Volcanic lightning: how an ash plume electrifies itself

01

What causes lightning in a volcanic plume?[1]

Charge separation, the same requirement as any lightning, reached by different means. Near the vent, rock fragmenting under enormous stress leaves particles charged, a process called fractoemission. Higher in the plume, water freezes and the collision-based charging of an ordinary thunderstorm takes over.

Fractoemission happens because breaking a solid does not divide its charge evenly. Magma tearing itself apart during explosive decompression produces enormous numbers of fresh fracture surfaces in a very short time, and the fragments carry away net charge. This is the dominant process in the first few hundred meters above the vent, where discharges are small, frequent and continuous.

The ice mechanism appears higher up, where the plume has risen and cooled below freezing. Eruption columns carry a great deal of water, both from magma and from any groundwater, snow or ice the eruption encountered. Once that water freezes, the plume charges like a thunderstorm and produces the long, branching flashes that show up in photographs.

Plume height therefore predicts flash character reasonably well. Short columns give vent discharges. Columns that punch above the freezing level give proper lightning.

02

What did Eyjafjallajokull show?[1]

The 2010 Icelandic eruption was monitored closely enough to separate the two mechanisms. Analysis of its discharges found both monopole and dipole charge structures in the plume, with evidence that ice formation contributed to the dipole structure through thunderstorm-style charging.

The dipole structure is the significant finding, because a dipole is the signature of an ordinary thunderstorm's charge arrangement rather than a bulk-charged cloud of ash. Its presence in a volcanic plume is direct evidence that the ice mechanism operates there and is not merely plausible.

Low-level discharges at that eruption were attributed to processes at or very near the vent, including magma interacting with water and fractoemission. Eyjafjallajokull sat under a glacier, so meltwater flashing to steam was continuously available.

03

Can lightning networks detect volcanic eruptions?

Yes, and this is the practically useful part. Volcanic lightning emits the same very low frequency radio pulses as ordinary lightning, so the same ground stations detect it thousands of kilometers away, without needing anything installed on the volcano.

That makes lightning detection a monitoring tool for volcanoes in remote regions where instrumentation is sparse, and at night or under cloud where satellites and cameras see nothing. A sudden cluster of discharges at a known volcano's coordinates is a strong signal that an explosive eruption has begun.

It also carries information about the eruption itself. Because the ice mechanism only turns on above the freezing level, a burst of large flashes implies a column tall enough to reach it, which is exactly the parameter aviation authorities need for ash advisories.

On a live map, volcanic lightning looks like a persistent tight cluster that does not move with the wind the way a thunderstorm does. Anak Krakatau's activity produced dense ash clouds that generated lightning in that pattern.

04

How is it different from thunderstorm lightning?

The physics of the discharge is identical, since both are electrical breakdown through air. What differs is the source of the charge and the scale. Vent discharges are far smaller and more frequent than cloud-to-ground lightning, and much of the activity never reaches the ground at all.

Charge source

fragmenting rock and ice, rather than ice alone

Flash size

vent discharges are meters to tens of meters; plume flashes are kilometers

Rate

vent activity can be near-continuous during vigorous eruption

Detection

same VLF signature, so ordinary lightning networks see it