Assessing the spatio-temporal variation of eruption hazard in a poorly known volcanic region : the Virunga Volcanic Province, Central-Africa

The Virunga Volcanic Province (VVP) lies within the western branch of the East-African Rift Valley between The Democratic Republic of Congo, Rwanda and Uganda. This highly active volcanic zone contains eight main edifices and hundreds of adventive cones. While recent studies mainly focus on the recently active volcanoes Nyamulagira and Nyiragongo, little knowledge is available on the six dormant edifices on the eastern rift shoulder. The VVP is a densely populated area, but recurrent military conflicts restrict detailed field works in time and place. In order to improve the eruption hazard assessment, we present here volcanological investigations into the spatio-temporal variation of eruptive activity in the entire VVP.
First, we studied the spatial distribution of eruptive activity by extending the volcano-structural map by Smets et al. (2010) for the entire VVP. This was done using colonial-time reports and maps, recent satellite images (SPOT, Pléiades) and a newly-created 5 m resolution TanDEM-X digital elevation model (Albino et al., submitted), and limited field observations. A volcanic susceptibility map was produced using the distributions of eruptive vents and fissures. We observed that volcanic features are generally concentrated along major topo-lineaments, potentially controlled by palaeo- and neo-tectonic structures. Secondly, contrasts in erosion grade and relative age were estimated for the main volcanoes and their adventive cones through a set of morphometric parameters calculated from the DEM. At last, detailed field surveys revealed the explosive nature of phreatomagmatic tuff cones along the Lake Kivu shoreline. Radiocarbon dating of selected palaeosols constrained recent maximum eruptive ages ranging from 300 to 2000 years before the present. This type of eruption could seriously affect the cities of Goma and Gisenyi (~1 million inhabitants).
Combining the morphometric data, the available eruptive ages and the volcanic susceptibility map , we propose a morphological classification of the main Virunga volcanoes : 1. Erosive stage : deeply incised erosion valleys, morpho-structural evidence of gravitational collapse; 2. Dormant stage : poorly eroded volcanic edifices with ‘smooth’ slopes and distinct eruptive deposits, but no historic eruptions observed; 3. Active stage : historic volcanic activity with cone-building eruptions and lava flow effusion, and pre-historic phreatomagmatic activity within the past 10.000 years. We note that a 1957 lava flow, at ‘Mugogo’ ~10 km north of Visoke volcano, is reportedly the only historic activity outside the rift valley.
In conclusion, the present work highlights the use of GIS mapping and geomorphometry, coupled with a thorough literature review and limited field observations, to offer an overview of the eruption hazards in a poorly studied and remote Volcanic Province.