Planetary Triggers · Chapter 1

The Champagne Column

Lake Nyos and the Hydrostatic Trap of Dissolved Gas

Cattle grazed quietly along the terraced volcanic hills above the village of Subum in northwestern Cameroon on the evening of August 21, 1986. Two kilometers upslope sat Lake Nyos, a 208-meter-deep crater lake formed inside an extinct volcanic maar. Shortly after nine o'clock that night, a deep rumbling sound echoed from the crater. Within minutes, a silent, invisible river of cold gas fifty meters thick poured over the natural rock spillway. It raced down the valleys at fifty kilometers per hour.

In Subum, survivor Joseph Nkwain woke gasping on the dirt floor of his house hours after the cloud passed. His daughter lay dead beside him. Nearly every oil lamp in the village had gone out simultaneously without smoke or fire. Recounting the ordeal to investigators days later, Nkwain explained that he lost his voice and blacked out almost instantly, unable even to open his mouth against the foul-smelling vapor. By sunrise, 1,746 people and more than 3,500 cattle across three valleys had died of acute carbon dioxide asphyxiation.

When geophysicist Haraldur Sigurdsson and geochemist William Evans arrived to sample the lake days later, they found the surface water stained rusty red by oxidized iron brought up from the bottom. A surge wave had also stripped vegetation twenty-five meters up the southern crater wall. Yet no volcanic ash or lava had erupted from the crater floor. Instead, the lake itself had functioned as a hydrostatic pressure vessel. Magmatic carbon dioxide seeping upward from a deep mantle conduit had dissolved into the cold bottom water over decades. According to Henry's Law, the maximum concentration of dissolved gas $C$ scales directly with hydrostatic pressure ($C = k_H P_{\text{hydrostatic}}$). At a depth of 208 meters, twenty-one bars of water pressure allowed every liter of bottom water to hold five liters of carbon dioxide in solution. Because Lake Nyos was meromictic—divided into a warm, lighter surface layer resting atop a dense, cold deep layer—the bottom water never circulated upward to vent its gas. Nearly one billion cubic meters of carbon dioxide sat trapped beneath the thermocline.

208m Deep Crater 21 Bars Dissolved CO2 Bubble Nucleation Column Density Drops 100m Gas Fountain Dense Valley Cloud Positive Feedback Siphon Loop Rising froth lowers bottom pressure and pulls deep water upward
FIG. 1.1 — Self-accelerating gas-lift siphon and dense CO2 gravity current at Lake Nyos (1986).

A minor rockslide along the western cliff—or a sudden overturning of surface water cooled by heavy rain—pushed a parcel of deep water a few meters above its saturation depth. Bubbles of carbon dioxide nucleated at once. As those bubbles expanded, the bulk density of the water-and-gas mixture dropped below the density of the surrounding lake water. Buoyancy drove the parcel rapidly upward. That ascent reduced hydrostatic pressure further and triggered runaway exsolution. Within seconds, a self-sustaining gas-lift column formed. It sucked saturated bottom water toward the surface like an uncorked champagne bottle and blasted a hundred-meter fountain into the night air.

Carbon dioxide has a molecular weight of 44 grams per mole, compared to 29 grams per mole for dry air. The cold gas cloud was therefore 1.5 times denser than the surrounding atmosphere. Instead of dispersing upward into the sky, the erupted gas collapsed back onto the crater rim and flowed downhill as a ground-hugging gravity current. It displaced breathable oxygen at ground level and suffocated every person and animal in its path before diluting twenty kilometers downstream. In January 2001, French engineer Michel Halbwachs installed the first polyethylene degassing pipe on a raft at Lake Nyos, priming a controlled 50-meter fountain that safely vented the bottom water into the open wind.