22 December 2011


There is a passing mention of lightning in Pliny's description of the destruction of Herculaneum and Pompey by Vesuvius's eruption in 79 AD, but no one made much of it at the time, nor for the two ensuing millennia. 

Then the eruption of Surtsey in 1979 (a basalt-andesite volcano forming a new island off the coast of Iceland) had people present with cameras - and the spectacular lightning associated with the eruption was actually recorded, and evaluated in subsequent scientific papers.

When Mount St Helens erupted in May, 1980, the pilot flying closest to the Plinian column reaching up to the Stratosphere commented repeatedly about lightning bolts coming from the top of the eruption column and flashing down into the caldera. (Yes, that name came from Pliny the Younger's description of the demise of his father, Pliny the Elder, in the Mediterranean Sea off Herculaneum. These eruptions also entrain the surrounding air, which drew Pliny's ship into the coast; there are reports of 100 kph winds roaring towards MSH on May 18, 1980). 

Current models have suggested that the rapid rise of silica-loaded particles in effect drew (or separated, depending on your point of view) a net unbalanced charge entrained in the ash to considerable heights. When sufficient charge imbalance accumulated, it relieved itself by bolts of lightning to the highest topographic point below it... the still-considerable remains of the edifice of the volcano (which lost 400 meters of its original elevation in the eruption). 

There are scientists in the Alaska Volcano Observatory who now track lightning along the Aleutian chain as an early warning of an eruption on volcanoes that we can't yet afford to instrument.  It turns out that the noise threshold for a reasonably reliable call on an eruption is around a VEI level 3-4. That's for Volcano Explosivity Index, and the numbers are approximately logarithmic: the 1980 MSH eruption was a VEI 5, which is about 10 times greater energy released than a VEI 4 event. 

As an aside: Managua, Nicaragua, is not an old city. Instead, there have been at least three versions of the city just in historic times. In between each, stupendous subduction earthquakes leveled most of the pre-existing city, and huge blankets of volcanic tephra and ash buried what remained. The same holds, more or less, for Guatemala City, the capitol of Guatemala. While we can visit the Parthenon in Greece and the Forum in ancient Rome, we cannot view what ancient Central America might have looked like 2,000 years ago... 

...for behold, the whole face of the land was changed, because of the tempest and the whirlwinds, and the thunderings and the lightnings, and the exceedingly great quaking of the whole earth...

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