Mount Nyiragongo is an active stratovolcano with an elevation of in the Virunga Mountains associated with the Albertine Rift. It is located inside Virunga National Park, in the Democratic Republic of the Congo, about north of the town of Goma and Lake Kivu and just west of the border with Rwanda. The main crater is about two kilometres wide and usually contains a lava lake. The crater presently has two distinct cooled lava benches within the crater walls – one at about and a lower one at about. Nyiragongo's lava lake has at times been the most voluminous known lava lake in recent history. The depth of the lava lake varies considerably. A maximum elevation of the lava lake was recorded at about prior to the January 1977 eruption – a lake depth of about. A recent very low elevation of the lava lake was recorded at about. Nyiragongo and nearby Nyamuragira are together responsible for 40% of Africa's historical volcanic eruptions.
Geology
The volcano partly overlaps with two older volcanoes, Baratu and Shaheru, and is also surrounded by hundreds of small volcanic cinder cones from flank eruptions. The lava emitted in eruptions at Nyiragongo is often unusually fluid. Nyiragongo's lavas are made of melilitenephelinite, an alkali-rich type of volcanic rock whose unusual chemical composition may be a factor in the unusual fluidity of the lavas there. Whereas most lava flows move rather slowly and rarely pose a danger to human life, Nyiragongo's lava flows may race downhill at up to. This is because of the extremely low silica content. Hawaiian volcanic eruptions are also characterized by lavas with low silica content, but the Hawaiian volcanoes are broad, shallow-sloped shield volcanoes in contrast to the steep-sided cone of Nyiragongo, and the silica content is high enough to slow most Hawaiian flows to walking pace.
Active history
Not much is known about how long the volcano has been erupting, but since 1882, it has erupted at least 34 times, including many periods where activity was continuous for years at a time, often in the form of a churning lava lake in the crater. The existence of the lava lake had been suspected for some time, but was not scientifically confirmed until 1948. At that time, it was measured at nearly. Subsequent expeditions showed that the lake fluctuated in size, depth, and temperature over time. The lava lake activity continued through 2010., the lake is mostly confined within a broad, steep-sided cinder cone on the crater floor.
1977 eruption
Between 1894 and 1977 the crater contained an active lava lake. On 10 January 1977, the crater walls fractured, and the lava lake drained in less than an hour. The lava flowed down the flanks of the volcano at speeds of up to 60 km/h on the upper slopes, overwhelming villages and killing at least 70 people. Within 30 minutes, the lava lake had emptied, flowing north, south, and west of the volcano. The hazards posed by eruptions like this are unique to Nyiragongo. Nowhere else in the world does such a steep-sided stratovolcano contain a lake of such fluid lava. Nyiragongo's proximity to heavily populated areas increases its potential for causing a natural disaster. The 1977 eruption raised awareness of the unique dangers posed by Nyiragongo, and because of this it was designated a Decade Volcano, worthy of particular study, in 1991. The 1977 eruption was preceded by the creation of a new small volcanic vent, Murara, a short distance away on the slopes of Nyamuragira.
2002 eruptions
Lava lakes reformed in the crater in eruptions in 1982–1983 and 1994. Another major eruption of the volcano began on January 17, 2002, after several months of increased seismic and fumarolic activity. A 13 km fissure opened in the south flank of the volcano, spreading in a few hours from 2800 m to 1550 m elevation and reaching the outskirts of the city of Goma, the provincial capital on the northern shore of Lake Kivu. Lava streamed from three spatter cones at the end of the fissure and flowed in a stream 200 to 1000 m wide and up to 2 m deep through Goma. Warnings had been given and 400,000 people were evacuated from the city across the Rwandan border into neighbouring Gisenyi during the eruption. Lava covered the northern end of the runway at Goma International Airport, leaving the southern two-thirds usable, and reached Lake Kivu. This raised fears that the lava might cause gas-saturated waters deep in the lake to suddenly rise to the surface, releasing lethally large amounts of carbon dioxide and methane – similar to the disaster at Lake Nyos in Cameroon in 1986. This did not happen, but volcanologists continue to monitor the area closely. About 245 people died in the eruption from asphyxiation by carbon dioxide and buildings collapsing due to the lava and earthquakes. At least 15% of Goma comprising 4,500 buildings were destroyed, leaving about 120,000 people homeless. Immediately after the eruption stopped, a large number of earthquakes were felt around Goma and Gisenyi. This swarm activity continued for about three months and caused the collapse of more buildings. Six months after the start of the 2002 eruption, Nyiragongo volcano erupted again. Activity at Nyiragongo is ongoing, but currently confined to the crater, where another lava lake has formed about 250 metres below the level of the 1994 lava lake.
Ongoing threat
Localized carbon dioxide toxicity, known locally as 'mazuku', has killed children even more recently. In locations where the gas seeps from the ground in relatively high levels, without the dispersing effects of wind, its effects can be deadly. On March 8, 2016, Goma Volcano Observatory discovered a new vent that opened in the northeast edge of the crater, following local reports of rumblings coming from the volcano. Some fear that this could lead to a flank eruption.
Monitoring
The volcano is monitored by a team of scientists at the Goma Volcanic Observatory. It is monitored 24/7 365 days a year with seismic data produced every four minutes and temperature data produced every ten minutes.
