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.
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There are three mechanisms of volcanic ash formation:
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]
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 |
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]
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]
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]
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]
Fluoride 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]
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:
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). Propeller-aircraft are also endangered.
Accumulation of ash can also block an aircraft's pitot tubes. This can lead to failure of the aircraft's air speed indicators.[23]
Volcanic ash particles are charged and disturb communication by radio.[24]
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).
Potential effects on the operation of a jet engine include:
High concentrations of volcanic gases in an eruption plume can cause other problems that should be distinguished from those caused by ash:
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.
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][28] 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][29]
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.[30][31] 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.[32] 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.[33]
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.[34] 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.
In June 2010, Easyjet airline company has unveiled a system based on infrared light that it says will allow pilots to detect volcanic ash plumes up to 100 kilometres (62 mi) ahead and so safely fly around them.[35][36] The system is based on 20-year old research by Fred Prata, then at the Australian research organisation CSIRO[37] and now based at the Norwegian Institute for Air Research.
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.
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.[38]