Stratigraphy and radiocarbon dating of phreatomagmatic deposits on the northern shoreline of Lake Kivu (DRCongo)

The active volcanoes Nyamulagira and Nyiragongo are part of the Virunga Volcanic Province (VVP), within the western branch of the East-African Rift. Nyiragongo lies to the immediate North of the cities of Goma (East-DRCongo) and Gisenyi (Rwanda) and its central crater holds a permanently active lava lake. Nyamulagira lies to the North-West of the former, and has been erupting lava flows every 1-4 years over the past century.
While many satellite spatter-and-scoria cones are scattered within these volcanoes’ lava fields, several satellite tuff cones and a maar volcano lie markedly within 1-2 km from the northern shoreline of Lake Kivu (Fig.1).
Denaeyer (1975) reported the presence of these satellite volcanoes, and more recently Capaccioni et al. (2003) analysed the overall geochemistry of the deposits. The dynamics and timing of these past eruptions hence remains poorly understood, while these eruptive centers are located in a densely urbanized area. We present the stratigraphy of the best exposed satellite edifices, along with radiocarbon dates of palaeosols retrieved below the lava flows that cover the cone-and-maar deposits.
Mt Goma is a complex tuff cone constructed during at least three eruptive stages with undefined time lags in between, around at least three craters, dissected to the South by Lake Kivu (see Fig.1A).
The oldest eruptive phase has deposited decimetric cross-laminated layers of low-vesicular and angular lapilli in an abundant matrix of fine ocher ash, and beds of centimetric ash-armored lapilli, with sporadic decimetric lithic older lava blocks. This points towards a significant magma:water interaction from the southernmost crater, with an instable ash column as the result.
The second phase displays a thick package of centimetric laminae of finer and coarser, ocher to black ash, with impact sags below up to m-size ballistic lithic blocks. Cross-lamination, wavy bed-forms and erosive channels testify the occurrence of base surge pulses from an instable ash column, as the result of pulsating magma:water interaction in the central crater.
The third phase consists of plane-parallel centimetric laminae of brown to black scoriaceous coarse ash to fine lapilli, with both lithic blocks and scoria bombs. While cross-laminations and wavy bed forms are lacking here, sporadic base surges eroded decimetric channels.
The Mt Goma eruptive sequence is interpreted to result from a decrease in water:magma ratio, and thus probably in explosive dynamics of the eruptive activity, with recurrent collapse of the eruptive column and formation of (mildly) wet base surges.
The pyroclastic deposits of Mt Goma are covered by more recent lava flows. Two samples of the uppermost mafic ash below the lava yield ages between 0.6 and 0.9 ka BP, thus a minimum age for the last phase that constructed Mt Goma.

Lac Vert is a maar volcano consisting of 2-3 craters - the deeper of which is filled with green water providing the maars name – and an adjacent tuff ring (Fig.1B). The ejecta tuff ring consists of 2 phases. The lowermost visible sequence starts with a chaotic polygenic lithic breccia, comprising several types of older lava flows, aggregates of older ash-and-lapilli tuff blocks and basement granites. Lithic blocks range in size from centimetric to metric, demonstrating a high explosivity through magma:water interaction during this phase. The breccia grades upward into a bedded lithic breccia with an overall fining-upwards trend of the matrix, and an upward-increasing content of low-vesicular juvenile lapilli. This bedded sequence contains wavy bed-forms with metric wavelengths, cross-laminations, and decimetric ballistic lithic blocks with impact sags.
An abrupt contact separates the bedded breccia, from a cover of brown to black, centimetric laminae of alternating coarse to fine scoriaceous ash without lithics. This contrast suggests a switch to a violent Strombolian phase at Lac Vert, possibly from a different crater than the first phase.
A terrestrial gastropod shell in the clearly weathered top of the bedded breccia, at the outer base of Lac Vert, was dated at 1.3 -1.4 ka BP, while its surrounding palaeosol was dated between 2.1 and 2.5 ka BP in two separate locations. Two samples from the upper scoriaceous ashes below a more recent lava flow covering the Lac Vert ash-and-lapilli tuffs date at 0.35 - 0.45 ka BP. We thus suggest a time gap of at least ~1000 years must exist between both eruptive phases within the maar – ejecta rim complex of Lac Vert, with the youngest and least explosive eruptive phase occurring more recently than ~1.3 ky.

At least seven additional tuff cones with each at least one incompletely exposed phreatomagmatic eruptive phase are present in the Bulengo area, to the immediate West of Goma (Fig.1C). Assuming Lake Kivu must have reached its approximate current water level ~10 ka BP, and at least 10 phreatomagmatic eruptive centers are present close to the lake shoreline, the average recurrent rate of a phreatomagmatic eruption originating from Nyiragongo volcano could be roughly estimated at ~1000 years in the area.
Additionally, Ross et al. (2014) recently identified numerous relatively older tuff cones on the northernmost lake floor of Lake Kivu, below the present water level and ‘off-shore’ of the above presented edifices.
In conclusion, bedding structures in satellite edifice deposits along the northern shoreline of Lake Kivu suggest the occurrence of phreatomagmatic eruptive activity and the occurrence of base surge events, with plausible severe impacts on the close surroundings of such eruptive centres. Due to the dense urbanization surrounding the observed tuff cones and maars, notably Mont Goma and Lac Vert, the potential intermediary explosive nature of future eruptive outbreaks close to Lake Kivu is a key element of volcanic hazard in the Saké-Goma-Gisenyi urban area. The eruptive history and dynamics of these phreatomagmatic cones should be completed and analyzed further, in order to provide a more robust assessment of the ‘explosive’ volcanic hazard in the area.
Acknowledgements

This work has been carried out in the framework of the project ‘Geo-Risk in Central-Africa’ (GeoRisCA; http://georisca.africamuseum.be), and is supported by the Belgian Science Policy (BELSPO). The GVO personel is thanked for logistic support during field work. Palaeosol and gastropod shell samples have been dated by radiocarbon at Beta Analytic lab, Florida, USA.

References

Capaccioni, B., Vaselli, O., Santo, A.P. and Yalire, M.M., 2003. Monogenic and polygenic volcanoes in the area between the Nyiragongo summit crater and the Lake Kivu shoreline. Acta Volcanologica, 14 :1-2, 129-136.
Denaeyer, M.E., 1975. Le glacis des volcans actifs au Nord du Lac Kivu. Mémoires du Museum national d’Histoire Naturelle, Série C, Sciences de la Terre, 33, 79 pp.
Ross, K.E., Smets, B., De Batist, M., Hilbe, M., Schmid, M. and Anselmetti, F.S., 2014. Lake-level rise in the late Pleistocene and active subaquatic volcanism since the Holocene in Lake Kivu; East African Rift. Geomorphology, in Press.
DOI:10.1016/j.geomorph.2014.05.010