Earth's Deadly Breath: A Scientific Mystery

It was May, 1947, near Hekla Crater in southwest Iceland, an onlooker would have observed a curious scene. Workers were digging seemingly aimless ditches across the landscape, as if trying to drain invisible ponds. In fact, that is exactly what they were doing.

During the Hekla eruption of 1947-48, streams issuing from beneath the new lava flows began to precipitate lime. At the same time, bird and animal carcasses began turning up in low areas of the topography around the crater. The implications were clear: the volcano was exhaling carbon dioxide. In the water, the carbon dioxide was combining with calcium to produce calcium carbonate, or lime, and there were areas on the ground where the gas collected in concentrations strong enough to suffocate the animals. Being heavier than air, carbon dioxide will flow like water across the terrain and collect in low spots, forming "ponds" that are invisible and odorless, but lethal. (The largest pond associated with the Hekla event covered an area of about two acres.)

Carbon dioxide in small amounts is nontoxic, but concentrations of eight percent will make people feel dizzy, vomit, and faint; higher concentrations quickly induce heart failure and death. Evidently, the Hekla ponds formed in completely calm weather, and usually at night. Animals that happened to be in low areas when the carbon dioxide accumulated didn't stand much of a chance. It was found that mice placed into a pond would suffocate in less than a minute.

The recent disaster in Cameroon, where a silent cloud killed about 1,700 people, is not so easily explained. There was no discernible volcanic activity associated with the expulsion of gases from the crater lake Nios, and the cause is still a matter for speculation.

But the incident is not without precedent. Almost exactly two years earlier, on August 16, 1984, a similar gas cloud was expelled from Lake Monoun, 70 miles to the southeast, and 37 people were killed.

There are many unanswered questions surrounding these events. One is that the nature of the gases involved has not been clearly established. Witnesses to the Monoun disaster state that they could see a smokelike cloud coming from the direction of the lake (the Nios event occurred during darkness). Survivors of both tragedies claimed that they could smell a strong rotten-egg or gunpowder odor. Neither of these descriptions sound like carbon dioxide, which is colorless and odorless. Rather, they suggest hydrogen sulfide or sulphur dioxide, both of which are common poisonous volcanic gases. Additionally, many of the victims had chemical burns on their bodies. This must mean that the fumes had reacted chemically to form sulfuric or nitric acids, although at least one group (the U.S. Agency for International Development) believes that the agent responsible might have been the caustic and deadly hydrogen cyanide. Most likely, one or more of these accompanied carbon dioxide, which is a major constituent of all volcanic emissions.

The biggest question is: "What caused the gases to be expelled from the lake in the first place?" There is little doubt that they were originally formed at the bottom through volcanic or organic processes, and that the stratified layers of water overturned," bringing the gases to the surface. In pressure at depth, they would have been dissolved and confined, but at lower pressures near the surface, they would have popped into the air (much like the bubbling that occurs when you open a pop-top soda can). That this process can occur suddenly is made evident by a trim line in the foliage caused by a wave that rose fifteen feet above the level of Lake Monoun.

In both instances, witnesses reported hearing a boom before the gases swept down. Could this have been a volcanic burp, a small earthquake, or an underwater landslide? As yet nobody knows. Whatever triggered the lakes' overturn remains a mystery.

The overturn of lake waters is actually a fairly common occurrence in Alaska during early winter. Near freezing temperatures, the water near the surface becomes colder and denser than the water near the bottom of the lake, and at a certain point, the layers will switch places. This could not have been a factor in Cameroon, which lies near the equator so the surface water is always warm. Since the two lakes overlie volcanic vents, however, the water near the bottom is also warm. The water layers are in a fragile state of equilibrium, requiring only a slight nudge--from whatever cause--to turn over.