volcanic ash 3

volcanic ash 3

Volcanic ash is made up of tiny, dust-like fragments of jagged rock, minerals and volcanic glass. Ash particles are 2 millimeters (.08 inches) across or smaller. These particles are sometimes called tephra.

Coarse ash looks and feels like grains of sand, and very fine ash is powdery. Ash forms as lava is thrown into the air during an explosive volcanic eruption. Gases in the volcano's molten lava expand during the eruption and shatter the lava into the tiny ash particles.

Unlike the soft ash created by burning wood, volcanic ash is hard, abrasive, and does not dissolve in water. It can conduct electricity when it is wet. This ability allows ash to "recharge" itself as it drifts into storms.

During an eruption, the wind can carry small ash particles great distances. Ash has been found thousands of kilometers away from an eruption. The smaller the particle, the further the wind will carry it. Ash deposits tend to be thicker or deeper closer to the eruption, and thinner as distance from the volcano increases.

In addition to shooting volcanic ash into the atmosphere, an explosive eruption can create an avalanche of ash, gases and rock, called a pyroclastic flow. These incredibly fast avalanches of volcanic debris can be impossible for humans to outrun. They are capable of razing buildings and uprooting trees.

After a violent eruption, the ash in the air can be thick enough to block sunlight. If inhaled, ash can cause breathing problems or suffocation. It also can disable machinery. Severe eruptions that have sent ash into the atmosphere blocked some sunlight and lowered temperatures worldwide for years. This phenomenon is an example of global cooling.

One of the most famous explosive volcanic eruptions occurred in 79 CE, when Mount Vesuvius buried the Roman (now Italian) cities of Pompeii and Herculaneum.

Pompeii was buried under 18 meters (60 feet) of ash. The ash buried the cities so completely that it preserved entire buildings, paintings, and artifacts. It also created very detailed molds around the bodies of people who were killed.

Starting in the 18th century, archaeologists began excavating Pompeii. They discovered the hollow impressions left by bodies in the hardened ash and developed a way to inject them with plaster to create casts of the bodies. Today the excavated city and its gruesome models of dead and dying people and animals are popular tourist attractions.

Impacts

Impacts


Population growth has caused the progressive encroachment of urban development into higher risk areas, closer to volcanic centres, increasing the human exposure to volcanic ash fall events.

Infrastructure is critical to supporting modern societies, particularly in urban areas, where high population densities create high demand for services. These infrastructure networks and systems support urban living, and provide lifeline services upon which we depend for our health, education, transport and social networking. Infrastructure networks and services support a variety of facilities across a broad range of sectors.

Volcanic ash fall events can disrupt and or damage the infrastructure upon which society depends. Several recent eruptions have illustrated the vulnerability of urban areas that received only a few millimetres or centimetres of volcanic ash. This has been sufficient to cause disruption of transportation, electricity, water, sewage and storm water systems. Costs have been incurred from business disruption, replacement of damaged parts and insured losses. Ash fall impacts on critical infrastructure can also cause multiple knock-on effects, which may disrupt many different sectors and services.

Volcanic ash fall is physically, socially and economically disruptive. Volcanic ash can affect both proximal areas and areas many hundreds of kilometres from the source, and causes disruptions and losses in a wide variety of different infrastructure sectors. Impacts are dependent on: ash fall thickness; the duration of the ash fall; the grain size and chemistry of the ash; whether the ash is wet or dry; and any preparedness, management and prevention (mitigation) measures employed to reduce effects from the ash fall. Different sectors of infrastructure and society are affected in different ways and are vulnerable to a range of impacts or consequences. These are discussed in the following sections.

Formation

Formation

Volcanic ash is formed during explosive volcanic eruptions, phreatomagmatic eruptions and during transport in pyroclastic density currents.

Explosive eruptions occur when magma decompresses as it rises, allowing dissolved volatiles (dominantly water and carbon dioxide) to exsolve into gas bubbles.[2] As more bubbles nucleate a foam is produced, which decreases the density of the magma, accelerating it up the conduit. Fragmentation occurs when bubbles occupy ~70-80 vol% of the erupting mixture.[3] When fragmentation occurs, violently expanding bubbles tear the magma apart into fragments which are ejected into the atmosphere where they solidify into ash particles. Fragmentation is a very efficient process of ash formation and is capable of generating very fine ash even without the addition of water.[4]

Volcanic ash is also produced during phreatomagmatic eruptions. During these eruptions fragmentation occurs when magma comes into contact with bodies of water (such as the sea, lakes and marshes) groundwater, snow or ice. As the magma, which is significantly hotter than the boiling point of water, comes into contact with water an insulating vapor film forms (Leidenfrost effect).[5] Eventually this vapor film will collapse leading to direct coupling of the cold water and hot magma. This increases the heat transfer which leads to the rapid expansion of water and fragmentation of the magma into small particles which are subsequently ejected from the volcanic vent. Fragmentation causes an increase in contact area between magma and water creating a feedback mechanism,[5] leading to further fragmentation and production of fine ash particles.

Pyroclastic density currents can also produce ash particles. These are typically produced by lava dome collapse or collapse of the eruption column.[6] Within pyroclastic density currents particle abrasion occurs as particles interact with each other resulting in a reduction in grain size and production of fine grained ash particles. In addition, ash can be produced during secondary fragmentation of pumice fragments, due to the conservation of heat within the flow.[7] These processes produce large quantities of very fine grained ash which is removed from pyroclastic density currents in co-ignimbrite ash plumes.

Physical and chemical characteristics of volcanic ash are primarily controlled by the style of volcanic eruption.[8] Volcanoes display a range of eruption styles which are controlled by magma chemistry, crystal content, temperature and dissolved gases of the erupting magma and can be classified using the Volcanic Explosivity Index (VEI). Effusive eruptions (VEI 1) of basaltic composition produce <105 m3 of ejecta, whereas extremely explosive eruptions (VEI 5+) of rhyolitic and dacitic composition can inject large quantities (>109 m3) of ejecta into the atmosphere. Another parameter controlling the amount of ash produced is the duration of the eruption: the longer the eruption is sustained, the more ash will be produced. For example, the second phase of the 2010 eruptions of Eyjafjallajökull was classified as VEI 4 despite a modest 8 km high eruption column, but the eruption continued for a month, which allowed a large volume of ash to be ejected into the atmosphere.

Volcanic ash

Volcanic ash

"Ash cloud" redirects here. For the Eyjafjallajökull disaster, see 2010 eruptions of Eyjafjallajökull.
Volcanic ash streams out in an elongated fan shape as it is dispersed into the atmosphere.


Volcanic ash consists of fragments of pulverized rock, minerals and volcanic glass, created during volcanic eruptions, less than 2 mm (0.079 inches) in diameter.[1] The term volcanic ash is also often loosely used to refer to all explosive eruption products (correctly referred to as tephra), including particles larger than 2mm. Volcanic ash is formed during explosive volcanic eruptions when dissolved gases in magma expand and escape violently into the atmosphere. The force of the escaping gas shatters the magma and propels it into the atmosphere where it solidifies into fragments of volcanic rock and glass. Ash is also produced when magma comes into contact with water during phreatomagmatic eruptions, causing the water to explosively flash to steam leading to shattering of magma. Once in the air, ash is transported by wind up to thousands of kilometers away.

Due to its wide dispersal, ash can have a number of impacts on society, including: human and animal health; disruption to aviation; disruption to critical infrastructure (e.g., electric power supply systems, telecommunications, water and waste-water networks, transportation); primary industries (e.g., agriculture); buildings and structures.