Volcanic lightning is an electrical discharge produced by a volcanic eruption. Because the static electricity is produced in an ash plume rather than a cloud, volcanic lightning is sometimes called a dirty thunderstorm.
The earliest historical record of volcanic lightning came from the ancient Roman, Pliny the Younger. He described lightning from the 79 AD eruption of Mount Vesuvius. In the 19th century, volcanic lightning was studied from the Vesuvius Observatory. In addition to Mount Vesuvius, volcanic lightning has occurred during eruptions of Eyjafjallajökull in Iceland, Chaiten in Chile, Mount Etna in Sicily, Colima in Mexico, Mount Augustine in Alaska, and Taal in the Philippines.
How Volcanic Lightning Works
Like regular lightning in a thunderstorm, volcanic lightning results from a charge build-up within the plume. The mechanisms that build electrical charge depend on the height of the ash plume, the temperature of the atmosphere, and potential water sources near the volcano. Four of the key mechanisms are ice charging, frictional charging, radioactive charging, and fractoemission.
- Ice charging: Ice charging is the mechanism that produces lightning in thunderstorms. Rapid upward movement of air produces super-cooled water, ice crystals, and hail or graupel. The super-cooled water and small ice crystals gain altitude from the updraft, while the graupel remains where it is or else falls due to gravity. Sometimes volcanic hail occurs. When ice crystals collide with graupel, the crystals become positively charged, while the graupel becomes negatively charged. Over time, the top of the plume gains a net positive charge, while the middle or bottom gains a net negative charge. Lightning occurs the charge overcomes the electrical insulation provided by air. Volcanic plumes contain a lot of water from magma. Water may also vaporize from nearby sources, such as glaciers, rivers, lakes, or the sea.
- Frictional charging: Frictional charging or triboelectricity is a major player in volcanic lightning. Rock, ice, and ash rub against each other during an eruption, producing static electricity. Convection separates the charges, leading to static discharge.
- Radioactive charging: In radioactive charging, radioisotopes from rocks or radon gas ionize particles within a volcanic plume. Although the effect of radioactive charging is thought to be small, ash from volcanoes does tend to emit more radioactivity than background radiation.
- Fractoemission: In fractoemission, the fracturing of rocks generates and separates charge. Fractoemission contributes to lightning that occurs near the vent of the eruption.
Eruptions surrounded by cold ambient temperature promote ice charging. Tall ash plumes also promote ice charging, as the plume naturally extends into cold air. Volcanic lightning in shorter ash plumes mainly occurs from frictional charging and fractoemission.
Volcanic lightning can reach a temperature of 30,000 °C . The extreme heat vaporizes or melts ash in the plume. The melted ash solidifies into spherical shapes as it cools. Volcanic spherules indicate lightning occurred, even if it wasn’t directly observed. The spherules are analogous to fulgurites formed when lightning strikes and melts sand.
Is Volcanic Lightning Dangerous?
According to National Geographic, the odds of being struck by lightning in any given year in the United States is 1 in 700,000. However, the odds of getting struck in a lifetime are 1 in 3,000! So, lightning poses a serious health risk. Fortunately, volcanic lightning is not likely to strike you (presumably because people flee a volcanic eruption). Two fatalities are known to have occurred in Iceland from volcanic lightning associated with the 1755 eruption of Katla. However, it’s worth noting the two victims were 30 km (18.6 mi) away from the volcano when they were struck.
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