Past Lake Nyos’ explosion has sent shock waves rippling through Cameroon highlands and parts of Nigeria causing floods and destruction. Spacewatchafrica reports.
STILL YOUNG by geologic standards, Lake Nyos emerged from a volcanic birth only within the past few centuries. One of more than 30 crater lakes in Cameroon, Nyos was formed when molten rock rising from deep within the earth heated ground water into an explosion of stream. Situated in between the valley, far down the grassy slope, Lake Nyos glistened in the dim light, stretching across much of this beautiful African nation. In Africa, the stunning lake had always drawn the special attention of every visitor from the village of Nyos that lay on the valley floor.
In August 21 1986, as night fell quietly, a cloud of gas about 50 meters (150 feet) high engulfed lower Nyos village. The silent cloud moved 16 kilometres (ten miles) down the valley, spreading death. Lower Nyos was wiped out; 1,200 people perished there and in nearby encampments. In the neighbouring villages, including Cha, Subum, and Fang, more than 500 died. Human bodies lay in heaps in doorways, on bed, on kitchen floors beside unfinished metal. It appeared as if a neutron bomb had struck, destroying the living but leaving buildings unharmed. Not a creature moved in lower Nyos that morning, not even small ants,” according to an eye witness. Carcasses of more than 3,000 cattle littered the area, bloating in the hot sun, untouched by flies or vultures, the scavengers were dead too.
As relief workers assisted the refugees, scientists from around the world rushed in to study Lake Nyos in search of explanations. Many were short on time and equipment; some even lacked camping gear. All found it difficult to work in the rainy season. The release of one billion cubic meters of gas dropped the lake level by more than a meter. Lake Nyos was stained with a rusting iron compound- ferric hydroxide- carried up from the bottom by the escaping gas and precipitated into the surface water.
A water surge accompanied the gas burst, gushing as high as 80 meters to rip vegetation from the shores and deposit it in the lake. For weeks after the explosion Lake Nyos lay like a festering and angry red eye in its crater socket. Field research focused on two central questions: where did the all that gas comes from, and why was the gas released so violently? Samples of gas charged lake water were analysed by researchers.
Bubbles emerging from the samples contained 99 per cent carbon dioxide (Co2). This dense, colourless gas weighs one and half times as much as air. It is the gas that produces the fixle in soft drinks and beer. Human and animals exhale carbon dioxide – a product of the metabolising of food. Green plants takes carbon dioxide from the air and in sunlight combines it with water to male food. Concentrated, the gas is also used in fire extinguishers – heavier than air, it pushed the air aside and robs the fire of the oxygen needed to burn. This is similar to what happened at Nyos. The burst of lake gas formed a ground hugging cloud that, as it moved, forced aside breathable air. The victims were robbed of oxygen –asphyxiated.
Isotopic analysis of gas charged water from the Lake Nyos suggested that the carbon dioxide had leaked slowly from deep inside the earth to the lake floor. This came as no surprise to investigators. Cameroon’s Crater Lake district is a volcanically active region. Cameroon Mountain, which rises 4,100 meters over the streaming coastal forests, last erupted in 1982. Faults and crater shatter the face of the land. Many may still leak fumes from subterranean cracks.
Experts disagreed on why the gas was released so violently. The majority of scientists, however, concluded that the gas had dissolved slowly into the lake water itself, as it had a Monoun. Nyos became a time bomb of dissolved carbon dioxide. The slightest disturbance could nudge it into massive degassing.
What event finally triggered the release? At the moment of expulsion, the carbon dioxide shot to the surface and was expelled as gas. Among possible causes of the upwelling: a landslide, an earth tremor, turbulence from a strong wind or rain, or a disruption of the stratification caused by a change in water temperature. Wisp of fog drifting off Lake Nyos follow one of the paths taken by the gas en route to populated valleys. A violent wave that accompanied the gas burst stripped vegetation from the lake basin and littered the lake with debris. An iron compound brought up with bottom waters oxidise and stained the lake reddish brown.
Once the gas charged water began to rise, the resultant decrease in pressure allowed the gas to escape from solution and form bubbles. As the bubbles rushed upward, they forced the surrounding water up with them. The pressure accelerated in a vicious cycle. The result: a violent, foaming eruption like that from an uncorked champagne bottle.
In many lakes, thermal stratification may break down naturally and allow the layers to mix – a process known as overturn. This occurred in the fall and the spring in temperate zone lakes when cool surface waters sink and stir up bottom waters. Tropical lakes turn over from strong winds or heavy rains. Often the deep water in stratified lakes loses its oxygen supply through bacterial action.
In such cases, overturns can triggers massive fish kills, driving fish to gulp air the surface in their struggle to survive. The timing of the Nyos and Monoun events, both at the height of the rainy season, suggested to some scientists that a partial overturn may have driven the bottom waters upward, releasing some of the trapped gas.
The natural dam on the lake’s north shore is weak and may soon collapse. If it does, the collapse would release a catastrophic flood into the valley. The resultant drop in lake level could also triggers another gas burst. Some experts feel that the dam should be reduced in height before evacuated residents return. The remaining carbon dioxide could be removed by piping deep water to the surface and letting the gas escape slowly and harmlessly.
The Lake Nyos explosion sent psychological shock waves rippling through the two dominant ethnic groups in the Cameroon highlands. The Fulani are Muslim cattle herders who began to move into the region from the north early in this century. The more numerous Bantu have made the highlands a prosperous and densely populated center of agriculture, trade, and ironworking for thousands of years. The myths of exploding lakes come from the Bantu and reflect a deep spiritual link with these waters. For the bantu the Lake Nyos disasters had more than physical repercussions.
Likewise, Lake Kivu, perhaps the world’s gassiest large lake, lies in the Rift Valley of East Africa, between Rwanda and Zaire. A hundred kilometres long and 485 meters deep, it holds water charged with more than 63 billion cubic meters of methane and five timers as much carbon dioxide. “Kivu’s methane is generated by bacteria in the sediments,” explained West German physicist Klaus Tietze as he recuperated in Bamenda after three strenuous weeks at Nyos. “The Co2 seeps slowly into the lake from volcanic hot springs. Kivu seems to be more stable than Nyos. But there are plans for commercial exploitation of its methane, and these must be carefully designed to avoid setting off a catastrophic gas burst.”
As a safeguard against future gas bursts, investigators recommended monitoring carbon dioxide levels and installing underwater pipes for the slow release of gas lowering the lake level would prevent a flash flood should the lake’s fragile spill-way collapse.
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