A volcanic plume (the cloud of ash that bursts out of an active volcano) are made up of highly compressed particles as they have been underground for many thousands of years.
When a volcano erupts and spits out its ash plume, these particles mix with the air in the atmosphere, which is much less dense. As the particles are bashed together violently within the plume, they create friction and become electrically charged and separate from other air particles causing a mix of positive and negative particles. The electric field that connects the positively and negatively charged particles results in lightning.
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