Toward Another Lava Lake in the Virunga Volcanic Field?

In April and since June, a red glow has been visible on top of Nyamulagira, the most western volcano of the Virunga Volcanic Province, in the western branch of the East African Rift. Helicopter flights and field observations quickly confirmed the presence of lava fountains that might evolve into what could become a new lava lake inside a 400- by 600-meter-wide pit crater, in the northeast sector of the central caldera (see Figure 1).

After more than 75 years without a lava lake in the region, the reemergence of one on top of Nyamulagira would mark a major change in its eruption style, which is usually characterized by lava oozing from the volcano’s sides. The impact of such a change on the neighboring Nyiragongo volcano, which potentially threatens more than 1 million inhabitants who live in its shadow, is unknown.

Nyiragongo also holds an active lava lake, one of the largest on Earth. Thus, the coming episode of eruptive activity on Nyamulagira could be the first time that two active lava lakes, nested 13 kilometers apart in two distinct volcanoes, can be simultaneously studied in the same volcanic chain.

Volcanic Setting

Nyamulagira (or Nyamuragira), in the Kivu region, Democratic Republic of Congo, is one of the most active volcanoes in Africa, with one eruption every 1 to 4 years. The partial collapse of its central caldera in 1938, accompanied by a 2.5-year-long eruption, marked the disappearance of its active lava lake. Since then, eruptions at Nyamulagira have mainly occurred along the gentle slopes of that shield volcano.

The last eruption took place on 6 November 2011 about 12 kilometers east-northeast of the summit. The voluminous eruption lasted 5 months and was accompanied by the progressive collapse of the 400-meter-wide pit crater located in the northeast sector of the central caldera.

In April 2012, the pit crater started emitting large amounts of volcanic gases. The massive degassing is ongoing, as observed in the field and by the NASA Global Sulfur Dioxide Monitoring Program. It has been attributed to the presence of magma at a very shallow depth, beneath the caldera floor.

Volcano Monitoring

Because of recurrent armed conflicts, the subsequent lack of infrastructures, and a deliquescent economy in the Kivu region, monitoring Nyamulagira and Nyiragongo volcanoes is a difficult task. Scientists from the Goma Volcano Observatory (GVO)—the Congolese scientific institute in charge of volcano monitoring in the Virunga—often have to operate with basic and limited means, sometimes at the risk of their lives if, for example, they are trapped in the field after nightfall because of mechanical failures of decrepit vehicles.

Since 2006, the Royal Museum for Central Africa in Belgium, in collaboration with the National Museum of Natural History and the European Center for Geodynamics and Seismology, both in Luxembourg, has led various scientific and capacity-building projects aimed at supporting GVO’s monitoring efforts. This collaboration helped to provide GVO with ground-based monitoring equipment and remotely sensed data. Collaborators also trained staff in geophysical analysis techniques.

Tireless and sustained international efforts allowed scientists to install several permanent stations, which include GPS, broadband seismometers, infrasound arrays, gas (carbon dioxide, radon) sensors, and weather stations. However, despite these initiatives, volcano monitoring in the Virunga remains heavily jeopardized by robberies, frequent damage from lightning strikes during storms, and the lack of local resources for regular basic maintenance, such as a permanent power supply and capabilities for data collection.

Will a Lava Lake Emerge?

Despite the difficulties in monitoring these volcanoes, GVO scientists noticed unusual swarms of long-period earthquakes in early April 2014. Reports of a red glow visible to civilians at night suggested the presence of molten lava in the pit crater. However, a dense gas plume hovering over the crater prevented any visual confirmation, even during helicopter overflights performed at night.

Beginning on 22 June 2014, the intense activity resumed, and this time, helicopter flights on 1 July and field observations performed in the caldera on 5 July confirmed the presence of lava fountains in the pit crater (images from the flights are available here). Field measurements performed by GVO and three-dimensional photogrammetric models indicate that the pit crater has extended roughly 200 meters southward since the 2011−2012 eruption, widening to an area of 600 by 400 meters that is nearly 500 meters deep.

If episodes of lava fountaining continue, this pit might indeed form a lava lake.

Implications of the New Activity

The ongoing activity differs from what has been observed since the 1940s and is more similar to what probably happened after another voluminous eruption that took place in 1912, when local testimonies reported episodic red glowing visible on top of Nyamulagira between 1913 and the 1920s. This activity in the central caldera progressively gave birth to the semipermanent lava lake that disappeared with the caldera collapse in 1938. While the lava lake was present at Nyamulagira, eruptions on the volcano’s flank did not occur, as the open conduit allowing the free circulation of gas and magma toward the surface hinders pressure increases that would cause lateral breaches in the shallow magmatic system.

The repetition of such a scenario today would mark a major change in the Nyamulagira eruption style. The potential consequences on the neighboring and less remote Nyiragongo volcano remain unknown, as some (but not all) major flank eruptions of Nyamulagira may have triggered activity at Nyiragongo in the past. However, because of the challenging security situation in that region, knowledge about their magma plumbing systems and (sub)crustal structures remains too sparse to allow interaction modeling.

What’s Next?

GVO and its Belgian and Luxembourgian partners will keep an eye on the Virunga volcanoes. Currently, the red glow at Nyamulagira is no longer visible, and sulfur dioxide emissions have dropped back to very low levels, similar to conditions during this eruptive episode when lava fountains were temporarily not observed. Time will likely be needed to confirm the highly plausible reemergence of the Nyamulagira lava lake.

—Benoît Smets, European Center for Geodynamics and Seismology, Walferdange, Luxembourg; email: benoit@ecgs.lu; Nicolas d’Oreye, National Museum of Natural History, Walferdange, Luxembourg; and François Kervyn, Royal Museum for Central Africa, Tervuren, Belgium