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.
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 in diameter which are ejected from a volcanic vent.
|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.
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.
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 leads to power outages, prevents communication, and disorients people.
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. 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. 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.
Volcanic ash (by itself) is not poisonous, but inhaling it 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 of ash leads to the collapse of weaker roofs. A fall of 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 jams machinery. 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. The plane descended from to before the crew could manage to restart the engines.