sixpenceee:

Volcanic Lightning occurs when lightning is produced in a volcanic plume. The process starts when particles separate, either after a collision or when a larger particle breaks in two. Some difference in the aerodynamics of these particles then causes the positively charged particles to separate from the negatively charged particles. Lightning occurs when this charge separation becomes too great for air to resist the flow of electricity. Volcanic eruptions also release large amounts of water, which can also help these thunderstorms along. (Source)