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.

Volcanic Ash : Rabaul Caldera, Papua New Guinea (Lauer, 1995)

Volcanic Ash : Rabaul Caldera, Papua New Guinea (Lauer, 1995)

Aerial view of Rabaul Town covered in volcanic ash, Papua New GuineaFollowing a 27-hour period of intense earthquake activity, Tavurvur and Vulcan volcanoes on opposite sides of the Rabaul caldera erupted on September 19, 1994, early in the morning. Nearby Rabaul Town (right, click for large-sized image) was covered with volcanic ash as thick as 1.5 m, and an estimated 90,000 people were displaced from the area.

"The volcanic ash was mushrooming out in thick clouds-but there was no noise. The earth had stopped moving. It was 6.15 am. We watched in awe. The clouds began to drift towards us... What a tremendous experience! But now the sky was darkening and black specks of ash were falling on us like light rain. There was also an overpowering small of sulphur."

7.30 am, after driving to a new location, about 8 km (5 mi) from Rabaul town. "Soon the clouds would reach Kulau too. Suddenly there were fears of being overcome by poisonous gas. The smell of sulphur was sickening and the air was a strange, yellowish colour. The the power went off. In a moment of panic, people, including our party which had now increased to eleven, decided to head for the safety of Kerevat, using the North Coast Road. Progress was extremely slow as the road was choked with half the population of Rabaul, all with the same idea. Eventually the vehicle traffic halted altogether. Due to unusual thermal patterns Kerevat was deluged by mud rain and the road quickly became impassable." The party returned to Kulua.

"The next day brought much of the same-but it seemed to drag on and on. The atmosphere was very heavy. There was no power. Every time we left the house we collected volcanic ash and pumice on our skin and in our hair. And the noise-the thunder and lightning was constant, with some almighty cracks. It was obvious that objects were being struck."

Five days later, describing a visit home to retrieve belongings: "There were parts of trees all over the road, flattened vegetation everywhere and everything covered in ash and mud. Our houseyard looked like a moonscape—house, water tanks, plants—all a drab grey. The trees along the bank had snapped and lost most of their foliage... inside the house was a depressing mess. The floors were covered in volcanic ash. There was even ash on the ceiling above the louvres and caked along the top of the curtains."

Nine days after the eruption: "The volcanic ash was dreadful all day yesterday. It was clearly visible in the air and I could feel and see it on my skin every time I stepped outside. I can't get used to wearing the mask and goggles. I found the smell of the masks nauseating so I've taken to wearing a folded handkerchief over my nose and mouth like a "baddie" in a Western movie."

AVolcanic Ash : Mount St. Helens, Washington

AVolcanic Ash : Mount St. Helens, Washington

"By noon, the City was engulfed in darkness by volcanic ash and communications by home telephone were impossible. It was like an eclipse of the sun that lingered, a blinding blizzard and an electrical storm all in one. Light-sensitive street lights came on automatically, traffic stopped, and a strange quiet fell on our community; and everywhere a talcum-like sandy gray powder kept accumulating. Cars, trucks, buses, and trains, all stopped, and planes were re-routed away from the ash cloud."

"From noon until 6:00 a.m. the following morning, the City was in total darkness. Three types of ash fell alternatively on the City: dark gray sand, medium gray sand, and a light gray cement-like dust. All three grades were gritty and light, difficult to sweep or shovel, especially when dry. To make matters worse, shifting winds blew the ash everywhere, severely impairing visibility and driving in our area. It was exceedingly harmful and abrasive to mechanical and electrical equipment, especially motors of vehicles, aircraft and electronic systems. Unlike snow, however, this "precipitation" was not going to melt!"

"The volcanic ash fall was especially crippling—Yakima received 5-8 cm (2-3 in) of this material in the first 24 hours following the explosion. We estimated that several million tons of volcanic ash was deposited on the entire region."

What's it like during an ash fall?

What's it like during an ash fall?

When ash begins to fall during daylight hours, the sky will turn increasingly hazy and "dusty" and sometimes a pale yellow color. The falling volcanic ash may become so dense that daylight turns to murky gray or even an "intense blackness" such that "it is impossible to see your hand when held up close to the eye." Loud thunder and lightning and the strong smell of sulfur often occurs during an ash fall. Furthermore, rain may accompany the volcanic ash and turn the tiny particles into a slurry of slippery mud. Most people also describe an intense quietness, except for thunder that may accompany the ash fall, giving a "deadness" to the normal sounds of life.

Volcanic Ash falls vary widely in intensity, size of the ash particles, and the degree to which light from the sun is obscured or blocked completely. Because of the unexpected darkness during daylight hours, loud thunder and lightning, and the sometimes strong smell of sulfur during a volcanic ash fall, many people describe the experience as eerie and frightening, disorienting and confusing, or dreadful. In extreme ash fall, for example when ash thickness is more than 5-10 cm (2-4 in), people may feel stunned and fearful of the conditions, have a difficult time breathing if a well-sealed shelter is not available. If caught outside during low visibility, people may become lost or extremely disoriented.

If heavy volcanic ash fall continues for 12-60 hours or more, roofs may collapse under the weight of the ash, resulting in more confusion, injuries, and even death.

Knowing what to expect during and after an ash fall can help people (1) reduce their anxiety and uncertainty when ash is falling to the ground; and (2) prepare their families and communities to deal with the ash effectively.