Congo volcano: The facts

Nasa satellite image five hours after the eruption began

Mount Nyiragongo in the Democratic Republic of Congo is one of Africa's most active volcanoes.

The volcano, one of eight along the borders of Rwanda, the Democratic Republic of Congo and Uganda, has a long history of activity.

Nyiragongo was last active in 1994, when a lava lake reappeared in its summit crater.

The important thing now is to ensure that the local populations is moved to safer ground

Eugene O'Connor, British Geological Survey

The latest eruption is more serious. Lava from Nyiragongo can travel at 60 kilometres per hour (40 miles per hour) and there are concerns that lava reaching a nearby lake could do further damage.

Bill Evans of the US Geological Survey said lava could react with gas in the lake, with catastrophic consequences.

"A hot lava flow flowing down into the deep waters of the lake could trigger this gas to come out very quickly," he told the BBC.

"The gas is composed of carbon dioxide and methane; so it would come up and create a suffocating and yet flammable cloud that could engulf communities living around the lake."

As yet there are no reliable ways of predicting volcanic eruptions

There is no record of such an event at Lake Kivu in the past. However, in 1986, Lake Nyos in Cameroon released a cloud of gas, which killed more than 1,700 people.

Nyiragongo, and nearby Nyamuragira, are the two active volcanoes among the eight volcanoes in Rwanda, the Democratic Republic of Congo and Uganda.

Both are in the Virunga mountain range, which straddles the border with Rwanda. The pair are responsible for nearly two-fifths of Africa's historical eruptions.

Nyamuragira erupted early last year. It caused no casualties but cultivated land was burnt out by ash, trees were destroyed and some water supplies were polluted.

Fracture faults

Scientists understand the geology of the area relatively well. Physical stretching of the continent gives rise to large fractures that allow volcanic magmas to rise up through the Earth's crust.

Eugene O'Connor, regional geologist for Africa at the British Geological Survey, said: "The volcano is located in part of the East African Rift Valley system, which is a chain of major fracture faults running along the eastern half of Africa."

Dr O'Connor said he was not aware of any local monitoring of volcanic activity, although several academic institutions in the United States carried out remote monitoring using seismographic records.

"The important thing now is to ensure that the local population is moved to safer ground," he told BBC News Online.

"That is the first consideration. The second one is to set up some sort of closer monitoring observatory," he added.