Do you know the gas released in the eruption next to an African village that suffocated the villagers? As I remember, those that were asleep in the village died because their heads were close to the ground and some survived because they were upright and working that evening. I think this eruption occurred at the bottom of a lake. Thanks for your help!

Emily

Hi Emily,

The gas that killed all those people was carbon dioxide. It had been slowly released into the bottom of the lake (which was formed within a deep volcanic crater) for many years. It was therefore highly concentrated in the deeper water, but the pressure of the water above kept it in solution. Some sort of disturbance, either a landslide, earthquake or big rock falling into the lake, disrupted this meta-stable situation, and caused a bunch of this carbon dioxide-rich water to be displaced upward. Here the pressure was no longer sufficient to keep the gas in solution and it exsolved in to a gas phase (just like you see bubbles forming in a soda out of nowhere when you take off the cap). At lake Nyos, this exsolving of carbon dioxide was very rapid.

Carbon dioxide is heavier than air so when this huge batch of gas arrived at the surface, it flowed over the land. I'm pretty sure that it was a layer thicker than the height of a person so that it didn't really matter if the folks were sleeping or not--they were down in a valley that led away from the crater. Something like 3000 people died. Unfortunately they had all gathered in that one town because the next day was market day.

It was an unusual and frightening volcanic disaster. At the moment there is a program to constantly circulate small amounts of the deep gas-rich water to the surface in the hope that the concentration won't build up to huge levels again.

Sincerely,

Scott Rowland

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