Natural hazards associated with Lake Kivu and adjoining areas of the Birunga volcanic field, Rwanda and Zaire, Central Africa; final report

Abstract

The Birunga Volcanic zone (Western African Rift Valley), is a site of complex extentional rifting that has resulted in episodic basaltic volcanism. Large amounts of carbon dioxide (CC>2) associated with the volcanism discharge from springs and dry vents (mazukus). When the springs discharge into nearby Lake Kivu, the CC>2 accumulates in saline, bottom waters were it is biologically cycled. Hazards associated with earthquakes, volcanism, and CO2~buildup in Lake Kivu are the primary geologic hazards in the Rift Valley near the boundaries of Rwanda, Zaire, and Uganda. A volcanologist and geochemist were sent to this area for two weeks to evaluate the hazards associated with the Birunga volcanism and the dissolved gases in Lake Kivu. The results of the field and laboratory investigations are: Lake Kivu is the only lake sampled that contains a sufficient amount of dissolved gas (CC>2 and CH4) to pose a threat to human lives if released. Concentrations of these gases are in part controlled by biologic processes, and may change dramatically over short intervals of time. * Mazukus (terrestrial gas vents) are common around Lake Kivu and discharge CC>2 that can accumulate in low lying areas (e.g. basements, sunken gardens, depressions in lava flows, etc.). Concentrations are high enough in these depressions to kill animals and, presumably people. * Volcanic hazards in the Lake Kivu area include lava flows, littoral eruptive vents, and subaqueous eruptive vents. The lava flows from the Virunga volcanoes are extremely fluid and travel long distances very quickly, leaving little or no time for evacuation. Future eruptive activity near Lake Kivu would undoubtedly result in extremely destructive explosions because of the interaction of magma and groundwater. Eruptive activity into the bottom waters of Lake Kivu could initiate degassing of dissolved gases in the lake by destabilizing the water column. Recommendations for remediating these hazards include monitoring of the chemistry of dissolved gases and water in Lake Kivu especially if CH4 mining is initiated; monitoring volcanic activity (both short-term and longterm plans considered); educating the local population about geologic hazards; establishing a warning system and evacuation plan in the event of a disaster, and providing specialized training for local scientists and technicians.