Volcanic Lightning

Lightning is not always confined to thunderclouds. For hundreds of years, people have observed lightning in the pillar of ash from an erupting volcano; these lightning displays are sometimes huge and much more fierce than the lightning from ordinary thunderstorms.

Scientists have long believed that these fearsome lightning displays are caused by friction between particles of soot and ash, which leads to the the development of static charges high inside the ash cloud. But recent information suggests that the lightning in a volcanic eruption may be caused by water and ice, just like the lightning in an ordinary thunderstorm.

The mechanism by which lightning forms in volcanic clouds is not well-understood, in part because it's never been a priority for geologists. Small flecks of dust and ash will hold static electric charges, so the working model in the past has been "Well, you get static electricity built up in the columns of ash, and that's that."

A new model of volcanic lightning proposed by researchers at MIT casts doubt on this idea, and proposes a mechanism where water, the same thing that is responsible for lightning on Jupiter and Saturn as well as during storms on earth, causes these huge lightning displays.

Magma, the molten rock that spews out of a volcano, contains a great deal of water–in fact, the water carried to the earth's surface by erupting volcanoes, called "juvenile water," makes up in some cases as much as six percent of the total mass of material ejected from the volcano.

This water is initially ejected as vapor–steam–but quickly condenses out. The explosive force of a large eruption sends material into the upper atmosphere, where the temperature is typically well below freezing; the huge quantity of water blasted out along with the explosion expands and condenses on the particles of dust and ash, coating them with liquid water and ice.

And, as we know, wherever you have strong updrafts through clouds containing water and ice crystals, you get enormous static differentials. Those static electrical charges produce lightning.

In fact, lightning activity is so strongly associated with volcanism that volcanologists are now using lightning detectors to indicate volcanic activity.