When Mt. Vesuvius erupted in the year 79, famously burying the town of Pompeii in ash, historian Pliny the Younger recorded that the volcano expelled "frightening dark clouds, rent by lightning twisted and hurled, opening to reveal huge figures of flame." It was the first description of a phenomenon that, almost 2,000 years later, is the stuff of viral Internet photos: lightning that arcs its way through the ash plumes of some volcanic eruptions.
It's a phenomenon that has, however, puzzled scientists, who have wondered exactly how such lightning is generated. Two new papers, published in Geophysical Research Letters, the journal of the American Geophysical Union (AGU), have helped provide clarity and, in the process, unify two competing schools of thought. (Yes, there are competing schools of thought on volcanic lightning.)
The basics of lightning are understood well enough: Particles within a cloud become electrified, generating a field in which positive and negative charges are separated, and the lightning restores the charges to balance. In thunderstorms, ice crystals are the particles that are electrified. What was unknown was whether ice was also required for volcanic lightning, or whether friction between particles of ash performed that role. The new studies show that both can be true, depending on the situation.
Volcanic Lightning: How Does It Work?
In one study, Alexa Van Eaton, a volcanologist at the U.S. Geological Survey Cascades Volcano Observatory in Vancouver, Wash., and colleagues analyzed lightning during the April 2015 eruption of the Calbuco volcano in Chile, using a global network of lightning sensors.
They found that, far downwind of the volcano, lightning followed the movements of the very top of the ash plume where it was cold enough for ice crystals to develop.
"The lightning basically decoupled from all the other ash particles that were falling to the ground," said Van Eaton. "Instead, they seem to follow the ice crystals that stayed high in the atmosphere."
VIDEO: Volcano Lightning: How Does It Happen?
So, mystery solved, right? Ice is after all required for volcanic lightning? Not so fast. Because Van Eaton and her team found that lightning also occurred during a second stage of the eruption, when ash and gas flowed close to the ground, far away from any ice crystals, suggesting that the collision of ash particles alone can create enough of a charge.
That latter finding was reinforced by the second study, in which a team led by Corrado Cimarelli, a volcanologist at Ludwig Maximilian University in Munich, Germany, recorded high-speed video observations of volcanic lightning at Sakurajima volcano on the island of Kyushu, Japan, which has been erupting persistently since 2009. Because ash plumes can obscure the lightning that is taking place within them, Cimarelli and his team also included data from nearby acoustic sensors and electromagnetic field measurements.
They found that during an explosion, particles of ash and debris rub up against each other, which creates a build-up of electrical charge. "Where there is ash violently ejected in the atmosphere, there will be electrical discharges, independent of the magnitude of the eruption," said Cimarelli.
Volcano Sparks New Kind of Lightning
Added Van Eaton: "What we can clearly say is that the electrification of the volcanic plume starts immediately. However, the evolution of charge, in a very big eruption, tends to be enhanced by the development of ice."
Such studies are important not just for esoteric reasons, but also because understanding and monitoring volcanic lightning seems likely to enable swifter detection of remote eruptions and calculation of the likely path of their ash plumes.
"Anything that helps give us an earlier warning of explosive activity -- particularly for remote volcanoes -- is going to make our airways a little bit safer," Van Eaton told Lauren Lipuma of the AGU. "Earlier warning means a faster ash dispersal forecast. We're finding that it's possible to detect eruptions earlier with volcanic lightning than is possible with satellite detection alone."