Webpage by:
Theresa Kayzar “T.K.”

Geologic Background of Lake Kivu

geology overview

Lake Kivu is a volcanic lake that is located on the border of The Democratic Republic of the Congo and Rwanda in Africa. The formation of the lake occurred in the late Pleistocene when eruptions from the Virunga volcanic field blocked a tributary to the Nile deepening and widening the lake (Degens et al, 1973; Lockwood, 1994). Lake Kivu is located along the East-African rift, a segment of the continent that is being slowly pulled apart and thinned by extension (Fig. 1).

volcanoes DRC02

Figure 2. Volcanoes in the Virunga Volcanic Field that pose a threat to the overall stability of Lake Kivu; Nyiragongo and Nyamuragira.

Figure 1. Simplified geologic setting of Lake Kivu. Top: view of Lake Kivu with respect to the cities of Goma and Gisenyi, and the countries of Rwanda and the Democratic Republic of the Congo. Bottom: Zones of rifting and trusting near Lake Kivu in East Africa. Modified from IRIS (2003).

General Statistics on Lake Kivu

Area of Lake Kivu

2,650 km2

Depth

485 meters

Fresh Water

top 200 meters

Gases in Lake Kivu

CO2

200 km3

CH4

power entire US for one month

(Kling, 1989; USAID, 2002; Witze, 2002)

The “hazard lurking beneath the lake”...

In addition to the external geology of Lake Kivu, the internal stratification is rather remarkable. The general structure of Lake Kivu shows sharp boundaries in vertical temperature and salinity that are explained by separate convecting layers within the lake. The top 200m of the lake consist of freshwater environments; however, the deeper waters of the lake contain large amounts of CO2 and CH4; the origin of which is highly debated. It is generally thought that the CO2 is of volcanic origin and that bacteria within the lake itself produce the renewing reserves of methane (Degens et al, 1973; Lake Kivu, 2002).  In 1973, the levels of CO2 and CH4 within Lake Kivu approached but did not exceed saturation, and there was no spontaneous exsolution of these gases (Degens et al, 1973). According to George Kling, a limnologist at the University of Michigan, approximately 200km3 of CO2 lies at depth, which he refers to as a “hazard lurking beneath the lake” (Witze, 2002). For comparison, the catastrophic eruption of Mt. St. Helens in 1980 produced only 1/3 of a cubic kilometer of ash.

Mt. St. Helens 1980 Eruption, ~1/3 km3

Lake Kivu stored CO2
 ~200km3

If the gas that is stored at depth within Lake Kivu were to be suddenly released, the gases could asphyxiate local residents of the lake. While scientists have never first-handedly seen a release of this kind, George Kling and Robert Hecky of the University of Michigan provide evidence in sediment cores from the lake showing a catastrophic die-off of living creatures in the lake. This evidence is consistent with a gas release and possible overturning of the lake (Witze, 2002; Lake Kivu, 2002).

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