EGU2020-13517
https://doi.org/10.5194/egusphere-egu2020-13517
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

How do cirques form in ocean island volcanoes: the case of Piton des Neiges (Réunion Island, Indian Ocean)

Camille Paquez1,2, Vincent Famin1, Nicolas Villeneuve1, Laurent Michon1, and Bhavani Bénard1
Camille Paquez et al.
  • 1Laboratoire GéoSciences Réunion, Université de La Réunion, Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Université Paris Diderot, UMR 7154 CNRS, F-97744 Saint Denis, La Réunion (camille.paquez@univ-reunion.fr)
  • 2Austral Energy, Saint Pierre, Réunion (paquez@akuoenergy.com)

“Cirques” are funnel-shaped, seaward-narrowing valleys commonly observed on many volcanic islands worldwide, such as Tahiti (French Polynesia), La Palma or Gran Canaria (Canary), Anjouan (Comores), and Maui or Molokai (Hawai’i). Because they contradict the basics of regressive erosion by rivers, these geomorphic structures have been interpreted in many ways, including the erosion of volcano-tectonic depressions (crater, caldera, rift zone), the formation of leaf grabens caused by volcano spreading, or the subsidence of dense plutonic bodies within edifices. Piton des Neiges volcano (Réunion Island) is dissected by three cirques (Salazie, Mafate and Cilaos) and thus provides an excellent case to study the processes that lead to the formation of these funnel-shaped valleys. To do so, we performed a detailed field and photogrammetric mapping of the volcanic and volcaniclastic products outcropping in the cirques using an updated chrono-stratigraphy.

Our mapping reveals that the three cirques of Piton des Neiges are not delimited by faults, which excludes vertical movements as the primary cause of their formation. Rather, the cirques are built on former horseshoe-shaped depressions filled with volcaniclastic breccias (mostly related to debris avalanches and debris flows), and later covered by lava flow units. Importantly, the breccia are several hundred meters thick in the innermost parts of the cirques, but thin out until complete disappearance toward the outer flanks of the volcano.

In consequence, we interpret the basal volcaniclastic breccias as playing a major role in the formation of the cirques, by offering a weaker resistance than the lava flow units. This contrasted resistance leads to greater erosion rates on the inside of the volcano than on the outer flanks and, hence explaining the reverted funnel shape of the cirques. In our model, cirques are therefore erosional structures mostly guided by past dismantling episodes rather than by tectonic or volcano-tectonic structures.

How to cite: Paquez, C., Famin, V., Villeneuve, N., Michon, L., and Bénard, B.: How do cirques form in ocean island volcanoes: the case of Piton des Neiges (Réunion Island, Indian Ocean), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13517, https://doi.org/10.5194/egusphere-egu2020-13517, 2020

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  • CC1: Questions and answers from the live chat during EGU2020, Michael Heap, 11 May 2020

    Q: So a weak, perhaps brecciated, layer is required to form cirques? Did you see evidence for hydrothermal alteration in these layers?

    A: Yeah, hydrothermal alteration in inner part and in old breccia layer

    Q: Great detailed mapping campaign! What do you mean by flank destabilization? Long-term gravitationally, or do you mean flank collapse producing that breccia? Do you think spreading helped create the cirques?

    A: A huge thick of breccia deposit in Mafate (Orangers breccia deposit) seems to play a major role to enhance erosion and shape Mafate cirque, because in downstream where breccia are thin out erosion is controlled by classic regressive erosion. This breccia deposit came from a major landslide on western flank of Piton des Neiges. Moreover, caldera influence is in the inner part of the edifice. So we propose that Mafate is mainly erosional structures and the reverted funnel shape is controlled by erosion of scars and breccias from this large landslide.

    Q: Do you think the hydrothermal alteration weakened the breccia layer?

    A: I think so but recent breccia look like a good candidate to enhance cirque

    Q: A side question, do you have idea of how the Syenite layer could have emplaced within the frame of your interpretation?

    A: The question ! I think syenite use fault caldera. This position wasn't clear before but with this detailed map it's seems to be in along caldera (recent caldera)

    Q: Impressive landforms! Tough fieldwork I can imagine! Do you believe this is the case at many other oceanic islands, i.e. that cirque-landforms may be misinterpreted?

    A: This landforms are very difficult to understand because on volcanic island there are lot of major structure (tectonic and not). I think yes, cirque landforms may be misinterpreted but in some island (like La palma) some models ara accepted