Interplay between rifting-induced extension and surface loading effects illustrated by lateral magma propagation at Nyiragongo volcano

The lateral magma propagation is a common feature of rift zones, with vertical dykes flowing parallel to the rift direction and opening against the minimum compressive stress. 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 favor lateral migration radially away from the edifice central area, thus feeding peripheral vents. Here, we use the case of Nyiragongo volcano, a volcanic edifice located in the western branch of the East African Rift and culminating at 3,470 meter above sea level to study such combined effect of rifting-induced extension and topographic loading on both the orientation of vertical dykes and on the balance between lateral versus vertical magma propagation within the propagation plane. Using analytical and numerical models taking into account the effect of topography and the local West-East extension stress field, we show that the path of a dike coming from the volcanic edifice is first influenced by the load of the volcano, leading to a radial propagation while beyond 5 km, the 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 geophysical observations for the last two eruptive events of the Nyiragongo volcano (2002 and 2021). Furthermore, the downward slope toward Lake Kivu and, to a lesser extent, the slight increase in southward rift extension both favour lateral magma propagation, but reduced magma buoyancy at shallow depths is required to explain the lateral propagation over more than 20 km, where the magma remains trapped beneath the lake.