Volcanic ash is the term for very fine rock and mineral particles less than 2 mm in diameter that are ejected from a volcanic vent. 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) results in the magma and perhaps solid rock surrounding the vent, being torn into particles of silt to sand size. The plume that is often seen above an erupting volcano is composed primarily of ash and steam. The ejection of large quantities of ash will produce an ash cone. A layer of volcanic ash tends to become cemented together (see pozzulana) to form a firm rock called tuff (pronounced “toof”). 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 “ash fall.” The term for any material explosively thrown out from a vent is ejecta or pyroclastic debris. If liquid magma is ejected as a spray, the particles will solidify in the air to small fragments of volcanic glass. The very fine particles may be carried many miles, settling out as a dust-like layer across the landscape. Ejecta particles of gravel size are termed cinder.
Unlike the ash that forms from burning wood or other combustible materials, volcanic ash is hard and abrasive, rather than soft and fluffy. It does not dissolve in water, and it conducts electricity, especially when it is wet. During a severe ash fall, the sky may seem hazy or yellow, and light may fade altogether. The approaching ash cloud may appear to be a weather cloud: lightning and thunder are often present. The air often smells of sulfur.
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. The abrasive texture can cause irritation and scratching of the surface of the eyes. People who wear contact lenses should wear glasses if they are experiencing an ash event.
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. In 1902, the city of St. Pierre in Martinique was destroyed by a pyroclastic flow which killed over 29,000 people.
Ash and aviation
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 may quit, and fuel and water systems may 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 36,000 feet to only 12,000 feet before the crew could manage to restart the engines.
Increasing numbers of events like that prompted the aviation industry to meet in 1991 to decide how to best distribute information about ash events. One solution was the creation of Volcanic Ash Advisory Centers. There is one VAAC for each of 9 regions of the world. VAACs can issue advisories and serve as liaisons between meterologists, volcanologists, and the aviation industry.
Very fine ash particles may remain high in the atmosphere for years, spread around the world by high-altitude winds. This suspended material contributes to often spectacular sunsets, as well as an optical phenomenon known as “Bishop’s Ring.” This refers to a corona or halo effect around the sun.