Programme de bourses "Echanges Universitaires"
Natural disasters in Cameroon Volcanoes
In the night of August 21, 1986 1746 people and a large number of animals died of suffocation from CO2 rich gases emitted from a volcanic maar, lake Nyos in NW-Cameroon (Figures 1 to 3). The gas burst occurred asymmetrically, uprooting the trees at the southern end of the lake approx. 30 m above water level. Animal cadavers were found to an elevation of 150 m above the lake.
Volcanic eruptions bring gas up from the interior of the earth crust and from the upper mantle. Primitive, alkaline magmas of basanitic, ankaramitic and basaltic composition are often accompanied by large amounts of volatiles, particularly of CO2, H2 and CH4. The origin of CO2 in the magma reservoirs and in the hydrothermal systems is rather complex. We favour the hypothesis of mantle origin by the reaction: CH4 + 2 H2O = 2 CO2 + 4 H2O. The production of CO2 runs parallel with the generation of primitive alkaline melts by partial melting from a lherzolitic mantle source.
In Lake Nyos, the sediments together with a fault system at the southern end of the lake as well as the water itself seem to have acted as a gas trap. A minor earthquake, internal CO2 overpressure or a water overturn of the CO2-supersaturated bottom water finally released the gas burst during the rainy season.
The co-operative project between the University of Yaounde and the ETH-Zurich comprises within the framework of several Ph.D. theses (these troisieme siecle and these d'etat) investigations on the magmatic and volcanic development of the Cameroon line. This major volcanic chain starts in the South Atlantic (e.g.island of St. Helena) and continues into central Africa over a distance of 1500 km. One major objective of the project is the understanding of the formation of maars in the vicinity of Lake Nyos (NW Cameroon). Five maar lakes (Enep, Elum, Wum, Nye and Nyos) and several cinder cones were formed during historical times.
Petrological and geochemical studies on the alkaline basalts and basanites shall clarify the eruption mechanism in space and time, as well as the origin of the lethal CO2-gases. The co-operation includes supervision of Cameroon students in the volcanic fields (e.g. geological mapping and interpretation of volcanologic and structural phenomena). During their dissertation, the students have the opportunity to accomplish all analytical needs in the laboratories of the Department of Earth Sciences at the Swiss Federal Institute of Technology (ETH) in Zurich. All final works as well as the thesis defence take place in Cameroun, the Swiss scientists acting as examinators and referees of the jury.
Contacts:
Prof. Dr. Volker. Dietrich, Dr. Eric Reusser Institute for Mineralogy and Petrography Department of Earth Sciences
Swiss Federal Institute of Technology (ETH-Zurich). ETH Zentrum
CH-8092 Zurich
Switzerland
volker.dietrich@erdw.ethz.ch and eric.reusser@erdw.ethz.ch
In collaboration with:
Prof. F. Tchoua, Dr. Richard Nana, Dr. Amidou Moundi Department of Earth Sciences
University of Yaounde
Yaounde
Cameroon
rnana@uycdc.uninet.cm and amoundi@uycdc.uninet.cm
Fig. 1 Locality map of the Wum-Nyos region (NW-Cameroon)
Fig. 2 Lake Nyos on August 30, 1986 is facing south.after the catastrophic gas burst. The clear waters have turned into a muddy suspension with reddish brown colours. The lake covers an area of approx. 1.4 km2 and lies at an altitude of 1100 m. The northern spillway shows no signs of the flood wave (see undisturbed tree at the edge of the waterfall), although the lethal gas cloud must have moved over this area. Photo Heldstab.