Volcanic Ash Cloud
Volcanic ash cloud - Wikipedia, the free encyclopedia
Volcanic ash cloud 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 cloud 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 coneFormation of Volcanic ash cloud
There are three mechanisms of volcanic ash cloud formation:
- Gas release under decompression causing magmatic eruptions;
- Thermal contraction from chilling on contact with water causing phreatomagmatic eruptions
- Ejection of entrained particles during steam eruptions causing phreatic eruptions. 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
Composition of Volcanic ash cloud
Light microscope image of ash from the 1980 eruption of Mount St. Helens, Washington.The term for any material explosively thrown out from a vent is tephra or pyroclastic debris. 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
Spread of Volcanic ash cloud
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. 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. Ashfall breaks down over time, forming highly fertile soil, which has made many volcanic regions densely cultivated and inhabited despite the inherent dangers.
In 1783, the Laki eruption killed about one-fifth of Iceland's population, and sent a huge toxic cloud of ash and sulphurous gases across Western Europe. In Britain alone, it has been estimated that 23,000 died from the poisoning
Atmospheric effects from Volcanic ash cloud
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.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.
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 , which reacts with OH and water in the stratosphere to create sulfate aerosols with an residence time of about 2–3 years.
Hazard of Volcanic ash cloud
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. 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. In 1902, the city of St. Pierre in Martinique was destroyed by a pyroclastic flow which killed over 29,000 people.
Fluorine poisoning and death can occur in livestock that graze on ash-covered grass if fluoride is present in high concentrations. 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. Accompanied by rain and lightning, ashfall leads to power outages, prevents communication, and disorients people
Volcanic Ash Cloud
