How lightning forms in general is still debated among scientists, and volcanic lightning is even less well understood. What is mostly agreed upon is that the process starts when particles separate, either after a collision or when a larger particle breaks in two. Then some difference in the aerodynamics of these particles causes the positively charged particles to be systematically separated from the negatively charged particles. Lightning is the electrical flow that results when this charge separation becomes too great for air to resist the flow of electricity. Some of the lighting strokes in these photos are at least 2 miles long, so the separation of charged particles must occur on this scale.
Here is an idealized sequence of events that leads to lightning:
- Starting state (particles might have already been charged by some previous process).
- Collisions lead to charge separation. For this to happen there has to be some difference in the electrical properties of the particles in the collisions.
- Some process, such as aerodynamic sorting, segregates the positively and negatively charged particles. This means that there are sections of the cloud that are more negative or positive than other sections.
- When the charge separation becomes too great, electricity will flow between the positive and negative regions of the cloud, forming lightning and neutralizing the charge separation.
These photos are of the Redoubt Volcano in Alaska and were taken by Bretwood Higman. He set his camera up to take 30 second exposures every two minutes.
I remember a few years ago there were some absolutely astonishing photos of lightning in a Chilean volcano. Here's one...
