Erupting Volcanoes Make Their Own Lightning, and Pliny Saw It in 79 AD

When Pliny the Younger watched Mount Vesuvius bury Pompeii in August of 79 AD, the detail he kept circling back to in his letters to Tacitus was the lightning. "The fitful gleam of torches," he wrote, was "obscured by the transient blaze of lightning." He was looking at one of the strangest electrical phenomena on the planet — a storm generated not by clouds but by an erupting mountain.

A volcanic plume is essentially a column of pulverized rock screaming upward at hundreds of meters per second. As ash particles grind against each other on the way up, electrons hop between them and the plume separates into regions of net positive and net negative charge. When the voltage difference gets large enough, it discharges as lightning. Researchers studying Sakurajima in Japan and Eyjafjallajökull in Iceland have shown two distinct regimes: tall plumes that loft moisture above the freezing line build charge much like a normal thunderstorm, while smaller plumes get most of their charge from "fractoemission" near the vent, where magma shatters into fragments and electrons are stripped clean.

A bolt of volcanic lightning reaches roughly 30,000 °C, which is hot enough to vaporize an ash particle outright or, more interestingly, to melt one and let it cool back into a tiny glass sphere. Geologists call these lightning-induced volcanic spherules and use them as a fingerprint of past eruptions in ancient ash deposits.