I have recieved an email asking me to go into a couple of things which I mentioned in this post.

The first question asked how water causes lightning in eruption columns, and whether this is similar to meteorologically generated lightning in thunder storms.
The boiling of water in plumes and consequent rapid disruption of water molecules causes them to shear (or break-apart) this can create a charge which is more positive in larger molecules and more negative in smaller molecules. In eruptions including seawater, the salt included becomes negatively charged. This process discussed is different to the thunderstorm charging mechanisms. However a paper by Williams and McNutt (2005) does propose that the thunderstorm charging mechanism may occur in volcanic plumes (particularly at volcanoes located at high atltitudes with ice). To my knowledge thunderstorm charging is to do with collision of particles and how fast these
 
It should be stressed that there is not one 'correct' reason for lightning charging in eruptions and it could well vary from plume to plume dependent on a variety of factors!

The second question asked how lightning in a volcanic plume is used to infer height.
There is an excellent overview of this located here. The Met Office (UK) has used this during the eruptions of Eyjafjallajökull and Grimsvotn. Research has found that the number of lightning strikes per hour (or the amount of activity) increases proportionally with plume height. Therefore a rough plume height can be inferred. 

I hope this helps!

We have all seen the spectacular photos of eruption columns and lightning but why does it happen?

The majority of study on lightning in eruption plumes has been done on silicate rich plumes. These are the dark grey plumes you see in more plinian type eruptions (such as pinatubo), phreatomagmatic and some vulcanian eruptions. It is within these columns that lightning is more common.  

In general within these columns the larger particles at the base of the plume with smaller at the top. Within the column the overall charge is moe positive at the top and more negative at the base. This creates what is termed a dipole and consequently lightning occurs. However there are other mechanisms based on the type of eruption, the content of the eruption and the location (altitude). 

There are two main ways which plumes are thought to become charged; these are during the fragmentation process (often termed fractoemision) and the boiling of water (often a common volatile in eruptions, but also an addition during phreatomagmatic eruptions). Fractoemission occurs during the fracturing of a material (in volcanic cases - magma) this then causes a the release of electrically charged atoms, ions and radiation in different circumstances. Fractoemission is thought to be the dominant charging process.

So why is volcanic lightning important to study? Firstly, it can be used to infer plume height. This was done during the recent eruption of Grimsvotn in combination with the UK met office. It is important on extraterrestrial bodies (other planets) and may have played a role in the origin of life on our own planet Earth. Volcanic lightning also represents a hazard to people and has killed people in the past during eruptions of Paricutin and Rabaul. 

This is just a brief, and simplified overview, for those of you with access to journals and want a more detailed explanation, the following are articles on lighting in volcanic plumes.

James et al. 2008. Electrical Charging of Volcanic Plumes. Space Science Reviews 137, pp. 399-418

Mather and Harrison, 2006. Electrification of Volcanic Plumes. Surv Geophys 27, pp. 387-432

McNutt and Williams, 2010. Volcanic lightning: global observations and constraints on source mechanisms. Bulletin of Volcanology 72, pp. 1153-1167

I have posted three of my favourites below. The last photo of Krakatau just goes to show that it doesn't have to be a violent eruption to produce lightning! My personal favourite though is the one of Chaitén - very atmospheric. 

Puyehue-Cordón Caulle continues to erupt but with diminshed activity, there are detailed reports in the Eruptions blog by Erik Klemmeti and The Volcanism blog.

I have posted a couple of the more spectacular photos of the eruption column, I will be posting tomorrow about what causes the volcanic lightning so often seen in eruption columns! It's something which is seen so frequently that I feel it should be explained!