H. Sigurdsson and W. Evans visited Lake Nyos 20-25 January. Their preliminary report is excerpted below.

"We made temperature profiles of the lake, conducted a bathymetric survey, collected samples of water and gas from various depths, and made excursions to Lake Njupi and nearby springs.

"By mid-September, the reddish color on the surface of Lake Nyos had disappeared, leaving the lake clear as before the August event. On 15 December, some Cameroonian soldiers reported that the surface had again turned red. This observation was confirmed by local residents. On 30 December R. Chevrier and Eyike Emannuel made camp at the outflow of the lake (11:12). At about 2000, Chevrier heard an unusual noise, followed by rumbling. Emmanuel heard waves on the lake a few minutes later. They then heard two more noises, from the direction of Lake Njupi (1 km to the NE), accompanied by flashes of whitish light. Neither geologist smelled any sulfurous odor, but Chevrier reported a smell of ozone. The following day they noted a reddish spot on the surface of Lake Nyos, and a day later, from a high vantage point, could see that Lake Njupi was also red. However, a helicopter pilot reported that this color change had occurred by 14 December and several residents stated that this lake turns red every dry season.

"During our stay, Lake Nyos was clear and quiet. On one calm day we noted that a thin film of reddish ferric hydroxide had formed on the lake surface in sheltered areas. Rockfalls occurred from the near-vertical cliffs surrounding the lake. We witnessed one of the rockfalls occurring from a canyon or fault on the SW side of the lake. Another rockfall that occurred at night was sufficiently loud to awaken us in our camp on a ridge at the NW end of the lake. Cliffs around the lake showed several scars from earlier rockfalls or landslides. These are potentially important as triggering mechanisms of the August 1986 overturn and gas burst. It is possible that the noise and rumbling heard by Chevrier and the waves noted by Emmanuel on 30 December were produced by a rockfall. This rockfall may have induced a minor upwelling of anoxic deep water, with the resulting oxidation causing a reddish stain of ferric hydroxides in surface water.

"Temperature readings were taken from the surface to the lake bottom at 1.5-m intervals with a thermocouple. Bottom water temperature was also determined with high-precision reversible mercury thermometers. All indicated a deep water temperature of 23.7-24°C. The water temperature of Lake Njupi was determined as 24.5°C.

"The bathymetry . . . was determined by five traverses over the deepest part of the lake with a recording fathometer, revealing a deep basin, with very steep or near-vertical sides and a horizontal, flat, uniform bottom at a depth of ~206 m. The survey showed no change since August 1986 (11:08). Thus, there has been no apparent disruption . . . from volcanic explosions or large-scale gas venting.

"Preliminary results of water samples showed a surface pH of 6.3 and bottom water pH of 5.4. Alkalinity at the surface was 161 mg/l, increasing to 840 mg/l at the lake bottom. Water sampled from a spring-fed creek 0.5 km N of the lake was gas-rich and reddish-colored from ferric hydroxide precipitates. Preliminary chemistry results show a high gas content, low pH, and high alkalinity, indicating that the derivation of the springs is seepage from Lake Nyos. This creek was clear when visited in September 1986. The report by Marechal (1976) of soda springs with this chemistry around Lake Nyos indicates that the current composition of Lake Nyos is not a recent feature, but that the lake has fed soda springs at least a decade before the August event. Gas samples were collected in prototype pressure cylinders for analysis in the U.S.

"Our studies of Lake Nyos 20-25 January show no evidence of recent volcanic activity in the lake. The consistent and low temperature of bottom waters shows that no significant thermal input has occurred, other than that associated with a normal geothermal heat flux. Similarly, the bathymetric surveys show that the lake floor remains flat and undisturbed. There is no evidence of recent cratering or a subaqueous volcanic cone, as would be expected in the case of an eruption or deep-seated explosion.

"Evidence from this and previous studies suggests that these events do not represent volcanic eruptions, but are due to overturn and large-scale exsolution of CO₂ from deep waters. Examination of the steep, >100-m cliffs bounding the lake to the W suggests that rockfalls may also have played an important role in the August 1986 episode."

Reference. Marechal, A., 1976, Geologie et geochimie des sources thermominerales du Cameroun: ORSTOM, Paris, 176 p.

Geological Summary. Numerous maars and basaltic cinder cones lie on or near the deeply dissected rhyolitic and trachytic Mount Oku massif along the Cameroon volcanic line. The Mount Oku stratovolcano is cut by a large caldera. The Oku volcanic field is noted for two crater lakes, Lake Nyos to the N and Lake Monoun to the S, that have produced catastrophic carbon-dioxide gas release events. The 15 August 1984 gas release at Lake Monoun 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 ~1 km3 of magmatic carbon dioxide has been attributed either 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.

Information Contacts: H. Sigurdsson, Univ of Rhode Island; W. Evans, USGS.