Ash cloud from the 2008 eruption of Chaitén volcano streching 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. Chaitén is a volcanic Caldera in diameter 17km west of the elongated ice-capped Michinmahuida volcano and northeast of Chaitén town in the Llao LLaojpg|thumb|250px| Lake Nahuel Huapi, near Bariloche, Argentina Tephra is air-fall material produced by a volcanic eruption regardless of composition or fragment size Plate tectonics and hotspots Divergent plate boundaries At the 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. Magma (Plurals magmas and magmata) is molten rock that sometimes forms beneath the surface of the Earth (or any other Terrestrial planet Clay is a naturally occurring material composed primarily of fine-grained Minerals which show plasticity through a variable range of Water content, and Sand is a naturally occurring Granular material composed of finely divided rock and Mineral particles Volcanic ash can lead to breathing problems, malfunctions in machinery, and from more severe eruptions, years of global cooling. Global cooling in general can refer to an overall cooling of the Earth.

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. Tuff (from the Italian "tufo" is a type of rock consisting of consolidated volcanic ash ejected from vents during a volcanic eruption Over geologic time, the ejection of large quantities of ash can produce an ash cone. The geologic time scale is a chronologic schema (or idealized Model) relating Stratigraphy to time that is used by Geologists and other Volcanic cones are among the simplest Volcano formations in the world

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. Pyroclastic rocks or pyroclastics (derived from the Greek πῦρ, meaning fire and κλαστός, meaning broken are Clastic rocks ^[1] Ash terminology is restricted to very fine rock and mineral particles less than 2 millimetres (0. In Geology, rock is a naturally occurring aggregate of Minerals and/or Mineraloids The Earth's outer solid layer the ‘ Lithosphere A mineral is a naturally occurring substance formed through geological processes that has a characteristic chemical composition a highly ordered atomic structure and specific 079 in) in diameter which are ejected from a volcanic vent. ^[3]

Table modified after^[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. A Phreatic eruption, also called an ultravulcanian eruption, occurs when rising Magma makes contact with ground or surface water Phreatomagmatic eruptions are defined as juvenile forming eruptions as a result of interaction between water and Magma. ^[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. Tuff (from the Italian "tufo" is a type of rock consisting of consolidated volcanic ash ejected from vents during a volcanic eruption 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] 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. Combustion or burning is a complex sequence of Exothermic chemical reactions between a Fuel and an Oxidant accompanied by the production of 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

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 leads to power outages, prevents communication, and disorients people. ^[8]

Hong Kong sunset c. 1992 after the eruption of Mount Pinatubo. Mount Pinatubo is an active Stratovolcano located on the island of Luzon

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. A Bishop's Ring is a diffuse brown or bluish halo observed around the sun in the presence of large amounts of dust in the Stratosphere. ^[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. Mount Pinatubo is an active Stratovolcano located on the island of Luzon ^[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. Mount Tambora (or Tomboro) is an active Stratovolcano, also known as a Composite volcano, on Sumbawa island Indonesia. The Year Without a Summer, also known as the Poverty Year, The Year There Was No Summer or Eighteen hundred and froze to death ^[11]

Dangers

Rainbow and volcanic ash with sulfur dioxide emissions from Halema`uma`u vent. A rainbow is an optical and meteorological phenomenon that causes a spectrum of Light to appear in the Sky when the Sun

The most devastating effect of volcanic ash comes from pyroclastic flows. A pyroclastic flow (also known as a pyroclastic density current) is a common and devastating result of some volcanic eruptions. 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. Martinique is an Island in the eastern Caribbean Sea, having a land area of 1128 km² ^[14]

Volcanic ash is not poisonous, but inhaling it may cause problems for people whose respiratory system is already compromised by disorders such as asthma or emphysema. Asthma is a chronic Condition involving the Respiratory system in which the airways occasionally constrict become inflamed, and are Emphysema is a chronic obstructive Pulmonary disease ( COPD) formerly termed a chronic obstructive Lung disease (COLD 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. A contact lens (also known simply as a contact) is a corrective, cosmetic, or therapeutic lens usually placed on the Cornea Glasses, also called eyeglasses or spectacles, are frames bearing lenses worn in front of the Eyes normally for vision correction, 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]

Aviation

Ash plume from Mt Cleveland, a stratovolcano

Volcanic ash jams machinery. Mount Cleveland is an active and currently erupting Stratovolcano that forms the western half of Chuginadak Island in the central Aleutian Islands A stratovolcano, also called a composite volcano is a tall conical Volcano composed of many layers of hardened Lava, Tephra, and Volcanic 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. Galunggung, Galoen-gong, or Gunung Galunggung is an active Stratovolcano in West Java, Indonesia. The Republic of Indonesia ( (Republik Indonesia is a Country in Southeast Asia. British Airways Flight 9, sometimes referred to as the Jakarta incident, was a scheduled British Airways flight from London Heathrow to Auckland 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]

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

• Azomite
• Global dimming
• Pozzolana

References

External links

• A map of the 9 VAAC regions
• Ash Thickness and Particle Size Downwind from Mount St. Helens, Washington, on May 18, 1980
• Tephra Falls of the 1991 Eruptions of Mount Pinatubo
• What to do during an ash fall event
• Social & Economic Costs of Volcanic Ash to Aviation from "NOAA Socioeconomics" website initiative