Ask Tom Why: Volcanic lightning

April 22, 2010

Dear Tom,

I keep seeing pictures of lightning with the Iceland volcano. What causes it?

—Charmaine Fortuna, Plainfield,

and Peter Gottstein, Chicago

Dear Charmaine and Peter,

Volcanic lightning, while frequently observed, is not fully understood. It appears that volcanic lightning formation is somewhat analogous to thunderstorm lightning — produced by a separation of positive and negative charges from colliding materials. In thunderstorms, it is the collision of hail and rain, while in a volcano the collision of ash and solid debris particles causes the positive and negative charges to separate. The smaller and lighter negative charges tend to rise while gravity causes the heavier positive charges to fall. When the charge differential is great enough, a static electricity discharge produces the lightning.