Morphological evolution of volcanic crater through eruptions and instabilities: The case of Ol Doinyo Lengaï since the 2007-08 eruption

Ol Doinyo Lengaï (OL) in north Tanzania is the only active volcano in the world emitting natrocarbonatite lavas. This stratovolcano (2962 m a.s.l) is mostly characterized by effusive lava emissions since 1983. However, on the 4^th of September 2007, explosive events marked the beginning of a new eruptive style that lasted until April 2008. This new phase involved short-lived explosive eruptions that generated volcanic ash plumes as high as 15 km during its paroxysmal stage. This explosive activity resulted in the formation of a 300 m wide and 130 m deep crater in place of the growing lava platform that had filled the crater since 1983. Since then the effusive activity at OL resumed within the crater and has been partially filling it over the last 14 years. Due to the remote location of the volcano there is a lack of monitoring of its activity and, hence, its eruptive and morphological evolution over the last years is not well constrained (e.g., emission rates, number of vents, unstable areas). This absence of monitoring, preventing the detection of features, such as instabilities of the summit cone, could have hazard implications for the tourists regularly visiting the summit area.

In this study, we quantify the evolution of OL crater area over the last 14 years by reconstructing its topography at regular time interval. We collated several sources of optical images including Unoccupied Aircraft Systems (UAS) images, videos and ground-based pictures that have been collected over the period 2008-2021 by scientists and tourists. Those data have been sorted by year and quality in order to reconstruct the most accurate topographical models using Agisoft Metashape Pro, a software for Structure from Motion (SfM) photogrammetry, and CloudCompare a 3D point cloud processing software. This enables estimating the emitted volume of lava, the emission rate and the remaining crater volume available before crater overflow. It also allows identifying punctual events, such as hornito formation or destruction, and partial crater collapses. Our results indicate that the main lava emission area has repeatedly moved over the years within the crater floor and that OL’s effusion rate has been increasing over the last few years, with more than two times higher lava emission in the period 2019-2021 compared to 2017-2019. Assuming a similar lava effusion rate in the coming years, the crater could again be filled within the next decade leading to new lava overflows. There is thus a need for periodic assessment of the situation at OL. New cost- and time-effective photogrammetry techniques, including UAS and SfM processing, offer a solution to improve the monitoring of such remote volcanoes.