FORMATION OF THE ISLAND - VOLCANICITY OF THE ISLAND - HISTORICAL ACTIVITY - THE VOLCANIC RISK OF THE ISLAND - FUTURE SCENARIOS

The Piton de la Fournaise – Reunion Island

The Piton de la Fournaise was built on the south-eastern side of the Piton des Nieges. During at least 500.000 years, the two volcanoes were active simultaneously. The Piton de la Fournaise forms a cone of 30 km in diameter, covering 26 % of the surface of the island. This building differentiates from the first by the uniformity of its products: It produce fluid lava from mainly effusive eruptions. It is formed by the stacking of lava flows attaining a thickness of thousands of meters, mostly aa and pahoehoe lava, alternating with layers of scoria, the whole reshaped by a network of more or less inter-connected dykes.

We can distinguish several different active phases:

The old shield

The old volcano, to the West of the current volcano, is only observable at the outcrop in the bottom of the valley of the Ramparts River and the East River. The oldest rocks are 530 000 years old (Gillot et al, 1990). A first caldera was formed roughly 290 000 years ago towards the current Rampart River, which served as barrier and directed the flows from the old volcano (Bachelery and Mairine, 1990).

The recent shield

The massif of the Piton of the Furnace. The three calderas are indicated in red. The first in dotted line was reconstituted by observations on the ground. The second can be observed at the pass of Sables. Its prolongation has been reconstituted in the East and the South. The third caldera corresponds to the cliffs of Bellecombe. Extract from Mairine, 1998.

About 150 000 years ago the activity shifted towards its current site. The new eruption covered the remainders of the old shield, before a new caldera, of more than 10 km wide, formed 65000 years ago; on the current days one of its margins corresponds to the cliffs of the Plain of Sands. From 65 000 to 4700 years, this caldera was partially filled, while the activity spread out to the East of the Langevin river and the East river. Following the successive intrusions, the Eastern part of the volcano was destabilized and produced landslides towards the sea, mobilizing a total of 600 km3 of material (Bachèlery, 1995). The formation of the last caldera around 4500 years ago ended this period (Abchir, 1996); this caldera is open towards the sea to the East, producing the depression of "Enclos Fouqué" and the slope of "Grand Brûlé". Initially this caldera was several hundred meters deep.

Current phase

For the last 4500 years the current final cone of the Piton of the Furnace has been at the center of the last caldera. For two centuries there has been observed an approximate average of one eruption per year. At the end of the 18th century, the top was crowned by two craters, as depicted by drawings of Bory of St-Vincent (1804): the Bory crater in the West and the Dolomieu crater in the East. If the Bory crater did not change much since the beginning of the 20th century, greater changes were observed in the Dolomieu crater. In 1911, the Dolomieu crater was completely filled creating a small plateau still visible in the southern part of the top. From 1927, of low amplitude collapses took place at the East of the plateau. Between 1930 and 1946 the plateau collapse continued and accelerated with the formation of the crater "Brûlant" of approximately 180 m deep (Lacroix, 1936). In the Western part the caldera "Enclos Vélain" remained in place and today it merges entirely with the bottom of the crater Dolomieu.
Since then the crater Dolomieu has been again partially filled by successive summit eruptions (Bachelery, 1981).

Most of the recent eruptions are limited to the Enclos Fouqué caldera, except for some lava flows outside the caldera that only accounts for 3% of the total eruptions: this includes the eruption of 1977 that crossed the village of Ste Rose and the eruption of 1986 in Tremblet. Following the eruption of 1998, a small eruption outside the caldera took place at upper part of "Bois Blanc". We should note that, for 25 years, approximately a quarter of the caldera "Enclos Fouqué" has been cover by an average of 2 to 3 meters of lava.