Scientists don't actually know what causes volcanic lightning even though they've been studying the phenomena since the 1960s. They've concluded there are two kinds: ones that occur at the mouth of the volcano, and another that occurs high up in the plume of ash. Video of Calbuco in Chile clearly shows the second kind AND IT'S AWESOME!
Scientists believe the first kind is caused by positively charged magma and gases bubbling up from the Earth's crust. It is not known how they become positively charged.

The second kind is the ash interacting with cold air in the atmosphere has the same result as a super cell (like in the famous Great Plains thunderstorms).
Watch for yourself to see some of the lightning coming out of Calbuco.

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