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Ash cloud from the 2008 eruption of Chaitén volcano stretching across Patagonia from the Pacific to the Atlantic Ocean

Volcanic ash consists of small tephra, which are bits of pulverized rock and glass created by volcanic eruptions,[1] less than 2 millimetres (0.079 in) in diameter. There are three mechanisms of volcanic ash formation: gas release under decompression causing magmatic eruptions; thermal contraction from chilling on contact with water causing phreatomagmatic eruptions, and ejection of entrained particles during steam eruptions causing phreatic eruptions.[2] The violent nature of volcanic eruptions involving steam results in the magma and solid rock surrounding the vent being torn into particles of clay to sand size. Volcanic ash can lead to breathing problems, malfunctions in machinery, and from more severe eruptions, years of global cooling.

Ash deposited on the ground after an eruption is known as ashfall deposit. Significant accumulations of ashfall can lead to the immediate destruction of most of the local ecosystem, as well the collapse of roofs on man-made structures. Over time, ashfall can lead to the creation of fertile soils. Ashfall can also become cemented together to form a solid rock called tuff. Over geologic time, the ejection of large quantities of ash can produce an ash cone.

Contents

Formation

There are three mechanisms of volcanic ash formation: gas release under decompression causing magmatic eruptions; thermal contraction from chilling on contact with water causing phreatomagmatic eruptions and ejection of entrained particles during steam eruptions causing phreatic eruptions.[2] The violent nature of volcanic eruptions involving steam results in the magma and solid rock surrounding the vent being torn into particles of clay to sand size.

Composition

Particle of volcanic ash from Mount St. Helens

The term for any material explosively thrown out from a vent is tephra or pyroclastic debris.[1] Ash terminology is restricted to very fine rock and mineral particles less than 2 millimetres (0.079 in) in diameter which are ejected from a volcanic vent.[3]

Clast Size Pyroclast Mainly Unconsolidated:

Tephra

Mainly Consolidated:

pyroclastic rock

> 64 mm Bomb, Block Agglomerate Agglomerate, pyroclastic breccia
< 64 mm Lapillus Layer, Lapilli Tephra Lapilli Tuff, Lapillistone
< 2 mm Coarse Ash Coarse Ash Coarse (ash) Tuff
< 0.063 mm Fine Ash Fine Ash Fine (ash) Tuff
Table modified after Heiken and Wohletz, 1985.[2]

Ash is created when solid rock shatters and magma separates into minute particles during explosive volcanic activity. The usually violent nature of an eruption involving steam (phreatic eruption or phreatomagmatic eruption) results in the magma and solid rock surrounding the vent being torn into particles of clay to sand size.[3]

Diamond Head, a well-known backdrop to Waikiki in Hawaii, is an ash cone that solidified into tuff

Spread

The plume that is often seen above an erupting volcano is composed primarily of ash and steam. The very fine particles may be carried for many miles, settling out as a dust-like layer across the landscape. This is known as an ashfall.[4]

Volcanic ash dunes near Tarvurvur Crater, Rabaul caldera

If liquid magma is ejected as a spray, the particles will solidify in the air as small fragments of volcanic glass. Unlike the ash that forms from burning wood or other combustible materials, volcanic ash is hard and abrasive. It does not dissolve in water, and it conducts electricity, especially when it is wet.

Ashfall can become cemented together by heat to form a solid rock called tuff.[5] Ashfall breaks down over time, forming highly fertile soil, which has made many volcanic regions densely cultivated and inhabited despite the inherent dangers.[6]

Atmospheric effects

Chichester Canal by J. M. W. Turner, whose brilliant colors may be due to ash from the eruption of Mount Tambora in Indonesia
Daytime Montserrat image during ash fall (1997)

When ash begins to fall during daylight hours, the sky turns hazy and a pale yellow color. The ashfall may become so dense that daylight turns the sky gray to pitch black, with the ash severely restricting visibility and deadening sound. A darkened ash sky lowers temperatures during daylight hours from what would otherwise be expected. Loud thunder and lightning as well as the strong smell of sulfur accompany an ashfall.[7] If rain accompanies an ashfall, the tiny particles turn into a slurry of slippery mud. Rain and lightning combined with ash can lead to power outages, breakdowns of communication, and disorientation.[8]

Hong Kong sunset c. 1992 after the eruption of Mount Pinatubo.
River eroding volcanic ash flow Alaska Southwest, Valley of Ten Thousand Smokes

Very fine ash particles can remain high in the atmosphere for many years, spread around the world by high-altitude winds. This suspended material contributes to spectacular sunsets, as well as an optical phenomenon known as "Bishop's Ring", which refers to a corona or halo effect around the sun.[9] High levels of ash high in the atmosphere causes climate change by cooling the globe for a few years following major eruptions. The last episode of ash-induced global cooling followed the Mount Pinatubo eruption of 1991.[10] The most documented case in recorded history of this phenomenon followed the epic eruption of Mount Tambora in 1815, which led to the year without summer in 1816.[11]

Hazards

Rainbow and volcanic ash with sulfur dioxide emissions from Halema`uma`u vent.

The most devastating effect of volcanic ash comes from pyroclastic flows. These occur when a volcanic eruption creates an "avalanche" of hot ash, gases, and rocks that flow at high speed down the flanks of the volcano. These flows can be impossible to outrun.[12] As well as being impossible to outrun, they are almost as difficult to predict. In many cases prediction has been based on the topography of a region, only to see a valley fill and overflow[13]. In 1902, the city of St. Pierre in Martinique was destroyed by a pyroclastic flow which killed over 29,000 people.[14]

Volcanic ash (by itself) is not poisonous, but inhaling it may cause problems for people whose respiratory system is already compromised by disorders such as asthma or emphysema. The abrasive texture can cause irritation and scratching of the surface of the eyes. People who wear contact lenses should wear glasses during an ashfall, to prevent eye damage. Furthermore, the combination of volcanic ash with moisture in the lungs can create a substance akin to liquid cement.

Therefore, people should take caution to filter the air they breathe with a damp cloth or a face mask when facing an ashfall. Ash is very dense, as only 100 millimetres (3.9 in) of ash leads to the collapse of weaker roofs. A fall of 300 millimetres (12 in) leads to the death of most vegetation, livestock, the wiping out of aquatic life in nearby lakes and rivers, and unusable roads.[15] Accompanied by rain and lightning, ashfall leads to power outages, prevents communication, and disorients people.[8]

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Aviation

Ash plume from Mt Cleveland, a stratovolcano

Volcanic ash jams machinery. This poses a great danger to aircraft flying near ash clouds. There are many instances of damage to jet aircraft as a result of an ash encounter. Engines quit as fuel and water systems become fouled, requiring repair. After the Galunggung, Indonesia volcanic event in 1982, a British Airways Boeing 747 flew through an ash cloud that fouled all 4 engines, stopping them. The plane descended from 36,000 feet (11,000 m) to 12,000 feet (3,700 m) before the crew could manage to restart the engines.[16]

Marine transportation

Similar to aviation, volcanic ash has detrimental effects on marine transportation machinery.

Advisories concerning ongoing events

Increasing numbers of airplane incidents from atmospheric ash prompted a 1991 aviation industry meeting to decide how best to distribute information about ash events. One solution was the creation of Volcanic Ash Advisory Centers. There is one VAAC for each of nine regions of the world. VAACs can issue advisories and serve as liaisons between meteorologists, volcanologists, and the aviation industry.[17]

See also

References

  1. ^ a b United States Geological Survey. Tephra: Volcanic Rock and Glass Fragments. Retrieved on 2008-01-23.
  2. ^ a b c Heiken, G. & Wohletz, K. 1985. Volcanic Ash. University of California Press, Berkeley
  3. ^ a b Smithsonian Institution's National Museum of Natural History. Types and Processes Gallery - Magma meets Water. Retrieved on 2008-01-23.
  4. ^ Merriam-Webster's Online Dictionary. Ashfall. Retrieved on 2008-01-23.
  5. ^ Merriam-Webster's Online Dictionary. Tuff. Retrieved on 2008-01-23.
  6. ^ Skwirk. Volcanic mountains and living in volcanic zones. Retrieved on 2008-01-23.
  7. ^ United States Geological Survey. What's it like during an ash fall? Retrieved on 2008-01-23.
  8. ^ a b United States Geological Survey. Volcanic Ash... What it can do and how to prevent damage. Retrieved on 2008-01-23.
  9. ^ Glossary of Meteorology. Bishop's Ring. Retrieved on 2008-01-23.
  10. ^ United States Geological Survey. The Cataclysmic 1991 Eruption of Mount Pinatubo, Philippines. Retrieved on 2008-01-23.
  11. ^ NASA. Volcanoes and Global Cooling. Retrieved on 2008-01-23.
  12. ^ United States Geological Survey. Generation of Pyroclastic Flows. Retrieved on 2008-01-23.
  13. ^ Branney M.J. & Kokelaar, B.P. 2002, Pyroclastic Density Currents and the Sedimentation of Ignimbrites. Geological Society London Memoir 27, 143pp.
  14. ^ Zananas. Saint-Pierre Martinique: Pelée Mountain and Eruption of 1902. Retrieved on 2008-01-23.
  15. ^ GNS Science. Volcanoes in New Zealand. Retrieved on 2008-01-23.
  16. ^ C. M. Riley Tephra. Retrieved on 2008-01-23.
  17. ^ NESDIS. Volcanic Ash Advisory Centers. Retrieved on 2008-01-23.

External links

Further reading


stretching across Patagonia from the Pacific to the Atlantic Ocean.]]

ic eruption event spreads its ashes to Scandinavia in 48 hours.]]

in the catacombs of Peter the Great's Naval Fortress in Estonia near Laagri. The diameter of objective cover is 58 mm.]]

Volcanic ash consists of small tephra, which are bits of pulverized rock and glass created by volcanic eruptions,[1] less than 2 millimetres (0.1 in) in diameter. There are three mechanisms of volcanic ash formation: gas release under decompression causing magmatic eruptions; thermal contraction from chilling on contact with water causing phreatomagmatic eruptions, and ejection of entrained particles during steam eruptions causing phreatic eruptions.[2] The violent nature of volcanic eruptions involving steam results in the magma and solid rock surrounding the vent being torn into particles of clay to sand size. Volcanic ash can lead to breathing problems and malfunctions in machinery, and clouds of it can threaten aircraft and alter weather patterns.

Ash deposited on the ground after an eruption is known as ashfall deposit. Significant accumulations of ashfall can lead to the immediate destruction of most of the local ecosystem, as well the collapse of roofs on man-made structures. Over time, ashfall can lead to the creation of fertile soils. Ashfall can also become cemented together to form a solid rock called tuff. Over geologic time, the ejection of large quantities of ash can produce an ash cone.

Contents

Formation

, a stratovolcano.]]

There are three mechanisms of volcanic ash formation:

  1. Gas release under decompression causing magmatic eruptions;
  2. Thermal contraction from chilling on contact with water causing phreatomagmatic eruptions
  3. Ejection of entrained particles during steam eruptions causing phreatic eruptions.[2] The violent nature of volcanic eruptions involving steam results in the magma and solid rock surrounding the vent being torn into particles of clay to sand size.

If a volcanic eruption occurs beneath glacial ice, cold water from melted ice chills the lava quickly and fragments it into glass, creating small glass particles that get carried into the eruption plume. This can create a glass-rich plume in the upper atmosphere which is particularly hazardous to aircraft[3]

Composition

.]]


The term for any material explosively thrown out from a vent is tephra or pyroclastic debris.[1] Ash terminology is restricted to very fine rock and mineral particles less than 2 millimetres (0.079 in) in diameter which are ejected from a volcanic vent.[4]

Clast Size Pyroclast Mainly Unconsolidated:

Tephra

Mainly Consolidated:

pyroclastic rock

> 64 mm Bomb, Block Agglomerate Agglomerate, pyroclastic breccia
< 64 mm Lapillus Layer, Lapilli Tephra Lapilli Tuff, Lapillistone
< 2 mm Coarse Ash Coarse Ash Coarse (ash) Tuff
< 0.063 mm Fine Ash Fine Ash Fine (ash) Tuff
Table modified after Heiken and Wohletz, 1985.[2]

Ash is created when solid rock shatters and magma separates into minute particles during explosive volcanic activity. The usually violent nature of an eruption involving steam (phreatic eruption or phreatomagmatic eruption) results in the magma and solid rock surrounding the vent being torn into particles of clay to sand size.[4]

Spread

image of the area around Karymsky. Ash from earlier eruptions has settled onto the snowy landscape, leaving dark grey swaths. The ash stains are confined to the south of the volcano’s summit, one large stain fanning out toward the south-west, and another toward the east.]]

The plume that is often seen above an erupting volcano is composed primarily of ash and steam. The very fine particles may be carried for many miles, settling out as a dust-like layer across the landscape. This is known as an ashfall.[5] If liquid magma is ejected as a spray, the particles will solidify in the air as small fragments of volcanic glass. Unlike the ash that forms from burning wood or other combustible materials, volcanic ash is hard and abrasive. It does not dissolve in water, and it conducts electricity, especially when it is wet.

Ashfall can become cemented together by heat to form a solid rock called tuff.[6] Ashfall breaks down over time, forming highly fertile soil, which has made many volcanic regions densely cultivated and inhabited despite the inherent dangers.[7]

In 1783, the Laki eruption killed about one-fifth of Iceland's population,[8] and sent a huge toxic cloud of ash and sulphurous gases across Western Europe.[9] In Britain alone, it has been estimated that 23,000 died from the poisoning.[10]

Atmospheric effects

When ash begins to fall during daylight hours, the sky turns hazy and a pale yellow color. The ashfall may become so dense that daylight turns the sky gray to pitch black, with the ash severely restricting visibility and deadening sound. A darkened ash sky lowers temperatures during daylight hours from what would otherwise be expected. Loud thunder, lightning, as well as the strong smell of sulfur accompany an ashfall.[11] If rain accompanies an ashfall, the tiny particles turn into a slurry of slippery mud. Rain and lightning combined with ash can lead to power outages, breakdowns of communication, and disorientation.[12]

Volcanic ash particles have a maximum residence time in the troposphere of a few weeks. The finest tephra particles remain in the stratosphere for only a few months, they have only minor climatic effects, and they can be spread around the world by high-altitude winds. This suspended material contributes to spectacular sunsets. The major climate influence from volcanic eruptions is caused by gaseous sulfur compounds, chiefly sulfur dioxide, which reacts with OH and water in the stratosphere to create sulfate aerosols with a residence time of about 2–3 years.[13][14][15]

Hazards

image during ash fall (1997)]]

The most devastating effect of volcanic ash comes from pyroclastic flows. These occur when a volcanic eruption creates an "avalanche" of hot ash, gases, and rocks that flow at high speed down the flanks of the volcano. These flows can be impossible to outrun.[16] They can also be difficult to predict. In many cases prediction is based on the topography of a region, but a valley may fill and overflow.[17] In 1902, the city of St. Pierre in Martinique was destroyed by a pyroclastic flow which killed over 29,000 people.[18]

Fluorine poisoning and death can occur in livestock that graze on ash-covered grass if fluoride is present in high concentrations.[19] Inhaling volcanic ash may cause problems for people whose respiratory system is already compromised by disorders such as asthma or emphysema. The abrasive texture can cause irritation and scratching of the surface of the eyes. People who wear contact lenses should wear glasses during an ashfall, to prevent eye damage. Furthermore, the combination of volcanic ash with moisture in the lungs can create a substance akin to liquid cement.

Therefore, people should take caution to filter the air they breathe with a damp cloth or a face mask when facing an ashfall. Ash is very dense, as only 100 millimetres (3.9 in) of ash leads to the collapse of weaker roofs. A fall of 300 millimetres (12 in) leads to the death of most vegetation, livestock, the wiping out of aquatic life in nearby lakes and rivers, and unusable roads.[20] Accompanied by rain and lightning, ashfall leads to power outages, prevents communication, and disorients people.[12]

Aviation

According to Dr. Dougal Jerram, an earth scientist at the Centre for Research into Earth Energy Systems, University of Durham, UK, "Eruptions which are charged with gas start to froth and expand as they reach the surface. This results in explosive eruptions and this fine ash being sent up into the atmosphere. If it is ejected high enough, the ash can reach the high winds and be dispersed around the globe, for example, from Iceland to Europe. These high winds are exactly where the aeroplanes cruise."[21] Volcanic ash can harm a plane mainly in four ways:

Sandblasting effect

Ash can "blind" pilots by sandblasting the windscreen requiring an instrument landing, damage the fuselage, and coat the plane (KLM Flight 867 and BA Flight 9).[22] In addition, the sandblasting effect can damage the landing lights, making their beams diffuse and unable to be projected in the forward direction (BA Flight 9). Propellor aircraft are also endangered.[citation needed]

Clogging of the plane's sensors

Accumulation of ash can also block an aircraft's pitot tubes. This can lead to failure of the aircraft's air speed indicators.[23]

Electromagnetic wave insulation

Volcanic ash particles are charged and disturb communication by radio.[24]

Combustion power failure

Volcanic ash damages machinery. The effect on jet aircraft engines is particularly severe as large amounts of air are sucked in during combustion operation, posing a great danger to aircraft flying near ash clouds. Very fine volcanic ash particles (particularly glass-rich if from an eruption under ice) sucked into a jet engine melt at about 1,100 °C, fusing onto the blades and other parts of the turbine (which operates at about 1,400 °C).

The effect on the operation of a jet engine is often to cause it to cut out—failure of all a plane's engines is common:

  • Volcanic ash particles can erode and destroy parts, drive it out-of-balance, and cause jams in rotating machinery.[25]
  • Also simple lack of oxygen is given as a probable cause of engine failure (fooling of the air/fuel control).[23]
  • Fooling of the engine temperature sensors (KLM Flight 867).
  • And compressor stall and flameout can be other reason.

The standard emergency procedure when jet engines begin to fail had been to increase power, which makes the problem worse. The best procedure is to throttle back the engines,[22] turn on engine and wing anti-ice devices (it helps to avoid compressor and wing stall),[26] and to lose height so as to drop below the ash cloud as quickly as possible. The inrush of cold, clean air is usually enough to cool, solidify, and shatter the glass, unclogging the engines.

Occurrences

There are many instances of damage to jet aircraft as a result of an ash encounter. After the Galunggung, Indonesia volcanic event in 1982, a British Airways Flight 9 flew through an ash cloud; all four engines cut out. The plane descended from 36,000 feet (11,000 m) to 12,000 feet (3,700 m), where the engines could be restarted.[27] On December 15, 1989 a KLM Boeing 747-400 (Flight 867) flying from Amsterdam Schiphol Airport to Anchorage International Airport encountered similar problems near Mount Redoubt (Alaska). The damage was 80 million US$; there was 80 kg ash in each turbine; it took 3 months work to repair the plane.[22][28]

In April 2010, airspace all over Europe was closed—which was unprecedented—due to the presence of volcanic ash in the upper atmosphere from the eruption of the Icelandic volcano Eyjafjallajökull.[29][30] On 15 April 2010 the Finnish Air Force halted training flights when damage was found from volcanic dust ingestion by the engines of one of its Boeing F-18 Hornet fighters.[31] On 22 April 2010 UK RAF Typhoon training flights were also temporarily suspended after deposits of volcanic ash were found in a jet's engines.[32]

Aviation risks of flight through downstream ash clouds

A distinction can be made between flight through (or in the immediate vicinity of) an eruption plume, and flight through the so-called affected airspace.[1] Volcanic ash in the immediate vicinity of the eruption plume is of an entirely different particle size range and density to that found in downwind dispersal clouds, which contain only the finest grade of ash. The actual level of ash loading which catastrophically affects normal engine operation has not yet been established, beyond the knowledge that relatively high ash densities must exist for this to happen. Whether this silica-melt risk still remains at the much lower ash densities characteristic of downstream ash clouds is currently unclear. This is however a serious safety hazard which requires preventive risk-management strategies, in line with other comparable aviation hazards.

Detection and Avoidance

In June 2010, Easyjet airline company has unveiled a system that it says will allow airlines to safely fly around ash clouds.[2][3] The system is based on 20-year old research by Fred Prata at the Australian research organisation CSIRO[4] and now based at the Norwegian Institute for Air Research.

Marine transportation

Similar to aviation, volcanic ash has detrimental effects on marine transportation machinery. However, it poses much less of a hazard—an aircraft encountering an ash plume has engines sucking in huge amounts of air, and cannot stop them until the plume passes, possibly days later.

Advisories concerning ongoing events

Increasing numbers of airplane incidents from atmospheric ash prompted a 1991 aviation industry meeting to decide how best to distribute information about ash events. One solution was the creation of Volcanic Ash Advisory Centers. There is one VAAC for each of nine regions of the world. VAACs can issue advisories and serve as liaisons between meteorologists, volcanologists, and the aviation industry.[5]

See also

References

External links

Further reading


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