Gas piston activity of the Nyiragongo lava lake: First insights from a Stereographic Time-Lapse Camera system

Nyiragongo volcano (D.R. Congo), in the western branch of the East African Rift System, is one of the most active volcanoes on Earth. Its eruptive activity is mainly characterized by the presence of a persistent lava lake in its main crater. As observed at other persistent lava lakes, the Nyiragongo lava lake level exhibits metric vertical variations in the form of minutes-to hour-long cycles, which we infer to be gas piston activity. To study this activity, we developed and tested a Stereographic Time-Lapse Camera (STLC) system, which takes stereo-pairs of photographs of the Nyiragongo crater at regular intervals. Each pair of gas- and steam-free images during daytime allows the production of a 3D point cloud. The comparison of the point clouds provides a measurement of topographic changes related to variations in lava lake level. The processing of a first dataset acquired between 18 and 20 September 2011, at an acquisition rate of 1 pair of images every 2 min, revealed cycles of vertical lava lake level variations reaching up to 3.8 m. Lava lake level variations >0.5 m are detected significantly. They are interpreted to result from gas accumulation and release in the lava lake itself. The limitations of the STLC approach are related to the number of cameras used and the atmospheric masking by steam and volcanic gas in the Nyiragongo crater. The proposed photogrammetric approach could be applied elsewhere or in other disciplines, where frequent topographic changes occur.