Lightning (Volcanic Trigger)

Primary reference(s)

Behnke, S.A. and S.R. McNutt, 2014. Using lightning observations as a volcanic eruption monitoring tool. Bulletin of Volcanology, 76:847. 10.1007/s00445-014-0847-1

Mather, T.A. and R.G. Harrison, 2006. Electrification of volcanic plumes. Surveys in Geophysics, 27:387-432.

McNutt, S.R. and R.J. Thomas, 2015. Volcanic lightning. In: Sigurdsson, H., B, Houghton, H. Rymer, J. Stix and S. McNutt (eds.), The Encyclopedia of Volcanoes, 2nd Ed. Academic Press, pp. 1059- 1067

Additional scientific description

Explosive injection of volcanic ash and gas into the atmosphere produces a wide range of electrical activity (Behnke and McNutt, 2014). The most hazardous electrical phenomenon is cloud-to-ground volcanic lightning, which creates a transient channel of hot plasma between a volcanic cloud and the ground. Exactly like ordinary thunderstorms, cloud-to-ground lightning from volcanic eruptions can produce thunder, trigger wildfires and destroy unshielded monitoring equipment or other infrastructure. Despite its potential impact, there are only a handful of documented cases where volcanic lightning resulted in injury or death (McNutt and Thomas, 2015).

In general, the hazards of volcanic lightning increase with eruptive intensity (McNutt and Williams, 2010; Behnke et al., 2013):

• Small eruptions: Low plumes (<1 km high) have been observed to create lightning (Cimarelli et al., 2016), including low-level steam plumes from lava flows entering the ocean. However, these flashes are sparse and only measurable with close-range sensors.
• Moderate eruptions: Slightly larger eruptions with plume heights 1–10 km (and ground-hugging ash flows if present) are likely to produce some lightning activity, but it tends to be weak and restricted to areas within about 20 km of the volcano (Behnke et al., 2013; Van Eaton et al., 2020). 
• Large eruptions: Plumes exceeding heights of 10–15 km above the vent tend to produce the highest rates of volcanic lightning. These volcanic events occur only a few times per year worldwide, and in some instances are capable of transporting lighting-rich plumes over 100 km from the volcano (Van Eaton et al., 2016). Volcanic lightning from large eruptions is detectable on a global scale using worldwide networks.

The origin of volcanic plume electrification is a topic of active investigation, but it is clear that at least two distinct processes are involved. Silicate charging occurs close to the eruptive vent, during magma fragmentation and high-energy collisions among airborne rock particles (Mather and Harrison, 2006). At higher altitudes, ice charging—which is responsible for lightning in ordinary thunderstorms—becomes active if the volcanic plume rises well above the freezing level (approximately -20°C), creating a mixed-phase region of ice crystals, soft hail, and supercooled liquid water (Behnke et al., 2013; Van Eaton et al., 2020). Once the particles undergo either or both of these charging mechanisms, they accumulate in oppositely charged regions due to turbulent flow and gravitational separation of particles based on their different sizes and settling speeds (Behnke et al., 2013). Charge separation builds an electric field until it exceeds the local breakdown threshold of surrounding air, resulting in lightning discharges.

Metrics and numeric limits

Not identified.

Examples of drivers, outcomes and risk management

The hazards of cloud-to-ground volcanic lightning are nearly always of second-order importance compared to the other volcanic hazards of ground-hugging ash flows, lahars, and ashfall (Blong, 2000). The exception is rare situations when a large eruption transports a lightning-rich cloud directly over a populated area, exposing people and infrastructure to cloud-to-ground lightning. In areas of the world where ordinary thunderstorms are rare (e.g., high latitudes), the local population may not be accustomed to moving immediately indoors during lightning activity.

Current methods for mitigating this hazard include developing near-real time alerts for volcanic thunderstorms using global or regional networks of radio antennas (Behnke and McNutt, 2014).

A well-established example includes the World Wide Lightning Location Network’s volcanic lightning monitor, which generates an alert when lightning initiates near an active volcano and progresses outward through time (University of Washington, no date).

Detection of radio emissions from electrical discharges can provide early warning of a lightning-rich eruption because the signal travels at the speed of light.