When Alaska's Redoubt Volcano rumbled to life three years ago, researchers rushed to the scene to deploy radio sensors to catalog and map lightning within the plume.

Lightning bolts shoot out of the Mount Redoubt volcano as it erupts in 2009. (Bretwood Higman)

The readings, the team now reports, reveal that volcanic lightning storms can be as powerful as giant Great Plains thunderstorms and that more lightning means a bigger plume. "If you get lightning," said report coauthor Stephen McNutt, "that's telling you there's a lot of ash."

Iceland's Eyjafjallajökull volcano erupts with flash and ash in 2010. (Peter Vancoillie/National Geographic Your Shot)

With the right instruments, McNutt said, "you can see electrical activity right at the onset of the eruption," inside the crater.

These otherwise invisible lightning bolts are produced by static electricity, generated when rocks are shattered and thrust skyward in an eruption.

The in-crater lightning bolts aren't huge, but they can strike thousands of times a second, creating a nearly continuous radio signal that would instantly mark the onset of the eruption, he said.

A giant bolt of lightning strikes Indonesia's Mount Merapi in 2010. (Jeffe Castan/National Geographic Your Shot)

"We sometimes refer to [volcanic plumes] as dirty thunderstorms," McNutt said. But, he added, there's a lot more lightning in the ash plumes than is visible in the pictures. "That's because ash clouds are opaque."

Lightning branches out from behind a veil of ash clouds at Iceland's Eyjafjallajökull volcano in April 2010. (David Jon/NordicPhotos)

Every bolt travels in a distinct direction, Martin Uman, a lightning expert at the University of Florida, told National Geographic News in 2012. A spark begins in an electrically charged spot and then travels either up, down, or sideways until it reaches an oppositely charged area.

More amazing displays of electrical discharges can be seen in the National Geographic photo slideshow, Volcano Lightning, Illuminated .