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Volcanoes And Lightning

June 9, 2011


This is an image from recent volcaninc eruption in Chile. On June 4th, Puyehue volcano started erupting sending plumes of volcanic ash as high as six miles leading to evacuation of more than 3,500 people and shutting down of the airports in the area. The ash has majorly affected nearby cities in chile and Argentina. The volcano which was dormant since 1960 also produced some spectacular and mighty lightning bolts (few miles long) as seen in the picture above. Such lightning shows have been also documented in other volcanic eruptions including Mt Vesuvius, Eyjafjallajökul,  Mt St Augustine volcano in Alaska, Chaiten volcano in Chile and many more. But not all volcanoes lead to lightning display. Volcanic lightning can be roughly divided into three types depending on the length of lightning bolt and how and where they are formed: i) Large volcanoes which spew out large amount of ash deep into the sky can lead to formation of lightning bolts many miles long. ii) Mid range lightning bolts can sometimes come directly from the volcanic vents. iii) Small range sparks (few meters long ) can occur in the plume which are short lived (few milliseconds). But what is the main mechanism of volcanic lightning. The answer is not clear yet.

For lightning to occur, a potential difference need to developed and then an ionization channel need to be created for the charges to flow from highe potential to lower potential. But how do you create a potential difference in the volcanic cloud or ash plume? Charge separation is the answer according to one theory. Volcanic plume is extremely hot and energetic and collision causes  particles to get charged. Positive and negative charged particles have different aerodynamic properties which lead to their separation in different zones in the volcanic cloud. Probably, positive charged particles settle down in the lower cloud while negative particles move in the upper regions of the cloud. This charge separation keeps on occurring until enough voltage difference has been created to form an ionization channel (streamer, leader head formation) and boom- lightning bolt occurs! Since the potential difference is between different regions in the clouds , you can see the bolts originating as well as ending up within the cloud itself.  But again, this is just one of the explanations and the process is still not clearly understood. This doesn’t explain how lightning sometimes start from the vents, during the beginning of eruption itself, as in this short time it’s not possible for charge separation to occur. Interestingly, there has not been many scientific studies explaining the phenomenon. It was not until 2007, when scientific observations were documented in a Science paper where the authors studied Mt St Augustine eruption. Here is an abstract of the article:

Electrical Activity During the 2006 Mount St. Augustine Volcanic Eruptions

Thomas et al | Science 23 February 2007:

DOI: 10.1126/science.1136091

Abstract

By using a combination of radio frequency time-of-arrival and interferometer measurements, we observed a sequence of lightning and electrical activity during one of Mount St. Augustine’s eruptions. The observations indicate that the electrical activity had two modes or phases. First, there was an explosive phase in which the ejecta from the explosion appeared to be highly charged upon exiting the volcano, resulting in numerous apparently disorganized discharges and some simple lightning. The net charge exiting the volcano appears to have been positive. The second phase, which followed the most energetic explosion, produced conventional-type discharges that occurred within plume. Although the plume cloud was undoubtedly charged as a result of the explosion itself, the fact that the lightning onset was delayed and continued after and well downwind of the eruption indicates that in situ charging of some kind was occurring, presumably similar in some respects to that which occurs in normal thunderstorms.

I will try to find more details about the process and would add more information in my later posts. For now, here is one more picture from the Puyehue volcano.

Inage credit: 1) Boston.com/Bigpicture|Carlos Gutierrez/Reuters 2) Boston.com/Bigpicture|Claudio Santana/AFP/Getty Images

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