Lightning in volcanic plumes is not completely understood, but there are several ideas about how it might work. Lightning is a giant spark that fires when large quantities of electric charge build up. Thunderclouds are made of water, ice and hail, and the electrification arises from collisions between light particles (ice) moving upwards in the cloud, and heavier water drops falling.
One way lightning in volcanic plumes could be generated is from ash particles colliding with each other. This is similar to the electric charge that can be generated by rubbing a balloon on a jumper.
Another way lightning can occur in volcanic plumes is from some volcanoes, like Eyjafjallajökull in Iceland, that have glaciers on top of them. When the volcano erupts, the glacier melts, which both causes huge floods and makes a big cloud of ice and water that becomes mixed with the volcanic plume. This mixture of cloud and plume can generate volcanic lightning by ice, water, and ash collisions in what’s called a ‘dirty thunderstorm’.
Credit: Karen Aplin, Pete Wilton