NATURAL HAZARDS.

The volcanic eruption of Nyiragongo in January 2002 is unique in the annals of vulcanology because it was caused by tectonic fracture between the active plates of the African rift.

The recent volcano-tectonic crisis of the Nyiragongo volcano (Democratic Republic of Congo) which destroyed part of the town of Goma on the 17 January 2002 highlighted a new kind of risk connected to the presence of dissolved gas in the depths of Lake Kivu.

In its normal state, this highly stratified lake is stable and not at all dangerous. But an important disturbance, caused by the volcanic activity of Nyiragongo on the north shore, could cause a rising up of gas-filled water from the deeps. This water would then release, either as localised and limited emanations or via a cataclysmic explosion involving a large part of the lake, a quantity of suffocating gas which would threaten the safety of local populations.

There are only three lakes in the world containing large concentrations of dissolved gas. Lake Nyos and Lake Monoun in Cameroon were the sites of gas explosions which caused 40 deaths in 1984 at Monoun and 1800 in 1986 at Nyos. The third lake is Kivu, which contains a thousand times more gas than Lake Nyos.

We have recently discovered the existence of ancient volcanoes on the lake floor, right in the middle of those gas-filled layers of water. A new sub-lacustrine eruption, or a magmatic intrusion at depth would inevitably trigger off, by a chain reaction, the liberation of an enormous quantity of gas, with probably catastrophic consequences for the entire region. A population of several millions (Goma, Bukavu, Gisenyi, even Bujumbura) are living under this threat.