Volcanic lightning above an eruption of Mount Rinjani
The earliest recorded observations of a dirty thunderstorm are from Pliny the Younger, describing the eruption of Mount Vesuvius in 79 AD. “There was a most intense darkness rendered more appalling by the fitful gleam of torches at intervals obscured by the transient blaze of lightning.” The first studies of volcanic lightning were also conducted at Mount Vesuvius by Professor Palmieri who observed the eruptions of 1858, 1861, 1868, and 1872 from the Vesuvius Observatory. These eruptions often included lightning activity.
Dirty thunderstorms earn their name from the ash, rock fragments, and other ejecta which collide during a volcanic eruption and generate static electricity within the volcanic plume. A study presented in the Bulletin of Volcanology stated that “27-35% of eruptions are accompanied by lightning, assuming one eruption per year per volcano.” This study also indicated that volcanic lightning has been observed with 212 eruptions from 80 different volcanoes.
A famous image of the phenomenon was photographed by Carlos Gutierrez and occurred in Chile above the Chaiten Volcano. It circulated widely on the internet. Another notable image of this phenomenon is "The Power of Nature", taken by Mexican photographer Sergio Tapiro in Colima, Mexico, which won third place (Nature category) in the 2016 World Press Photo Contest. Other instances have been reported above Alaska's Mount Augustine volcano, Iceland's Eyjafjallajökull volcano and Mount Etna in Sicily, Italy.
Triboelectric charging from fragmentation of rocks near the vent and within the plume of a volcano during eruption offers another mechanism for electrical charging.
A study in the journal Science indicated that electrical charges are generated when rock fragments, ash, and ice particles in a volcanic plume collide and produce static charges, just as ice particles collide in regular thunderstorms.
Volcanic eruptions are sometimes accompanied by flashes of lightning. However, this lightning doesn’t descend from storm clouds in the sky. It is generated within the ash cloud spewing from the volcano, in a process called charge separation.
As the plume started going downwind, it seemed to have a life of its own and produced some 300 more or less normal [lightning bolts] ... The implication is that it has produced more charge than it started with. Otherwise [the plume] couldn't continue to make lightning.
—Martin A. Uman, co-director of the University of Florida Lightning Research program
Large amounts of water are released as vapor during volcanic eruptions. A study performed by McNutt & Williams in 2010 found that the water content of volcanic plumes is much greater than the water content of thunderstorms. This study also found “there may be a threshold of water substance concentration required in a plume for lightning to occur (…) this appears to be a function of the large amount of water in the magma and not the smaller amount in the entrained air”.
Naturally occurring radioisotopes within ejected rock particles may cause self-charging of volcanic plumes. During an eruption, a large amount of fragmented sub-surface rock is ejected into the atmosphere. In a study performed on ash particles from the Eyjafjallajökull and Grímsvötn eruptions, scientists found that radioisotopes were an unlikely source of self-charging in the Eyjafjallajökull plume. However, there was the potential for greater charging near the vent where the particle size is larger. A second study conducted on volcanic ash from the same eruptions found that both samples possessed a natural radioactivity above the background level.
The height of the ash plume appears to be linked with the mechanism which generates the lightning. In taller ash plumes (7–12 km) it appears that the large concentrations of water vapor are contributing to lightning activity, while smaller ash plumes (1–4 km) appear to gain more of their electric charge from fragmentation of rocks near the vent of the volcano through charge separation. The atmospheric temperature also plays a role in the formation of lightning. Colder ambient temperatures will create greater amounts of ice inside of the plume thus leading to more friction and electrical activity.
In a study presented in the Bulletin of Volcanology in 2010, it was stated that “Seasonal effects show that more eruptions with lightning were reported in winter (bounded by the respective autumnal and vernal equinoxes) than in summer”. This finding indicates that the lightning activity is driven by magma-derived water rather than atmospheric.
It has been suggested that a by-product of volcanic lighting are lightning induced volcanic spherules, (LIVS). These small glass spherules are thought to be formed during high-temperatures processes such as cloud-to-ground lightning strikes. The temperature of a bolt of volcanic lightning can reach 30,000 °C. When this bolt contacts ash particles within the plume it will cause them to melt and then quickly solidify as they cool, forming orb shapes. The presence of volcanic spherules aids in providing evidence for the occurrence of volcanic lightning when the lightning itself was not observed directly.
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