Volcanic lightning (sometimes referred to as a "dirty thunderstorm") is a phenomenon in which a volcano eruption generates a thunderstorm -- and only recently have researchers been able to understand how this happens.
Table of contents
It's probably safe to assume that since the dawn of mankind, man has been fascinated by lightning and volcanoes. These are two of the most brutal and spectacular phenomenon you can observe in nature, and to see them together is simply stunning. But this is no coincidence, nor is it a divine intervention -- there is a good reason why volcano plumes are sometimes associated with lightning.
The first recorded mention of the phenomenon comes from Pliny the Younger, who described the dramatic 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 ancient scholar wrote.
More recently, in the mid-late 19th century, renowned geophysicist and meteorologist Luigi Palmieri documented several eruptions of Vesuvius, noting that lightning often accompanies them. A 2008 study found that “27-35% of eruptions are accompanied by lightning, assuming one eruption per year per volcano.” The same study noted that there have been over 200 recorded instances of volcanic lightning around over 80 different volcanoes.
But why does this happen? Researchers have long suspected that the volcanic eruption somehow changes the electrical properties of the air around the volcano, but the mechanism has only recently been properly explained. Let's take it step by step.
What is lightning
Lightning is essentially a sudden atmospheric electrostatic discharge. It can happen between two areas of the same cloud, two different clouds or, as we're most familiar, between a cloud and the ground.
The main driving force behind lightning is a combination of a quick air updraft and low temperatures between −15 to −25 degrees Celsius. This combination of updraft and cold air produces supercooled cloud droplets (small water droplets below freezing), small ice crystals, and graupel (soft hail).
Since these particles move very differently, they often collide. When the rising ice crystals collide with the falling graupel, the ice crystals become positively charged and the graupel becomes negatively charged. As a result, the top part of the cloud becomes positively charged, while the bottom part becomes negatively charged, creating the perfect conditions for an electrical discharge.
It's estimated that on Earth, lightning strikes 40–50 times a second, which equates to nearly 1.4 billion flashes per year. Flashes last from around 0.003 seconds to 0.2 seconds.
What is a volcano
The Earth can be roughly divided into a nucleus, a mantle, and an outer crust. The crust isn't an impermeable layer, but is split into several pieces we call tectonic plates. At the edges of the tectonic plates (as well as in some other spots on the crust), magma from the mantle can flow towards the planet's surface. A volcano is essentially a rupture in the planet's surface that allows magma to escape.
However, magma doesn't flow outside on its own -- it's typically accompanied by volcanic ash and gases -- and therein lies the key to the connection between volcanoes and lightning.
In order for lightning to take place, there needs to be a charge separation between two air masses -- only if the charge is big enough to overpower the air resistance, it can conduct electricity in the form of lightning. The ash of the volcano starts out as electrostatically neutral, but through friction, and especially in the presence of heat and movement within the volcano, the ash can facilitate the electron flow and can allow masses of air to quickly become charged relative to one another. It's similar to the childhood experiment where you rub a balloon against your head -- the same type of electrostatic charge is accumulating within the ash cloud, but it all happens at a much larger scale.
A somewhat similar process happens with ice storms, where the fragments of ice move and collide to create a similar phenomenon.
So essentially, volcanic lightning (which is colloquially called a "dirty thunderstorm") happens when volcanic ash "paves" the way for the electric discharge to take place. A 2008 study found that “27-35% of eruptions are accompanied by lightning, assuming one eruption per year per volcano,” and reported that volcanic lightning has been observed in 212 eruptions from 80 different volcanoes.
The process has also been carried out experimentally in a lab. A recent study carried out by researchers working in Germany has confirmed that volcanic ash can help generate lightning.
“Volcanic lightning is associated with many different types of volcanoes and many different types of eruptions,” says Corrado Cimarelli, a volcanologist at the Ludwig Maximilian University in Munich, Germany, and lead author of the study.
Triboelectric (frictional) charging within the plume of a volcano during an eruption is the major electrical charging mechanism. However, volcanic lightning is still a phenomenon actively researched, with the impact of different elements still being investigated.
A few recent studies have identified another process which can cause or exacerbate volcanic lightning: fractoemission. Fractoemission is the generation of charge through the break-up of rock particles; we’ve known about the phenomenon since at least the 1980s. It's only natural that there is a lot of material break-up during a volcanic eruption, but the exact impact that this process has on lightning generation near the erupting vent remains uncertain.
Another potential source of electrocstatic charge is radioactive charging. Volcanic eruptions eject plumes, rock, and ash -- all of which can carry a significant amount of radioisotopes. Studies have been inconclusive about the influence of this type of charging, but it seems that the bigger the eruption, the bigger the influence it can have.
Another factor which seems to influence volcanic lightning is the water content of the eruption. It may seem counterintuitive but, volcanic eruptions can contain huge amounts of water. It seems that the higher the water content, the more likely it is to generate lightning. While this mechanism isn't exactly understood, the correlation has been demonstrated. The fact that volcanic lightning is more common in winter than summer also supports this hypothesis.
Lastly, plume height also seems to play an important role in the generation of volcanic lightning -- particularly regarding the influence of other factors. Specifically, studies have shown that in taller ash plumes (7-12 km), large concentrations of water vapor are the main driver of lightning activity, whereas in smaller ash plumes (<4 km), fractoemission seems to be the decisive factor.
Bonus! Volcanic spherules
As if volcanic lightning wasn't spectacular enough, it can also generate a unique type of rocks. When lightning hits any type of rock and sediment, it can turn them into special kinds of rocks called fulgurites. But volcanic lightning, which can reach temperatures of 30,000 °C, can create another type of rock called volcanic spherules.
The spherules form when the lightning bolt hits ash particles within the plume, causing then to melt and quickly cool down, solidifying and forming distinctive orb shapes.
These rocks are useful because they can serve as evidence for volcanic lightning even when it hasn't been directly observed.