Same reason that you get shocked when you shuffle your feet on the carpet and touch a metal door knob.

Lightning in volcanic eruptions is, I believe, rather common. The ash sent up in the volcanic plume is flowing upward at a furious rate - as the dust particles bump into each other, they rub off electric charges, and eventually set up what is effectively a big voltage between the cloud and the surrounding air (or the cloud higher up). Eventually that voltage difference becomes large enough that the air can break down and BAM - you got lightning.

Same thing happens in thunderheads - the air currents driving water molecules in a convection pattern inside thunderheads generates sufficient voltage differences that lightning (air breakdown and electrical connection across that broken-down air path) can strike.

__________________
Todd K. Pedlar
member, First Congregational Church, (CCCC) Cresco, IA
http://semperubi.rtrc.net

"Many men, after a long conversion, see more of the workings of sin in their hearts than ever they did before or at their first conversion. Now, such men have not an increase of sin, but an increase of illumination and light" (Christopher Love)


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