On the Road to the volcano, the Ravine of the Rivière des Remparts is an interesting site, but not always easy to understand ... it is necessary to go back the history.
The road to the volcano - location of the Riviere des Remparts and the crater Commerson (stop 7) - a click to enlarge - Doc. cybergeo.revues.org
The River of the Remparts, seen from the Belvedere of the Nez de Boeuf - picture © Bernard Duyck / June 2017
Presently visible relief formation:
320,000 years ago, the eruptive center of the volcano is located in the present Plaine des Sables. The rainy waters are guided by the relief towards Plaine des Cafres; A collapse occurs at the level of the present valley 300,000 years ago; By creating a barrier to the flow to the west, it promotes erosion.
Geological history of the Riviere des Remparts - after an explanatory panel of the Nez de Boeuf - photo © Bernard Duyck / juin 2017
Between -280,000 and -110,000 years, erosion deeply digs the valley, the bottom of which is lined with a few lava flows.
Between 70,000 and 65,000 years, successive lava flows fill the valley.
65,000 years ago, the collapse of the Plain des Sables formed a barrier to the flow of lavas that no longer reach the valley, and erosion resumed.
Dissymmetry of the slopes of La Rivière des Remparts - in light gray, at the bottom of the Valley, the "dam" of Mahavel - photo © Bernard Duyck / June 2017
Seen from the Belvedere of the Nez de Boeuf at 2.136 meters, the site presents asymmetric slopes: continuous, rectilinear and rectified to the west, indented by multiple confluences on the east side. The bed is enclosed by 1,000 meters, but encumbered with terraces and alluvial tablecloths.
These ramparts intersect volcanic materials which are not very coherent and therefore unstable. Their collapse gives muddy flows, lahars. This phenomenon is illustrated by the lahar of May 6, 1965, in the Mahavel Arm; The lahar came to bump against the right bank of the Riviere des Remparts, five kilometers below leaving gray "badlands". The volume of the excavation is estimated at 30 million m³.
Curiosity: a valley where water rarely flows are visible. The river is present, but it has dug its bed in the lava of the permeable flows of the Commerson, the last flows to have covered the bottom of the valley. Little fed, it infiltrates into the cracks of the lava flow and is flowing through lava tunnels. During periods of heavy rain, water does not have time to infiltrate, and flows into its minor bed.
The last lava flows in the Rivière des Remparts, those of the crater Commerson.
This crater is part of a recent eruptive set of three craters lined up on a one-kilometer long eruptive fissure, oriented N145 ... appeared in 1.825 BP +/- 74 years (fixed date of reference: 1950 - dating of one Charred wood) during a violent phreato-magmatic eruption outside Enclos.
The Commerson Crater is an explosion crater described by Philibert Commerson, physician and naturalist of the King, who left him his name. It has a diameter close to 200 meters and a depth of 235 meters, not filled by the waters.
Border of the Commerson crater and on the right, the western side of the ravine of the Riviere des Remparts - photo © Bernard Duyck / June 2017
Crater Commerson - explanatory panel of the outbreak of the eruption - a click to enlarge - photo © Bernard Duyck / June 2017
The eruption began in a classic way: the primitive magma of deep origin, very rich in gas, and loaded with cumulates of olivine, is pushed towards the surface and fountains and lava flow came out of the crack.
A second phase is marked by the fall of the magma level in the chimney ... the water imbibing the surrounding lands is drawn, by aspiration, towards the depths. It is a phreato-magmatic eruption which violently projects rocks and lavas sprayed, mixed with steam, gases and ashes, creating an explosion crater.
In a third phase, the calm returns, the magma ascends and a lake of lava is set up. A crack appeared in the left upstream slope of the Rivière des Remparts, leaving millions of m³ of fluid lava to escape. The flow will follow the valley for 22 km, to the coast.
The end of the eruption is accompanied by an emptying of the chimney, and releases the crater.
The Ravine of the River of the Remparts ... simply invaded by some clouds - photo © Bernard Duyck / June 2017
A number of eruptive events, resulting from a deeper feeding than that of the "ordinary" magmatic eruptions of Piton de La Fournaise, have been classified as "Chisny-type eruptions".
They are characterized by the formation of cones of slags, long and thick flows, and deposits of tephra on large surfaces
The flows of the Piton Chisny - dated a thousand years - have traveled along the Langevin River for 17 km. ; Those of the Commerson, following the course of the Riviere des Remparts on 22 km.
There are many similar cases, and the probability of a localized eruption to the west of the La Founaise massif is far from zero, and could affect more densely populated areas, such as Le Tampon.
Pitons of the Massif of La Fournaise, inherited from events of "Chisny type" - doc. Cynergéo / Villeneuve & Bachèlery.
- BRGM - Geological itinerary: the road to the volcano, discovery of ancient La Fournaise - link
- Cybergeo - Review of the typology of eruptions of Piton de La Fournaise, volcanic processes and associated risks - by N.Villeneuve and P.Bachèlery / 2006.
- BRGM - Geological knowledge of Reunion Island - link
- SVT - Academy of Reunion Island - Geology - the Volcano:
The Riviere des Remparts - link
The Commerson Crater - link