The exploding lakes of Cameroon

Dear Readers: I'm going to be taking a couple of days off just to catch my breath.  Keep those topics coming!  The next Skeptophilia post will be on Wednesday, October 11.  Cheers!

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Prompted by yesterday's post, about the bizarre, beautiful, and deadly hot springs of Dallol, Ethiopia, a reader commented, "Yes, but have you heard about the exploding lake of Cameroon?"

Being a bit of a geology buff, I had indeed, but I don't believe I've ever written about them here.  So, without further ado, meet Lake Nyos:

Its peaceful appearance belies its behavior.  It's a crater lake, a water-filled depression in the middle of the remnant of a (in this case, very active) volcano.  Lake Nyos is part of the Oku Volcanic Field, a relatively poorly-studied geological feature in the Cameroon Line.  The latter is (as you'd guess) a nearly straight line of faults and volcanoes running the entire length of Cameroon from southwest to northeast.  This, in turn, appears to be part of the Central African Shear Zone, a region of unexplained thin crust and high torque that is prone to catastrophic ruptures.

So at first glance, it seems like nothing more than a seismically-active fault zone, which (after all) are nothing unusual.  But the Oku Volcanic Field, and the part of it under Lake Nyos, has a feature that is nothing short of wild.

The magma underneath Oku holds tons (literally) of dissolved carbon dioxide.  Why this is so is not understood, but it's probable that the magma has been in contact with layers of carbonate rocks, such as limestone.  When magma contacts carbonates, the intense heat breaks them down -- the metal ions (usually calcium and magnesium) bond to the aluminum and silicon oxides present in the magma, forming pyroxenes, while the carbon is released as carbon dioxide.  

As long as this carbon dioxide is under pressure, it remains dissolved (whether in the magma or in water the magma is in contact with), and all is well.  But the problem with Lake Nyos is twofold.  First, the bottom of the lake is very deep, and has seams and cracks extending far down toward the magma chamber.  Second, its water is highly stratified -- the top is warm and buoyant, the lowest layers cold and dense.

And it's into that cold, dense layer at the bottom that the carbon dioxide has been seeping for centuries.

The result is something like a bottle of champagne.  Keep the cork on, and nothing happens.  Shake it, then pop the cork...

The shake came with a relatively small underwater landslide on August 21, 1986.  This jostled the water in the lake -- as you'd expect -- and in an ordinary lake, this probably wouldn't have caused anything other than some bottom mud being stirred to the surface.  But remember that the lower strata of the water column in Lake Nyos were supercharged with carbon dioxide, with the pressure of all the upper layers keeping it in solution.  As soon as it started to rise, the dissolved carbon dioxide came bubbling out.

Bubbles expand as they rise.  Disturbing the water more, and bringing more water up.  Releasing more carbon dioxide.  The result?

The lake exploded.

An enormous blast of carbon dioxide blew out of the lake, and as carbon dioxide is heavier than air, it filled the valley and then poured over the margin, rushing downhill at an estimated one hundred kilometers an hour.  Within minutes, over seventeen hundred people living in villages downhill from the valley rim had been smothered to death.

Since then, efforts have been made toward degassing the lake -- running vertical pipes all the way down to the bottom, attached to pumps the bring the supersaturated water to the surface and allow the carbon dioxide to fountain off gradually and harmlessly.  How effective this will be in preventing another deadly explosion is unknown -- and there are at least two other lakes, Lake Monoun in Cameroon and Lake Kivu, between the Democratic Republic of the Congo and Rwanda, that have been shown to have supersaturated bottom strata as well.

Lake Kivu, by the way, is two thousand times bigger than Lake Nyos.

So that's a little alarming.  Especially given how many people live near these bodies of water.  It's the usual problem; volcanic soils are good for agriculture, and the gas eruptions don't happen that often... and people have short memories.

Yet another reason I'm glad I live where I do.  The climate may be a little dismal -- it's been described as nine months of expectations followed by three months of disappointments -- but I'll take that over exploding lakes any day.

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