Primary Landslide Type Associated with Rapid Landscape Evolution of Volcanic Islands as Exemplified by the Mahavel Case of Réunion Island (Indian Ocean)
Abstract
Since Mount St Helens' lateral collapse in 1980 and the observation of its debris/rock avalanche deposit, mass failures are considered as the main process in the destruction of volcanic reliefs, especially in hotspot volcanic islands. In Réunion island, the debris avalanche deposits observed in the topographic depressions that deeply incise the volcanic edifice, have been interpreted as the result of recurrent flank collapses in the volcano evolution (Oehler et al., 2008). However, erosion in Réunion is one of the world's most active (Gayer et al., 2019) and may actually have played an important role in the dismantling history of Réunion (Salvany et al., 2012) through processes such as river incision and landsliding (Gayer et al. 2019). Here we analyze the deposit of the 1965 Mahavel landslide that occurred in one of the steepest canyon of Réunion, where 30 to 50 Mm3 of thick lava flows and scoria collapsed from a head scarp at 2340 m asl and flew down over 5 km in the valley whose floor lies between 1000 and 600 m asl. We seek (i) to characterize the different types of deposits and (ii) to identify the related landslide dynamics in order to determine the category of landslide that would have primarily controlled the landscape evolution at the island scale. The joint analysis of historical photographs, geomorphology, granulometry measurements reveals the succession of two main flow dynamics. The landslide was first extremely dynamics with a lateral run-up of 150-200 m and an estimated velocity of 68 m/s. It presents sedimentological characteristics (facies, grain size distribution and fractal dimension values) typical for rock avalanches. This avalanche was stopped by high scarp building a 200-m-high pile of deposit, which immediately fed a secondary km-long flow whose surface presents faults and bulges that require a slow velocity to develop, like in the slow dry debris flows. Additional photogrammetry and images analyses on historical aerial images (1961-1966) will allow us to estimate the actual volume of the landslide and to model its flow dynamics. These preliminary results show that Mm3 size landslides can produce deposits similar to the ones cropping out in the large topographic depressions of Réunion and could then be primarily responsible for the rapid landscape evolution of Réunion.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMEP43D2395M
- Keywords:
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- 1810 Debris flow and landslides;
- HYDROLOGY;
- 1815 Erosion;
- HYDROLOGY;
- 1825 Geomorphology: fluvial;
- HYDROLOGY;
- 1826 Geomorphology: hillslope;
- HYDROLOGY