DESCRIPTION:
Volcanic Lakes and Gas Releases

Only three lakes in the world are known to contain high concentrations of dissolved gas in their bottom waters: Lakes Nyos and Monoun in Cameroon and Lake Kivu in East Africa. The release of large quantities of gas from lakes is very rare; however, massive carbon dioxide gas (CO2) releases from Lake Monoun in 1984 (Bulletin v. 9, no. 8) and Lake Nyos in 1986 (Bulletin v.11, no. 8) resulted in the loss of nearly 1,800 lives.

Numerous maars and basaltic cinder cones lie on or near the deeply dissected Mount Oku massif along the Cameroon volcanic line. Two of these crater lakes, Lake Nyos to the north and Lake Monoun to the south (~100 kilometers ESE of Lake Nyos, part of the Bambouto Volcanic Field), have produced catastrophic gas release events. The 15 August 1984 gas release at Lake Monoun that killed 37 people (Sigurdsson and others, 1987) was attributed to overturn of stratified lake water, triggered by an earthquake and landslide. The Lake Nyos event on 21 August 1986 caused at least 1,700 fatalities. The emission of around 1 cubic kilometer of magmatic CO2 has been attributed to overturn of stratified lake waters as a result of a non-volcanic process, or to phreatic explosions or injection of hot gas into the lake.

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In March 1985, Haraldur Sigurdsson, J. D. Devine, and F. Tchoua investigated the August 1984 event. The following briefly summarizes their findings; Sigurdsson and others have submitted a paper for publication and they will also present their data in December at the American Geophysical Union meeting in San Francisco.

On 15 August at about 2330, several people heard a loud noise or explosion from the Lake Monoun area and there were unconfirmed reports that an earthquake was felt that day at a town 6 kilometers N of the lake. The gas cloud was emitted from the E part of the lake, where a crater about 350 meters in diameter and at least 96 meters deep is located. Victims of the cloud were in a low-lying area and had apparently died between 0300 and dawn. No autopsies were performed and the exact causes of death are unknown; all bodies had suffered skin damage [corrected from first-degree burns]. Persons on the fringes of the cloud reported that it smelled bitter and acidic. From 0630 until it dissipated by 1030, the whitish, smoke-like cloud remained 0-3 meters above the ground. Vegetation was flattened within 100 meters of the lake's east end, indicating that a water wave as much as 5 meters above lake level was associated with the event.

Lake Monoun is near the center of a volcanic field that includes at least 34 recent craters, and there is evidence that eruptive activity has occurred there as recently as a few hundred years ago. However, the chemistry of the lake water and sediments, the uniformly low lake temperature (23-24=B0C), and the absence of new tephra in or around the lake suggested to the research team that the August 1984 event was not the result of an eruption or a sudden ejection of volcanic gas from the lake. Gradual emission of CO2 from volcanic vents is thought to have led to a buildup of HCO3 in the lake. An earthquake or internal seiche is thought to have upset the density stratification of the lake, triggering its overturn and catastrophic exsolution of CO2, which suffocated the victims. Explanations of the cloud's acid odor and the agent of victims' skin damage are uncertain.

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The August 12, 1986, sudden release of approximately one cubic kilometer of CO2 gas from Lake Nyos in Cameroon resulted in death of at least 1,700 people by asphyxiation (Kling and others, 1987). The source of CO2 at Lake Nyos was prolonged degassing of subsurface magma. This event drew attention to the potential for dissolved lethal gas to accumulate in the deepest parts of volcanic crater lakes. Depressurization of this water if the lake overturns suddenly results in rapid exsolution of the gas and its liberation to the atmosphere. The high density of cold CO2 relative to ambient air allows the gas cloud to flow down topographic depressions and accumulate locally.

Numerous maars and basaltic cinder cones lie on or near the deeply dissected Mount Oku massif along the Cameroon volcanic line. Two of these crater lakes, Lake Nyos to the north and Lake Monoun to the south (~100 kilometers ESE of Lake Nyos, part of the Bambouto Volcanic Field), have produced catastrophic gas release events. The 15 August 1984 gas release at Lake Monoun that killed 37 people (Sigurdsson and others, 1987) was attributed to overturn of stratified lake water, triggered by an earthquake and landslide. The Lake Nyos event on 21 August 1986 caused at least 1,700 fatalities. The emission of around 1 cubic kilometer of magmatic CO2 has been attributed to overturn of stratified lake waters as a result of a non-volcanic process, or to phreatic explosions or injection of hot gas into the lake.

Lake Nyos is ~1,400 meters long, 900 meters wide, and 208 meters deep. Lava fountain deposits and a basaltic flow are exposed along the NE rim of the lake, and pyroclastic deposits that include abundant peridotite xenoliths are on the N and E flanks. The crater is located on the N boundary of the NE-trending Cameroon Volcanic Line, a zone of crustal weakness containing Triassic to Recent, generally alkaline volcanoes.

During the evening of 21 August a toxic gas cloud emerged from a lake within a young volcanic crater in NW Cameroon, killing more than 1,700 people and uncounted numbers of animals. Scientists investigating the disaster suggested that the lake may [be underlain by] a recent diatreme [Kling and others, 1987].

Scientists interviewed many of the survivors (more than 300 were hospitalized), and took gas and water samples from the lake and streams. Medical and geological investigations suggest that victims were suffocated by CO2 gas; there was no evidence that HCN or CO were present. ...

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