Volcanic ash
Volcanic ash consists of fragments of rock, mineral crystals, and volcanic glass, produced during volcanic eruptions and measuring less than 2 mm in diameter. The term volcanic ash is also often loosely used to refer to all explosive eruption products, including particles larger than 2 mm. Volcanic ash is formed during explosive volcanic eruptions when dissolved gases in magma expand and escape violently into the atmosphere. The force of the gases 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 kilometres away.
Due to its wide dispersal, ash can have a number of impacts on society including: animal and human health problems, disruption to aviation, disruption to critical infrastructure, primary industries, and damage to buildings and other structures.
Formation
Volcanic ash is formed during explosive volcanic eruptions and phreatomagmatic eruptions, and may also be formed during transport in pyroclastic density currents.Explosive eruptions occur when magma decompresses as it rises, allowing dissolved volatiles to exsolve into gas bubbles. 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. 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.
Volcanic ash is also produced during phreatomagmatic eruptions. During these eruptions fragmentation occurs when magma comes into contact with bodies of water 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. 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, 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. Within pyroclastic density currents particle abrasion occurs as particles violently collide, 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. 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. 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. Effusive eruptions of basaltic composition produce <105 m3 of ejecta, whereas extremely explosive eruptions of rhyolitic and dacitic composition can inject large quantities of ejecta into the atmosphere.
Properties
Chemical
The types of minerals present in volcanic ash are dependent on the chemistry of the magma from which it erupted. Considering that the most abundant elements found in silicate magma are silicon and oxygen, the various types of magma produced during volcanic eruptions are most commonly explained in terms of their silica content. Low energy eruptions of basalt produce a characteristically dark coloured ash containing ~45–55% silica that is generally rich in iron and magnesium. The most explosive rhyolite eruptions produce a felsic ash that is high in silica while other types of ash with an intermediate composition have a silica content between 55 and 69%.The principal gases released during volcanic activity are water, carbon dioxide, hydrogen, sulfur dioxide, hydrogen sulfide, carbon monoxide and hydrogen chloride. The sulfur and halogen gases and metals are removed from the atmosphere by processes of chemical reaction, dry and wet deposition, and by adsorption onto the surface of volcanic ash.
It has long been recognised that a range of sulfate and halide compounds are readily mobilised from fresh volcanic ash. It is considered most likely that these salts are formed as a consequence of rapid acid dissolution of ash particles within eruption plumes, which is thought to supply the cations involved in the deposition of sulfate and halide salts.
While some 55 ionic species have been reported in fresh ash leachates, the most abundant species usually found are the cations Na+, K+, Ca2+ and Mg2+ and the anions Cl−, F− and SO42−. Molar ratios between ions present in leachates suggest that in many cases these elements are present as simple salts such as NaCl and CaSO4. In a sequential leaching experiment on ash from the 1980 eruption of Mount St. Helens, chloride salts were found to be the most readily soluble, followed by sulfate salts Fluoride compounds are in general only sparingly soluble, with the exception of fluoride salts of alkali metals and compounds such as calcium hexafluorosilicate. The pH of fresh ash leachates is highly variable, depending on the presence of an acidic gas condensate on the ash surface.
The crystalline-solid structure of the salts act more as an insulator than a conductor. However, once the salts are dissolved into a solution by a source of moisture, the ash may become corrosive and electrically conductive. A recent study has shown that the electrical conductivity of volcanic ash increases with increasing moisture content, increasing soluble salt content, and increasing compaction. The ability of volcanic ash to conduct electric current has significant implications for electric power supply systems.
Physical
Components
Volcanic ash particles erupted during magmatic eruptions are made up of various fractions of vitric, crystalline or lithic particles. Ash produced during low viscosity magmatic eruptions produce a range of different pyroclasts dependent on the eruptive process. For example, ash collected from Hawaiian lava fountains consists of sideromelane pyroclasts which contain microlites and phenocrysts. Slightly more viscous eruptions of basalt form a variety of pyroclasts from irregular sideromelane droplets to blocky tachylite. In contrast, most high-silica ash consists of pulverised products of pumice, individual phenocrysts and some lithic fragments.Ash generated during phreatic eruptions primarily consists of hydrothermally altered lithic and mineral fragments, commonly in a clay matrix. Particle surfaces are often coated with aggregates of zeolite crystals or clay and only relict textures remain to identify pyroclast types.
Morphology
The morphology of volcanic ash is controlled by a plethora of different eruption and kinematic processes. Eruptions of low-viscosity magmas typically form droplet shaped particles. This droplet shape is, in part, controlled by surface tension, acceleration of the droplets after they leave the vent, and air friction. Shapes range from perfect spheres to a variety of twisted, elongate droplets with smooth, fluidal surfaces.The morphology of ash from eruptions of high-viscosity magmas is mostly dependent on the shape of vesicles in the rising magma before disintegration. Vesicles are formed by the expansion of magmatic gas before the magma has solidified. Ash particles can have varying degrees of vesicularity and vesicular particles can have extremely high surface area to volume ratios. Concavities, troughs, and tubes observed on grain surfaces are the result of broken vesicle walls. Vitric ash particles from high-viscosity magma eruptions are typically angular, vesicular pumiceous fragments or thin vesicle-wall fragments while lithic fragments in volcanic ash are typically equant, or angular to subrounded. Lithic morphology in ash is generally controlled by the mechanical properties of the wall rock broken up by spalling or explosive expansion of gases in the magma as it reaches the surface.
The morphology of ash particles from phreatomagmatic eruptions is controlled by stresses within the chilled magma which result in fragmentation of the glass to form small blocky or pyramidal glass ash particles. Vesicle shape and density play only a minor role in the determination of grain shape in phreatomagmatic eruptions. In this sort of eruption, the rising magma is quickly cooled on contact with ground or surface water. Stresses within the "quenched" magma cause fragmentation into five dominant pyroclast shape-types: blocky and equant; vesicular and irregular with smooth surfaces; moss-like and convoluted; spherical or drop-like; and plate-like.