Volcanology: No Thundercloud Needed

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The hot plumes of steam and ash that rise from many volcanoes are often shot through with brilliant lightning flashes. But these violent bursts of electricity are usually blamed on conventional thunderclouds pushed up by the heat of the eruption. Curious to learn whether a volcano can make its own lightning, without "thunderclouds, a team of U.S. and Icelandic scientists studied the volcano that is forming the new island of Surtsey off the coast of Iceland. In Science, they report that they rode a fishing boat to within 250 yds. of the roaring vent, and flew in an airplane close to the hot, black plume. Once they saw rocks a foot in diameter tossed above their plane. They escaped injury and satisfied their curiosity: volcanoes do manufacture lightning.

Great flashes often struck out of the volcanic plume, usually hitting the newborn island near the active vent, and almost every time, the scientists' instruments showed a strong charge of positive electricity in the plume just before the flash. After the flash, the plume was neutral or negative, building up a positive charge before the next flash. This happened repeatedly without any thundercloud forming in the vicinity, proving that the volcano alone was generating the electricity. Just how it did this is still uncertain. In some cases, the positive electricity was created when a high-speed jet of ash-laden volcanic steam shot up through sea water; yet clean steam, formed when lava flowed quietly into the sea, also contained a charge. To satisfy themselves about the final details of the volcanic lightning generator, the scientists will have to brave a second and still closer look.