Spatio-temporal dynamics of eruptions in a youthful extensional setting: Insights from Nyamulagira Volcano (DR Congo), in the western branch of the East African Rift

Detailed historical records of rift volcanism are scarce. Such information is, however, of a great importance to study the role of magmatism and volcanism as forces contributing to continental rifting. Nyamulagira (or Nyamuragira) volcano, located in the western branch of the East African Rift System, is one of the most active volcanoes on Earth and, hence, represents a good case study to investigate the spatial and temporal evolution of eruptive activity in a youthful extensional setting. Since 1882, Nyamulagira produced at least 42 eruptions, offering a dense historical record for that time period. Using scientific literature, including colonial scientific documents archived at the Royal Museum for Central Africa in Belgium, field data and a detailed mapping of the volcanic field, the spatial and temporal evolution of the eruptive activity of Nyamulagira is studied in order to 1) review the current state of knowledge on eruption history and dynamics at that volcano, and 2) infer the influence of the rift on factors controlling the location, occurrence and characteristics of Nyamulagira eruptions. The results show that the duration and location of eruptions allow the distinction of four eruption groups: summit/upper-flank, classical flank, long-lived flank and remote eruptions. The interpretation of these groups suggests that Nyamulagira eruptions are related to the main crustal magmatic system and are strongly influenced by the gravitational stress field induced by the main central edifice. However, remote eruptions (1904, 1912, 1948, 1991-1993) are not influenced by this edifice loading and seem directly related to deeper magma reservoirs. The 1938-1940 eruption is also atypical, but is related to an important drainage of the upper magma plumbing system, which triggered a caldera collapse. No tectonic trigger of eruption is evidenced, but cone and eruptive fissure alignments suggest that rift structures can also influence the eruption location. In addition, the present study suggests that the major eruptions of Nyamulagira along structural axes close to Nyiragongo can induce a change in the eruptive activity of the latter hazardous volcano. Hence, the historical eruptive activity of Nyamulagira highlights the complex interplay between the magmatic system and the local and regional stress fields in controlling the location and characteristics of eruptions, in this youthful extensional tectonic setting where main volcanic edifices developed. (C) 2015 Elsevier B.V. All rights reserved.