La Réunion Island is the subaerial part of a 7 km high oceanic shield volcano with a diameter of 220–240 km. Considering the historical magma production rate and the oldest dated subaerial basalts (about 2 Ma [MacDougall, 1971]), an age of around 5-8 Ma was estimated for the beginning of the edifice growth.
The initial evolution was characterized by the development of two adjacent volcanoes (Piton des Neiges and Alizés volcanoes), which encountered recurrent flank destabilizations [Lénat et al., 2001; Bachèlery et al., 2003; Oehler et al., 2004].
Around 530 ka ago, Piton de la Fournaise appeared west of the Alizés volcano which
stopped its activity. Between 530 ka and 12 ka (date of the last eruption of Piton des Neiges [Deniel et al., 1992]) Piton des Neiges and Piton de la Fournaise showed contemporaneous activity. Finally, since 12 ka, eruptions are restricted to Piton de la Fournaise.
Piton des Neiges
Piton des Neiges results from a more than 2 m.y. long activity during which construction and dismantling phases alternated. The oldest parts correspond to La Montagne, the Dimitile, and Takamaka massifs. The other volcano flanks consists on piles of differenciated lava flows which date from the volcano’s late activity, some 350 ka ago. Large explosive eruptions occurred during this period [Fretzdorff et al., 2000]. The volcano’s central part exhibits three major depressions: the cirques of Cilaos, Mafate, and Salazie, which originate from an intense erosion controlled by structural limits such as large landslides [Oehler et al., 2004]. In the depressions, most of the outcropping formations are intensively weathered rocks and debris avalanche deposits, intruded by a large number of dykes [Chevallier, 1979; Maillot, 1999].
Piton de la Fournaise
Piton de la Fournaise is the active volcano of La Réunion Island. Geochronological and geological data allow defining two main phases of construction, separated by a main collapse event: the ‘‘ancient Fournaise’’ (530–150 ka) and the ‘‘recent Fournaise’’ (150 ka to present day) [Gillot and Nativel, 1989; Bachèlery and Mairine, 1990; Gillot et al., 1990]. This large landslide led to an eastward shift of the volcano center from the present-day location of the Plaine des Sables to the current position of the active volcano. During the last 0.15 Ma, the ‘‘recent Fournaise’’ was affected by at least two caldera collapses whose exact origin is still under debate [Bachèlery, 1981; Duffield et al., 1982; Gillot et al., 1994; Labazuy, 1996; Merle and Lénat, 2003; Michon and Saint-Ange, 2008]. The resulting structures are the Plaine des Sables and the large U-shaped structure composed of the Enclos depression and the Grand Brûlé. Since the formation of the Enclos depression 4.5 ky ago, the volcanic activity is mainly restricted to the caldera. Only few eruptions occurred along the NE and SE rift zones, in the Plaine des Sables and in the Rivière des Remparts [Bachèlery, 1981].
The current activity of Piton de la Fournaise is now restricted in the April 2007 caldera which cut the summit of the active cone during the largest historical lateral eruption [Michon et al. 2007].The caldera formed in 4 days by reccurent collapse events that were precisely recorded by the monitoring network of the Piton de la Fournaise Volcano Observatory. Pulsating collapse seems to be the classical way of formation of basaltic calderas like Fernandina (1968), Miyakejima(2000) and Piton de la Fournaise (2007) [Michon et al. 2009].