Subsurface lateral magma propagation from Nyiragongo volcano in the Western Rift Zone of the East African Rift

Lateral magma propagation is a common feature of rift zones, with opening against the minimum compressive stress and vertical dykes flowing parallel to the rift direction. Depending on the competition between vertical and lateral magma migration, these dykes may either feed an eruption or not. In this context, the topography which includes the edifice load acts against the rise of the magma and favours lateral migration radially away from the edifice central area, thus feeding peripheral vents. Here, we focus on the Nyiragongo volcano, a volcanic edifice located in the western branch of the East African Rift and culminating at 3,470 meters above sea level. In practice, we study the combined effect of the extension induced by the rifting of the EAR and the topographic loading of the Nyiragongo volcano on the orientation of the dyke propagation plane and on the balance between the lateral and vertical propagation of the magma in this propagation plane. Using analytical and numerical models taking into account the effect of topography and the rift-induced west-east extensional stress field, we show that the path of a dyke originating from the volcanic edifice is first influenced by the load of the volcano, leading to a radial propagation. Beyond 5 km, however, the rift-induced extensional stress field dominates leading to a north–south propagation towards Lake Kivu. These results are consistent with the path of the magma deduced from the position of the vents and geophysical observations for the last two eruptive events of the Nyiragongo volcano (2002 and 2021). On the other hand, the lateral propagation over more than 20 km, where the magma remains trapped beneath the lake, is shown to be controlled by depth-dependent extension and reduced magma buoyancy and, to a lesser extent, the downslope towards Lake Kivu and the slight increase in rift extension towards the south.