An electrifying phenomenon
Volcanic lightning is an electrical discharge caused by an eruption, creating a spectacle often called a "dirty thunderstorm". The 2010 eruption of Iceland's Eyjafjallajökull volcano provided a well-documented example of this powerful event. Unlike a typical thunderstorm where lightning is generated by colliding ice crystals, volcanic lightning has multiple charging mechanisms. Near the volcanic vent, the violent fragmentation of rocks and magma generates an electrical charge in a process called fractoemission. Higher in the plume, triboelectric charging occurs as particles of volcanic ash and rock fragments collide, stripping electrons from one another and building static charges.
The height of the ash plume determines which charging mechanism dominates. In shorter plumes, typically 1 to 4 kilometers high, fractoemission and the friction between rock particles are the primary drivers. In taller ash plumes, like those from Eyjafjallajökull which reached heights of 7 to 12 kilometers, another process occurs. The abundant water vapor from the magma and melted glacial ice rises into the cold atmosphere, where it freezes. Collisions between these new ice crystals and the ash particles create charge separation, similar to a regular thunderstorm. This interaction between magma and ice water, a phreatomagmatic eruption, can lead to significantly more lightning.
Reading the sparks
Scientists study volcanic lightning to better understand the dynamics of an eruption. The intensity and type of electrical activity can provide information about the ash concentration, plume height, and eruption intensity, which are difficult to measure directly. Researchers at the 2010 Eyjafjallajökull eruption used very high frequency (VHF) radio sensor networks, called Lightning Mapping Arrays, to detect and analyze the electrical discharges inside the dense ash cloud. These instruments revealed continuous radio frequency signals caused by a high rate of small discharges near the vent during the first explosive phase from April 14-18. Later, during a second phase in May, hundreds of larger flashes were recorded, with some bolts stretching up to 15 kilometers long.
The extreme heat of a lightning bolt, which can reach 30,000°C, can melt or even vaporize ash particles. As these molten droplets cool rapidly, they form tiny glass spheres called lightning-induced volcanic spherules (LIVS). These microscopic spherules, sometimes only a few micrometers in diameter, fall with the ash and become part of the geological record. Finding LIVS in ancient ash deposits allows scientists to confirm that volcanic lightning occurred during eruptions thousands of years ago, even with no human observers.