Landslide Processes Involved in Volcano Dismantling From Past to Present: The Remarkable Open‐Air Laboratory of the Cirque de Salazie (Reunion Island)

Abstract

Volcanic edifices often exhibit an eventful geological history characterized by constructive and destructive phases (Carracedo et al., 1999;Gayer et al., 2021;Thouret, 1999;Zernack & Procter, 2021). Volcanic terrains are characterized by a combination of specific environmental factors that make them prone to landslides, such as high relief, steep slopes, weathered and fractured materials, alternating layers of lavas and volcaniclastic deposits with different mechanical characteristics, and preferential shear surfaces (Roverato et al., 2015;Scott et al., 2001;Tibaldi et al., 2008). They are also exposed to processes known to trigger failures, such as volcanic activity with hydrothermal circulation, fluid injection, earthquakes (Di Traglia et al., 2018;Roverato et al., 2021;Schaefer et al., 2019) and high precipitation rates, especially in tropical environments.Volcanic edifices are prone to the full spectrum of landslide sizes ranging from giant debris or rock avalanches (RA) of several km 3 to small rockfalls, including:1. Large-scale volcanic landslides (e.g., slumps and debris avalanches [DA]) with various magnitudes and kinematics may occur several times during the evolution of a volcano. These landslides can reach volumes of several km 3 , which can correspond to the destabilization of an entire volcanic flank (Blahůt et al., 2019;

How Can the Morphometric Characteristics and Failure Conditions of a Historic Gully Caused by Intense Rainfall Be Reconstructed?

“…No precise chronology of the event has been described, except that the whole gully formed in the last days of the cyclonic storm. The damage caused by the cyclone is reported in Humbert et al (1981) [6] and Pinchinot (1984) [7], in which a photo of the déboulé taken just after the cyclone is presented (Figure 1) and the following field observations and interpretations can be read. The photo (Figure 1) shows that the resulting channel was covered by blocs of up to several meters in size, and finer materials.…”

“…The gully formed on the eastern part of the Grand Ilet landslide (Figure 2). This large compound landslide moves northeast slowly at about 40 to 50 cm•yr −1 in the area of the Ravine de l'Eglise [7,46]. The mean slope of the Grand Ilet landslide is about 10 • .…”

“…http://www.meteofrance.re/climat/ pluies-annuelles (accessed on 12 December 2021)). This heavy rainfall caused thousands of slope instabilities throughout the island [6,7]. It also led to the formation of gullies on steep slopes, but also on gentle slopes [6][7][8].…”

“…This heavy rainfall caused thousands of slope instabilities throughout the island [6,7]. It also led to the formation of gullies on steep slopes, but also on gentle slopes [6][7][8]. The Ravine de l'Eglise, formed during Cyclone Hyacinthe, is the most impressive gully that has formed over the last 40 years.…”

“…The substratum of the Grand Ilet landslide, an indurated layer of volcaniclastic deposits that crops out in the Ravine Roche à Jacquot, where the river has steep sides [7,8,47].…”