What causes lightning during a volcano?

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Dear Tom,
What causes lightning during a volcano?
Thanks,
Nan Doudican

Dear Nan,

The clouds of gases, steam and rock debris that rise above erupting volcanoes do, indeed, generate lightning. The closely studied 1963 eruption of Surtsey, an Icelandic volcano, produced a tremendous amount of lightning. Electrical activity in volcanoes is caused by the ejection of material carrying large positive charges. The following description appeared in a 1965 article in Science Magazine, entitled “Electricity in Volcanic Clouds”: “Because of concentrations of fine, black tephra particles, the volcano cloud was far more opaque than ordinary clouds and the appearance of lightning over the volcano was somewhat different from that in thunderstorms. Discharges inside the cloud did not provide much illumination, and they were entirely screened from view if they were at any depth.”

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