One of nature’s most explosive combinations occurs when an erupting volcano generates an electrical storm. The reason for this lightning bolt from the blue is still not fully understood.
Lightning is caused by a build up of static electric charges. Scientists are not sure what creates the charge during a volcanic eruption, but think that hot ash particles in the volcanic cloud may rub together, producing a charge of static that triggers sparks of lightning. This is similar to what happens inside storm clouds, where ice particles collide and create a charge.
When ice particles in a storm cloud rub together, they gain electrons (becoming negatively charged) or lose electrons (becoming positively charged). The positively charged particles accumulate at the top of the cloud, while the negatively charged ones settle at the bottom. When the difference in charge becomes great enough, the energy is discharged as lightning.