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SciShow takes you inside a limnic eruption, a natural disaster that’s as deadly as it is rare.

Hosted by: Hank Green
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     In the evening of August 21, 1986, eighty million cubic meters of carbon dioxide gas suddenly erupted from Lake Nyos in the West African country of Cameroon. This massive cloud expanded at 50 kilometers an hour, suffocating everything within a 25 kilometer radius, killing more than 1,700 people.

    The only known similar event happened at nearby Lake Monoun two years earlier, where 37 people died. And scientists weren't really sure, at first, what was going on. They figured this was some kind of volcanic eruptions, because both lakes are close to Cameroon's Oku Volcanic Field, which includes four major volcanoes.

    But even though volcanoes were partly to blame for these explosions, it turns out that they were no volcanic eruptions. Instead, what made this lakes explode were limnic eruptions, which are like popping open the largest, most pressurized soda can of all time.

    The volcanoes near the lakes are a sign that there's magma, or molten rock, close beneath the surface, and magma releases huge volumes of carbon dioxide, which can filter up through the cracks of the earth into the bottoms of the lakes.

    Now, these lake bottoms are highly pressurized and very cold, which is exactly what gasses need in order to dissolve in water. So, lots of carbon dioxide can dissolve in the lakes and just build up, sitting there. But as the CO2 continues to build, those lakes get to be like cans of soda pumped up with way too much fizz.

    Most lakes don't explode, thankfully, even if they're sitting on top of a whole lot of magma. That's because the water gets mixed around when the temperature changes throughout the year. In the fall and spring, the changing surface affects the water's density and keeps it circulating and, in turn, helps release the gas.

    But in Lake Nyos and Lake Monoun, things are very different. It's hot pretty much all the time, so the shallower, less dense water stays at the top of the lake, where the gas-infused deeper layers stay below. As a result, tens of millions of cubic meters of carbon dioxide can accumulate.

    No one's sure what exactly triggered Cameroon's limnic eruptions, but a seemingly minor event that another lake might not even notice, like a heavy rainfall or a heavy landslide or a small earthquake could send some of that gas-rich deep water up into the shallows, in which case it's like poking a hole in that over-pumped soda can and suddenly everything is covered in liquid and fizz.

     Except that instead of getting a little sticky, the liquid is a 25-meter wave of water and the fizz is a giant, deadly cloud of carbon dioxide.

    For obvious reasons, no one wants this to happen again, so once scientists figures out what was going on, pipes were installed in Lake Nyos and Lake Monoun, allowing the carbon dioxide to escape before enough of it built up to cause another explosion.

    Now, though, there's concern about third lake, Kivu, on the border of Rwanda and the Democratic Republic of Congo. It also has a lot of gas buildup, and the sediment at the bottom of the lake contains evidence of sudden lake-wide extinctions every few thousand years, probably because of limnic eruptions.

    Lake Kivu is extra dangerous, though, because it also contains enormous amounts of methane, produced by bacteria that incest the carbon dioxide. If that lake explodes, there could be both fiery explosions and widespread suffocation, and there are two million people living on the shores of Lake Kivu.

    So in 2011, plans were put into place to start siphoning off some of that methane and then using it to generate electricity for Rwanda. The hope is that we can harness the terrifying power of limnic explosions before they can occur; maybe even put it to good use.

    Thanks for watching this SciShow Dose. Isn't the world fascinating?
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