The Strangest Disaster of the 20th Century, pt. 2

In the last post, we discovered that all living things withing 12 miles of Lake Nyos in Cameroon died mysteriously overnight, without any indication of cause.

And now, the conclusion. (Story taken from here).

Scientists began testing water samples taken from different depths of the lake. The murky red color turned out to be dissolved iron, normally found at the bottom of the lake. Somehow, sediment from the bottom had been churned up to the top, where the iron turned red after coming in contact with oxygen.

Scientists also found unusually high levels of CO2 dissolved in the water. As they took samples from deeper and deeper, the CO2 levels steadily increased. At 600 feet, the levels suddenly shot off the charts. Beyond that depth, the containers burst from the pressure of the gas contained within them.

The theory formed that though the volcano had long since gone extinct, the magma chamber still simmered deep below the surface of the Earth and was still released carbon dioxide into the lake and the surrounding environment.

CO2 shows up in lakes all over the world. The CO2 in Lake Nyos, however, instead of floating to the surface and evaporating into the air, was accumulating at the bottom of the lake.

Normally lakes and other water bodies convect: the surface layers cools because of rain or wind, it becomes denser than the warmer water below, sinks, and displaces the warmer, CO2 rich layers at the bottom, which then move high enough for the CO2 to bubble to the surface and disappear.

Convection example. Pretend it's a lake.

Lake Nyos is one of the stillest lakes in the world. It is surrounded by tall hills that block the wind and because it is in a tropical climate, the water temperature stays consistent throughout the whole year. Also, because the lake is so deep (690 ft), when the surface is agitated, very little disturbance reaches its depths. The unusual stillness of the lake is what made it so deadly.

There is a physical limit to the amount of CO2 water can absorb. As the bottom layers become saturated, the CO2 is pushed upward to where the pressure is low enough for it to start coming out of solution. At this point, a disturbance such as an earthquake or a landslide can cause the CO2 to bubble to the top.

These bubbles cause a “chimney” effect – basically, they create a tube through which the lake can disgorge all of the CO2 that has been accumulating at its bottom for decades.

CO2 is odorless, colorless, and non-toxic; your body produces it and you exhale some every time you breathe. The air you breathe consists of about 0.05% CO2. What makes it deadly (in some circumstances) is that it is heavier than air: if too much is released at once, it displaces the air on the ground, suffocating everything in the area. 10% CO2 in the air can be fatal – as little as 5% can smother a flame…which is why the lamps were snuffed out.

As they monitored the lake, the scientists came up with an estimate of how much CO2 was released. On August 17, eyewitnesses said the lake began bubbling strangely, causing a misty cloud to form over the surface. Then on August 22, the lake exploded; water and gas shot a couple of hundred feet into the air.

The lake ejected 1.2 cubic kilometers into the air – enough to fill 10 football stadiums – in as little as 20 seconds. (If you know anything about the 1980 eruption of Mount Saint Helens, you remember the ash that circled the planet. That was only 1/3 of one kilometer of ash – a mere quarter of the CO2 released by Lake Nyos).

Dead cattle were found as high as 300 feet above the lake, indicating the gas had reached at least that high below settling down to suffocate everything at ground level.

Some survivors said it sounded like a gunshot or an explosion; other described it as a rumble. Those who ignored it and stayed in their houses were generally safe, but those who went outside to find the source quickly died on their front doorstep. The sight of these victims brought their family members to the doors and they too passed out and were killed.

Shut doors and windows offered the most safety, but there were still cases of CO2 seeping into homes through the floors and walls and smothering those who had laid down to sleep, but those who stood with their heads above the gas survived.

Since the disaster, officials have placed a large tube in the lake to artificially create convection and channel the CO2 up into the air.

'Degassing' Lake Nyos

And thus the mystery of the strangest disaster of the 20th century was solved.

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