Volcanic Lightning

Volcanic lightning is an electrical discharge caused by a volcanic eruption, rather than from an ordinary thunderstorm. Volcanic lightning arises from colliding, fragmenting particles of volcanic ash , which generate static electricity within the volcanic plume. Volcanic eruptions have been referred to as dirty thunderstorms due to moist convection and ice formation that drive the eruption plume dynamicsand can trigger volcanic lightning. Volcanic lightning can also occur before any ice crystals have formed in the ash cloud. 

Conventional lightning makes an appearance when layers of warm air get a little frisky with duvets of considerably chillier air. As the warm air rises, its water droplets agglomerate; as the air cools, these form ice crystals which sink to the bottom of the cloud. As this process takes place, sinking ice crystals and rising water droplets rub together in the cloud layer, and an electrical charge separation takes place. The upper, damper segment of this mix ends up having a positive electrical charge, whereas the basal section, imbued with ice crystals, is negatively charged.

When the gradient between the positive and negative charge becomes too steep, nature gets a little furious and seeks to balance it out – and there’s no faster way to remedy this charge imbalance than via a lightning bolt. The insulating qualities of the air can’t keep up with the need to restore balance, and it gives way to a dramatic flash.

source: Wikipedia and Forbes mags.