Sunday, 13 November 2011

Eruptions on Mount Nyamuragira in the Virunga National Park, November 2011.

On the evening of 6 November 2011 a fissure opened on the northern flank of Mount Nyamuragira, from which a lava fountain emerged and lava spread out over a wide area. Nyamuragira is in the Virunga National Park in the east of the Democratic Republic of Congo, close to the Rwandan border, and the glow from the volcano was clearly visible that night from the city of Goma, across the border in Rwanda.

Footage of the November 2011 eruption, taken from the Virunga National Park offices.

Eruptions of this sort are not unusual on Mount Nyamuragira, which has a reputation for being Africa's most active volcano. Nyamuragira is a shield volcano, with a broad shallow dome structure made up layers of lava, unlike the more famous stratovolcanoes which have a cone shape, made up of layers of ash and lava. Eruptions from Nyamuragira are very frequent, the last being in January 2010, when a fissure opened on the southeast flank of the volcano. Eruptions from fissures on the flanks of Nyamuragira are at least as common as those from the (small) caldera at the summit. Small cones are often formed by these fissure eruptions, but they are seldom long lived.

The lava produced by Mount Nyamuragira is a low silicone basalt. This tends to have a very low viscosity, i.e. it flows very freely, rapidly covering a wide area. Because of this few people choose to live near the mountain, so it is seldom a threat to human life, though it has in the past caused problems for vulnerable wildlife populations, such as the human-familiarized chimpanzees and gorillas of Virungu National Park.

The location of Nyamuragira, and the extent of it's lava fields.

Basalts are more usually associated with volcanoes on oceanic rifts that with continental volcanicity. The presence of basaltic volcanoes in the African interior is taken as evidence that the continent is slowly splitting apart along it's great rift valleys (not on any timescale likely to threaten human life).

Nyamuragira lies on the Eastern Branch of the East African Rift; most volcanoes in this area produce more typical siliclastic lavas, but there are several other basaltic volcanoes in the area, such as the nearby Mount Nyiragongo (Africa's second most active volcano - between them the two volcanoes account for 40% of all volcanic activity on the continent), the less active Visoke, which last erupted in 1957, and the inactive Karasimbi, Mikeno and Muhavura volcanoes.

The Western Branch of the Rift Valley also has basaltic volcanoes, most notably Ol Doinyo Lengi, to the south of Lake Natron in Tazania, the worlds only active carbonitic volcano (i.e a volcano that produces lavas containing carbonate compounds), and the nearby extinct, but dramatic, Mount Shombole.

The plume underlying the East African Rift, which appears to be slowly pushing the continent apart.

There are also basaltic volcanoes along the 'Cameroon Line' in West-Central Africa, including Mount Cameroon and the volcanoes of the Oku Volcanic Field. The Cameroon Line extends under the Atlantic Ocean, forming the volcanoes of Sao Tome, Principe, Annobon and Fernando Poo islands.

Map showing the distribution of volcanoes in Africa and the surrounding area.

Eruptions from Mount Nyamuragira also tend to be unusually rich in Sulphur Dioxide (SO₂), with the volcano producing a large proportion of all the volcanic SO₂ that enters our atmosphere. To give this a sense of proportion, about 75% of all the SO₂ entering our atmosphere originates from the burning of fossil fuels. The effects of SO₂ in the atmosphere are a little complicated. Firstly it is poisonous, though this is only a danger close to the source where it is concentrated. Since most people avoid going close to active volcanoes, this usually only presents a hazard to professional volcanologists. Secondly it mixes with water to form sulphuric acid, i.e. acid rain, which can be a major problem downwind of volcanic eruptions and large industrial centers, with the rain causing severe damage to vegetation and aquatic life. Thirdly SO₂ droplets act as coolant. Since both volcanic and industrial SO₂ is usually produced alongside larger quantities of Carbon Dioxide (CO₂), which is a greenhouse gas, this is usually a negligible effect, but cooler weather can be produced downwind of major eruptions, as the CO₂ disperses more rapidly than the SO₂.

Satellite image of the plume emerging from Mount Nyamuragira, taken on 7 November 2011 by the NASA Earth Observatory.

See also Eruptions on Mount Ebro, Ongoing volcanic activity on El Hierro, Snowfall in Namibia and Volcanoes on Sciency Thoughts YouTube.

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