/tectonics.html|access-date=25 September 2024|publisher=United States Geological Survey}}</ref><ref name="dynearth"/> [[Volcanic ash|Ashfalls]] are a threat to health when inhaled and are also a threat to property. A square yard of a 4-inch thick volcanic ash layer can weigh 120–200 pounds and can get twice as heavy when wet. Wet ash also poses a risk to electronics due to its [[Electrical conductor|conductive]] nature.<ref name=":22"/> Dense clouds of hot volcanic ash can be expelled due to the collapse of an [[Eruption column|eruptive column]], or laterally due to the partial collapse of a [[volcanic edifice]] or [[lava dome]] during [[explosive eruption]]s. These clouds are known as [[pyroclastic surge]]s and in addition to volcanic ash, they contain hot [[lava]], [[pumice]], [[Volcanic rock|rock]], and volcanic gas. Pyroclastic surges flow at speeds over 50&nbsp;mph and are at temperatures between 200&nbsp;°C – 700&nbsp;°C. These surges can cause major damage to property and people in their path.<ref>{{Cite web|title=Pyroclastic flows move fast and destroy everything in their path {{!}} U.S. Geological Survey|url=https://www.usgs.gov/programs/VHP/pyroclastic-flows-move-fast-and-destroy-everything-their-path|access-date=16 October 2024|publisher=United States Geological Survey}}</ref>

===Lava===
{{main|Lava}}
[[File:Mayon 0052.jpg|right|thumb|[[Mayon|Mayon Volcano]] in [[Philippines]] extruding lava flows during its eruption on 29 December 2009]]

[[Lava|Lava flows]] from stratovolcanoes are generally not a significant threat to humans or animals because the highly [[Viscosity|viscous]] [[lava]] moves slowly enough for everyone to evacuate. Most deaths attributed to lava are due to related causes such as [[explosion]]s and [[asphyxia]]tion from [[toxic gas]].<ref name=":12">{{Cite web|title=Lava flows destroy everything in their path|url=https://www.usgs.gov/programs/VHP/lava-flows-destroy-everything-their-path|access-date=16 October 2024|publisher=United States Geological Survey}}</ref> [[Lava|Lava flows]] can bury homes and farms in thick [[volcanic rock]] which greatly reduces property value.<ref name=":12" /> However, not all stratovolcanoes erupt viscous and sticky lava. [[Nyiragongo]], near [[Lake Kivu]] in [[central Africa]], is very dangerous because its [[magma]] has an unusually low [[Silicate|silica content]], making it much less viscous than other stratovolcanoes. Low [[viscosity]] lava can generate massive [[lava fountain]]s, while lava of thicker viscosity can solidify within the vent, creating a [[volcanic plug]]. Volcanic plugs can trap [[volcanic gas]] and create pressure in the magma chamber, resulting in violent eruptions.<ref>{{Cite web|title=Eruption styles|url=https://www.bgs.ac.uk/discovering-geology/earth-hazards/volcanoes/eruption-styles/|access-date=24 October 2024|publisher=British Geological Survey}}</ref> Lava is typically between 700 and 1,200&nbsp;°C (1,300–2,200&nbsp;°F).<ref>{{Cite web|date=23 September 2024|title=Lava {{!}} Types, Composition, Temperature, & Facts {{!}} Britannica|url=https://www.britannica.com/science/lava-volcanic-ejecta|access-date=24 October 2024|website=Encyclopædia Britannica}}</ref>

===Volcanic bombs===
{{main|Volcanic bomb}}

[[Volcanic bomb]]s are masses of unconsolidated rock and lava that are ejected during an eruption. Volcanic bombs are classified as larger than 64mm (2.5 inches). Anything from 2 to 64mm is classified as [[lapilli]].<ref name=":03">{{Cite web|title=Volcanic Bombs: Overview|url=https://www.sandatlas.org/volcanic-bomb/|access-date=24 October 2024|website=sandatlas.org}}</ref> When erupted, volcanic bombs are still molten and partially cool and solidify on their descent. They can form ribbon or oval shapes that can also flatten on impact with the ground.<ref>{{Cite web|title=Bomb {{!}} Explosive Eruption, Pyroclastic Flow, Ash Cloud {{!}} Britannica|url=https://www.britannica.com/science/bomb-volcanic-ejecta|access-date=24 October 2024|website=Encyclopædia Britannica}}</ref> Volcanic bombs are associated with [[Strombolian eruption|Strombolian]] and [[Vulcanian eruption|Vulcanian]] eruptions and [[basaltic lava]]. Ejection velocities ranging from 200 to 400&nbsp;m/s have been recorded causing volcanic bombs to be destructive.<ref name=":03" />

===Lahar===
{{main|Lahar}}

[[Lahar]]s (from a [[Javanese language|Javanese]] term for volcanic mudflows) are a mixture of volcanic debris and water. Lahars can result from heavy rainfall during or before the eruption or interaction with ice and snow. Meltwater mixes with volcanic debris causing a fast moving mudflow. Lahars are typically about 60% sediment and 40% water.<ref name=":04">{{Cite web|title=Volcanic hazards|url=https://www.bgs.ac.uk/discovering-geology/earth-hazards/volcanoes/volcanic-hazards/|access-date=24 October 2024|publisher=British Geological Survey}}</ref> Depending on the abundance of volcanic debris the lahar can be fluid or thick like concrete.<ref name=":13">{{Cite web|title=Plate tectonics and people [This Dynamic Earth, USGS]|url=https://pubs.usgs.gov/gip/dynamic/tectonics.html|access-date=24 October 2024|publisher=United States Geological Survey}}</ref> [[Lahar]]s have the strength and speed to flatten structures and cause great bodily harm, gaining speeds up to dozens of kilometers per hour.<ref name=":04" /> In the [[Armero tragedy|1985 eruption]] of [[Nevado del Ruiz]] in [[Colombia]], [[Pyroclastic surge]]s melted snow and ice atop the 5,321&nbsp;m (17,457&nbsp;ft) high Andean volcano. The ensuing lahar killed 25,000 people and flooded the city of [[Armero]] and nearby settlements.<ref name=":13" />

=== Volcanic gas ===
{{Main|Volcanic gas}}
As a volcano forms, several different [[Volcanic gas|gases]] mix with [[magma]] in the volcanic chamber. During an eruption the gases are then released into the [[atmosphere]] which can lead to toxic human exposure. The most abundant of these [[Volcanic gas|gases]] is [[Water|H<sub>2</sub>O]] ([[water]]) followed by [[Carbon dioxide|CO<sub>2</sub>]] ([[carbon dioxide]]), [[Sulfur dioxide|SO<sub>2</sub>]] ([[sulfur dioxide]]), [[Hydrogen sulfide|H<sub>2</sub>S]] ([[hydrogen sulfide]]), and [[Hydrogen fluoride|HF]] ([[hydrogen fluoride]]).<ref name=":04"/> If at concentrations of more than 3% in the air, when breathed in [[Carbon dioxide|CO<sub>2</sub>]] can cause dizziness and difficulty breathing. At more than 15% concentration [[Carbon dioxide|CO<sub>2</sub>]] causes death. [[Carbon dioxide|CO<sub>2</sub>]] can settle into depressions in the land, leading to deadly, odorless pockets of gas.<ref name=":06">{{Cite web|title=Volcanic gases can be harmful to health, vegetation and infrastructure {{!}} U.S. Geological Survey|url=https://www.usgs.gov/programs/VHP/volcanic-gases-can-be-harmful-health-vegetation-and-infrastructure|access-date=24 October 2024|publisher=United States Geological Survey}}</ref> [[Sulfur dioxide|SO<sub>2</sub>]] classified as a respiratory, skin, and eye irritant if come into contact with. It is known for its pungent egg smell and role in ozone depletion and has the potential to cause acid rain downwind of an eruption.<ref name=":06" /> [[Hydrogen sulfide|H<sub>2</sub>S]] has an even stronger odor than [[Sulfur dioxide|SO<sub>2</sub>]] as well as being even more toxic. Exposure for less than an hour at concentrations of over 500 ppm causes death.<ref name=":06" /> [[Hydrogen fluoride|HF]] and similar species can coat [[ash]] particles and once deposited can poison soil and water.<ref name=":06" /> Gases are also emitted during volcanic degassing, which is a passive release of [[gas]] during periods of dormancy.<ref name=":06" />