Lake Nyos ( NEE-ohs) is a crater lake in the Northwest Region of Cameroon, located about 315 km (196 mi) northwest of Yaoundé, the capital. Nyos is a deep lake high on the flank of an inactive volcano in the Oku volcanic plain along the Cameroon line of volcanic activity. A volcanic dam impounds the lake waters. A pocket of magma lies beneath the lake and leaks carbon dioxide (CO2) into the water, changing it into carbonic acid. Nyos is one of only three lakes known to be saturated with carbon dioxide in this way, and therefore prone to limnic eruptions (the others being Lake Monoun, also in Cameroon, and Lake Kivu in the Democratic Republic of Congo and Rwanda). In 1986, possibly as the result of a landslide, Lake Nyos suddenly emitted a large cloud of CO2, which suffocated 1,746 people and 3,500 livestock in nearby towns and villages, the most notable one being Chah, which was abandoned after the incident. The limnic eruption not only devastated human and livestock populations but also had a profound impact on the diverse aquatic life, including tilapia, crabs, snails, and frogs, leading to a significant loss of biodiversity in and around the lake. Though not completely unprecedented, it was the first known large-scale asphyxiation caused by a natural event. To prevent a recurrence, a degassing tube that siphons water from the bottom layers to the top, allowing the carbon dioxide to leak in safe quantities, was installed in 2001. Two additional tubes were installed in 2011. Today, the lake also poses a threat because its natural wall is weakening. A geological tremor could cause this natural levee to give way, allowing water to rush into downstream villages all the way into Nigeria and allowing large amounts of carbon dioxide to escape.
Geography
Lake Nyos lies within the Oku Volcanic Field, located near the northern boundary of the Cameroon Volcanic Line, a zone of volcanoes and other tectonic activity that extends southwest to the Mt. Cameroon stratovolcano. The field consists of volcanic maars and basaltic scoria cones.
Formation and geologic history Lake Nyos is located south of the dirt road from Wum, about 30 km (19 mi) to the west, to Nkambé in the east. Villages along the road in the vicinity of the lake include Cha, Nyos, Munji, Djingbe, and Subum. The lake is 50 km (31 mi) from the Nigerian border to the north, and lies on the northern slopes of the Massif du Mbam, drained by streams running north, then northwest, to the Katsina-Ala River in Nigeria which is part of the Benue River basin. Lake Nyos fills a roughly circular maar in the Oku Volcanic Field, an explosion crater caused when a lava flow interacted violently with groundwater. The maar is believed to have formed in an eruption a maximum of 12,000 years ago, and is 1,800 m (5,900 ft) across and 208 m (682 ft) deep. The area has been volcanically active for millions of years—after South America and Africa were split apart by plate tectonics about 110 million years ago, West Africa also experienced rifting, although to a lesser degree. The rift is known as the Mbéré Rift Valley, and crustal extension has allowed magma to reach the surface along a line extending through Cameroon. Mount Cameroon also lies on this fault line. Lake Nyos is surrounded by old lava flows and pyroclastic deposits. Although Nyos is situated within an extinct volcano, magma still exists beneath it. Approximately 80 kilometres (50 mi) directly below the lake resides a pool of magma, which releases carbon dioxide and other gases; the gases then travel upward through the earth. The fumes then dissolve in the natural springs encircling the lake, ultimately rising to the surface of the water and leaching into the lake. But with advances in technology now there is machinery placed at the bottom of the lake to remove the gases, so as to make the inhabitants of the area free from danger. The lake waters are held in place by a natural dam composed of volcanic rock. At its narrowest point, the wall measures 40 metres (130 ft) high and 45 metres (148 ft) wide.
Gas saturation Lake Nyos is one of only three lakes in the world known to be saturated with carbon dioxide, the others being Lake Monoun (also in Cameroon) and Lake Kivu, which is split between the Democratic Republic of the Congo and Rwanda. A magma chamber beneath the region is the root cause of the saturation, with carbon dioxide seeping up into the lake. The lake is also thermally stratified; the lower the depth, the more CO2-dense and colder the water becomes. Over long periods of time, carbon dioxide gas is absorbed in great amounts. Usually, the lake is stable and the CO2 remains in the lower layers. Over time however, the water becomes supersaturated, and if an event such as an earthquake or landslide occurs, large amounts of CO2 will escape.
1986 disaster
Although a sudden outgassing of CO2 had occurred at Lake Monoun in 1984, a similar threat from Lake Nyos was not anticipated. However, on August 21, 1986, a limnic eruption occurred at Lake Nyos, triggering the sudden release of about 100,000–300,000 tons (some sources state as much as 1.6 million tonnes) of CO2. This gas cloud rose at nearly 100 kilometres per hour (62 mph) and spilled over the northern lip of the lake into a valley running roughly east–west from Cha to Subum. It then rushed down two valleys branching off to the north, displacing all of the air and suffocating 1,746 people within 25 kilometres (16 mi) of the lake, mostly rural villagers, as well as 3,500 livestock. The villages most affected were Cha, Nyos, and Subum.
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