Volcanic eruptions are among the most powerful natural events on Earth. They can send huge clouds of ash high into the atmosphere, where electrical charges build up and produce spectacular flashes of lightning. But volcanic lightning may do more than create a dramatic display. New experimental research suggests that these lightning strikes can rapidly melt volcanic ash, change its shape, break particles apart, and even remove important chemical elements.
A study by Mueller and colleagues has investigated this process in laboratory experiments designed to reproduce the extreme heating caused by volcanic lightning. Their findings provide new evidence that lightning can actively transform volcanic ash during an eruption, potentially influencing both the behavior of ash clouds and atmospheric chemistry.
Volcanic Lightning: More Than a Spectacular Display
Lightning in volcanic ash plumes occurs when ash particles, gases and other materials interact and become electrically charged. As particles collide and move within an eruption column or pyroclastic density current, electrical charges can separate. When the difference in charge becomes large enough, an electrical discharge occurs, producing lightning.
Volcanic lightning has been reported during hundreds of historic eruptions. Researchers believe the actual number is probably much higher because volcanic lightning has not always been consistently observed or recorded.
These electrical discharges are scientifically valuable. Lightning can sometimes be detected from great distances, including by satellites. In remote regions, it may provide one of the few clues that an eruption is occurring. Measurements of lightning frequency, intensity and location can also help scientists understand the height and development of an ash plume, eruption explosivity and ash concentration.
However, lightning may also change the ash plume itself. Electrical discharges can encourage ash particles to stick together, influence how particles spread through the atmosphere and affect how quickly ash is removed from the plume.
Why Would Lightning Melt Volcanic Ash?
The answer lies in the extraordinary temperature of lightning.
The central region of a natural lightning plasma channel can reach temperatures of roughly 12,000 to 28,000 K. Although volcanic ash is exposed to these temperatures for an extremely short period, the energy released can still produce major changes.
Previous observations of ash from eruptions such as Iceland's 2010 Eyjafjallajökull eruption and Alaska's 2009 Mount Redoubt eruption identified glassy spheres and aggregates. Researchers proposed that these unusual particles may have formed when volcanic ash was rapidly melted by lightning.
Mueller and his team wanted to test this idea experimentally and understand exactly what happens to ash when it encounters an intense electrical discharge.
Recreating Volcanic Lightning in the Laboratory
The researchers developed an experimental system using an arc-welding machine as a source of intense electrical energy. Volcanic ash particles of different sizes and chemical compositions were directed toward an artificially produced electrical arc.
This setup allowed the researchers to observe what happens when ash experiences extremely rapid heating similar to the conditions associated with volcanic lightning.
The experiments produced several distinct types of altered particles. Some ash grains became completely melted and formed nearly spherical glass particles. Others were only partly melted. The researchers also observed particle aggregates and vesiculated particles containing bubble-like structures.
These shapes are important because similar spherical particles and aggregates have previously been found in natural volcanic ash deposits. The laboratory results therefore strengthen the argument that volcanic lightning can produce these distinctive ash morphologies.
High-Speed Cameras Capture Rapid Changes
One of the most interesting aspects of the research was the use of high-speed imaging.
The cameras captured processes occurring over extremely short timescales, from milliseconds down to microseconds. The observations showed ash particles melting and becoming rounded as surface tension pulled the molten material into spherical shapes.
In water-rich ash, heating could also cause foaming and vesiculation. As water and other volatile components were rapidly released from the molten material, bubbles formed inside the particles.
The researchers also observed explosive fragmentation. In some cases, rapidly heated particles broke apart, creating new smaller fragments.
This means that lightning does not simply melt ash. It can simultaneously reshape particles, produce bubbles, cause fragmentation and trigger chemical changes.
Lightning Can Change Ash Chemistry
The physical transformation of ash was only part of the discovery.
Chemical analyses showed that rapid heating caused significant losses of several elements, particularly chlorine (Cl), sulfur (S), phosphorus (P) and sodium (Na).
These elements can vaporize when exposed to extreme temperatures. During the short-lived melting event, material from inside a molten ash particle can move toward its surface, where volatile elements escape into the surrounding environment.
The researchers found evidence that convection plays an important role in this process. Movement caused by bubbles within the molten droplet can transport elements from its interior toward the outer surface. Once these elements reach the surface, they can be lost through thermal vaporization.
This process is especially significant because chlorine, sulfur and other volatile elements can influence atmospheric chemistry.
How Hot Did the Ash Become?
Using the amount of sodium lost from the experimental particles, the researchers modeled the maximum temperatures reached by ash grains measuring approximately 70–130 micrometers.
Their calculations produced estimated melt temperatures between 3,290 and 3,490 K.
These temperatures are lower than the maximum temperature of the central lightning plasma channel, which is expected because ash particles are not necessarily exposed directly to the hottest part of the discharge for the entire duration.
Nevertheless, the temperatures are high enough to rapidly melt volcanic material and cause substantial chemical alteration.
What Does This Mean for Volcanic Ash Clouds?
The findings suggest that volcanic lightning should be considered an active process within explosive eruptions rather than simply an electrical side effect.
Lightning can change the size, shape and chemistry of ash particles while they are still suspended in the eruption cloud. These changes could influence how particles aggregate and settle, potentially affecting the way volcanic ash moves through the atmosphere.
The chemical changes may also have broader environmental consequences. Elements released or removed during lightning-induced heating can participate in atmospheric reactions and contribute to aerosol formation. Some volatile compounds associated with volcanic emissions can also influence processes such as ozone chemistry.
However, the exact effects in nature are likely to vary. Natural volcanic plumes differ widely in ash composition, particle size, water content and atmospheric conditions.
A New Piece of the Volcanic Puzzle
Mueller and his colleagues' experiments provide strong evidence that volcanic lightning can rapidly transform ash both physically and chemically.
The formation of glassy spheres and aggregates in the laboratory closely resembles features found in natural volcanic deposits. At the same time, high-speed observations reveal previously less-recognized processes, including partial melting, vesiculation and rapid fragmentation.
The study also shows that extreme electrical heating can cause substantial loss of chlorine, sulfur, phosphorus and sodium from molten ash. By modeling sodium loss, the researchers were able to estimate temperatures of up to about 3,490 K in their experiments.
Future research could investigate how factors such as oxygen availability, ash composition and particle size influence these reactions under natural conditions.
Ultimately, volcanic lightning may be doing far more than illuminating an eruption. Each flash could act as a brief, high-temperature chemical reactor, reshaping volcanic ash in a fraction of a second. Understanding this hidden process could help scientists better interpret volcanic deposits, improve ash-cloud models and understand how explosive eruptions interact with Earth's atmosphere.
Reference: Mueller, S.P., Helo, C., Keller, F. et al. First experimental observations on melting and chemical modification of volcanic ash during lightning interaction. Sci Rep 8, 1389 (2018). https://doi.org/10.1038/s41598-018-19608-3
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