Réunion

From 24th September to 5th October 2000 MINSA staged a very successful volcanology excursion to the largest, and only active, Mascareigne archipelago volcanic island of Réunion in the South Indian Ocean. The excursion was first suggested and subsequently organised by Thinus Cloete (Council for Geoscience) in conjunction with Luc Chevalier (Council for Geoscience) who organised the technical programme - Jill Richards (MINSA secretary) assisted with administration, finances etc. The excursion was attended by 17 participants from South Africa and Germany and included earth scientists from the minerals industry, Council for Geoscience and the Universities of Durban-Westville, Pretoria and Wits. An excellent guidebook on the geology and other aspects of Réunion was compiled by Luc and Thinus.

Luc was the first geologist to map the active volcano of Piton de la Fournaise for his PhD thesis in the late seventies, and we were extremely fortunate to have him as our logistics and scientific leader, as well as translator. Réunion is very much part of France and virtually no English is spoken.

Réunion is a classic intra-oceanic plate 'hot spot' shield volcano of the Hawaian type and forms the leading edge of a submarine volcanic ridge, which includes extinct volcanic islands such as Mauritius.The island is 70 km x 40 km and represents the emerged portion (only 1/32) of a much larger volcanic pile resting on the oceanic floor at - 4000m.The island is formed by two large and partly coalescing shield volcanoes, the extinct or dormant (last eruption 12 000 years ago), Piton des Neiges (Peak of Snow) in the west and the active (last eruption 2 months ago) Piton de la Fournaise (Peak of the Furnace) in the east.

Piton des Neiges displays a differentiated sequence of older (>2.1 Ma) oceanites (olivine rich basalts) and picrites overlain by a younger series of differentiated lavas dating from 350 000 years to 12 000 years, including hawaiites, mugearites, and benmoreite that evolve to trachytes and have a clear alkaline composition.The volcano was marked by a succession of eruptive growth stages and associated dynamic events including caldera collapses, landslides and periods of erosion. Since the last volcanic activity at 12 000 years ago, strong erosion probably aided by caldera collapse has excavated a coalescing tri-lobate cluster of spectacular deep ovoid valleys (cirques) with awesome exit gorges that together are said to resemble the uterus of a rabbit. The highest point of Piton des Neiges (3069 m) is on the central divide close to the centre of the three deep cirques termed Cilaos, Salazie and Mafate. Layered gabbros occur in places in the river valleys and could represent evidence for differentiated magma in shallow reservoirs within the deeply eroded cirques. The deep valleys also show evidence for extensive hydrothermal alteration.

Western edge of Cirque Mafate with landslides in background and cliff face of alkali basalts overlying oceanites and olivine basalts

An abnormal geothermal flux, historic fumaroles, thermo-mineral springs and hydrothermal deposits testify to the fact that the volcano is not entirely extinct.

Piton de la Fournaise commenced its activity just before the eruption of the differentiated alkaline lavas on Piton des Neiges. Because of its young age, morphostructural features are very well preserved. These include a classic caldera collapse structure, called the Enclos Fouque caldera (see map), with a central cone containing two twin craters, the small Bory crater and the adjoining larger 1-km-diameter Dolomieu crater. The central cone has a height of 2631 m. Slump and collapse of the 10-km-diameter Enclos caldera has taken place on the east side of the volcano to form the Grand Brûle sector collapse. Virtually all of the more recent lava flows - approximately one per year since 1972 - have emanated from the flanks of the central cone and from within the Enclos Fouque caldera, and have flowed generally eastward down the Grand Brûle (Big Burn). The map shows Piton de la Fournaise with the extent of the 1985/86 flows and the 1977 flow. Both of these magmatic episodes also witnessed eruption from fissures outside of the Enclos caldera that reached the sea.

Edge of Bory summit crater spatter cone (2631 m) looking towards western edge of Enclos Fouqúe caldera (visible in background)

Part of excursion group in Bory crater - crest of Piton de la Fournaise. From left: Luc Chevalier, Patrick Bachélery, (Université de la Réunion), Uwe Horstman, Roger Scoon, Cécile Martine (geology graduate Univ. Réunion), Hanna Horsch, Andrew Mitchell, Jill Richards, Thinus Cloete and Sybrand De Waal

We also examined the 1977 and 1986 flows in detail where they cut the coastal road and where they reached the sea (particularly at Pointe de la Table) thereby increasing the size of the island. Classic lava tunnels and associated aa and pahoehoe flows were seen along the east coast and we walked in a spectacular lava tunnel for nearly 100 metres. At Ste Rose the 1977 flow partly enclosed a church, but stopped before destroying it. The Ste Rose church has now become a religious shrine.

Rim fractures of the Dolomieu caldera cutting the floor of the adjacent Bory crater

Part of the 1986 lava flow with aa edges approaching the coast at Pointe de la Table

Volcanic features are classically developed on Piton de la Fournaise and include abundant pahoehoe and as flows, lava tubes and tunnels, ash cones, cinder cones, and spatter cones and we had a memorable day climbing and examining the recent volcanic terrane on probably the world's most active volcano.

Pahoehoe lava flows draping over a spatter cone and forming a pahoehoe lake - Enclos Fouqúe caldera	Twisted rope pahoehoe lava - Enclos Fouqúe
Large solfatata vent near rim of Dolomieu crater	Part of the 1 km wide Dolomieu crater, with the floor covered by 1985-86 lava

Prof Bernard Robineau of the Department of Geology of the Universite de la Réunion has recently documented an important process of huge avalanche debris slides. These are considered to be high speed gravity slides with a heterogeneous clast assemblage (thereby distinguishing them from pyroclastic flows) ranging from olivine rich picrites to gabbros. As the flow slowed, the weight caused friction and striations of the underlying lava took place to form a striated pavement remarkably similar to that of a glacially striated pavement. We saw classic examples of striated pavements with the overlying debris termed the 'Saint - Gilles breccia' at Cap la Houssaye near Saint - Gilles.

Since the 1980 Mount St Helens eruption, huge landslides on volcano flanks, usually initiated by magmatic intrusions, are now recognised as catastrophic but normal events. Some factors invoked as the trigger for these landslides include dyke intrusions, caldera collapse, volcanic spreading and the presence of low cohesion layers such as pyroclastic or hyaloclastic layers. Classic evidence for debris avalanches include island volcanoes showing a U-shaped caldera open to the ocean such as the Grand Brûle of the Piton de la Fournaise. Bathymetric surveys have confirmed the offshore occurrence of debris avalanche deposits adjoining the Piton de la Fournaise, as well as on the west coast.

Because of the relatively young age of Réunion Island, coral reefs of the type that envelop much of Mauritius are not as abundant around Réunion. Nevertheless some superb coral beaches with their miniature offshore barrier reefs and lagoons occur in places on the west coast such as at Boucan Canot, a top rate resort town attracting many overseas visitors. Of geological interest are black volcanic sand beaches, as well as green olivine beaches - particularly those at Òtang Salé on the southwest coast where we enjoyed an excellent swim.

While touring the island we stayed at interesting and invariably spectacularly located lodges, small hotels and guesthouses or gites. The hotel Maido is situated high up on the western slope of Piton des Neiges on a spectacular drive through different vegetation zones from forest (Port Jackson,Tamarin and others) to a fynbos type of montane vegetation on the spectacular western rim of the Cirque Mafate. The Auberge de Jeunesse is situated in the quaint town of Éntre-Deux strategically situated at the entrance to two spectacular gorges, the westerly one being the only access to the Cirque Cilaos and the spectacular Cirque town of Cilaos where we stayed at the Gite Nassibou. Unfortunately about a quarter of the group developed rather severe food poisoning which layed them low for a day and provided an ideal excuse for not undertaking a river traverse which involved a 600m vertical climb into and out of a gorge! The Refuge Brancard in the town of Hell Bourg within the Cirque Salazie was described by one of the participants as a house made of packing cases and tin sheets. Some of the ablution facilities were in the main entrance (passageway). We had the standard Creole meal on our first night at Brancard, but on the second night we had a superb fish and seafood meal, undoubtedly the best of the whole trip.The highest point at which we stayed was at the Gite du Volcan in fynbos-type montane vegetation on the rim of the Enclos Fouque caldera of the Piton de la Fournaise. The food throughout was fairly standard Creole type consisting of rice, chicken (generally curried) and lentils.

Two places of great interest that we visited were the volcanology observatory (ObservatoireVolcanologique Du Piton De La Fournaise) and the volcano museum (Maison du Volcan). The observatory came about in 1980 as a result of the efforts of the two legendary volcano observers Maurice and Katie Kraft who were tragically killed in a nuee ardente incandescent ash flow in Japan in 1991. The superb museum is dedicated to the memory of the Krafts and their outstanding films of volcanoes and volcanic eruptions. The Observatory monitors a number of parameters from 160 data capture points in order to predict the onset of volcanic eruptions. These include seismicity caused by the rising magma, surface tilting and bulging, distinctive magnetic signatures and an increase in radon gas prior to eruption.The volcano is also continually under observation by two television monitors. Indications are that an eruption could occur in the near future.

Several of the party visited the University and were introduced to staff and research projects were briefly reviewed. As might be expected, the main focus of their research is volcanology, but important additional topics also include ground water and engineering geology. Engineering geology is of great importance in assisting with the design of structures to support highly unstable coastal road cuts and the island is very advanced in this form of engineering geology. Of interest along coastal roads was the use of dolosse to build up and protect the road at the base of sea cliffs. Profs Bernard Robineau and Patrick Bachélery, as well as Cécile Martino joined us at a number of field localities and provided valuable insights into various aspects of both the volcanoes.

Occupation of Réunion started when a few mutineers from Fort Dauphin (Madagascar) were left stranded on the island in 1646.They lived in historic caves near the present town of St Paul where there was plentiful water, edible plants and wildlife. The island became prosperous under a new governor 'Mahe de la Bourdonnais' in 1735. He was a great sailor and entrepreneur and his statue is prominently displayed in St Dennis, the capital city. In 1792, three years after the French Revolution the island was renamed Ille de la Réunion. In 1825, as a result of a thriving coffee trade the island had 25 000 free settlers and 71 000 slaves, the latter constituting the foundations of the present day Creole population. With the abolition of slavery in 1848 free workers from Africa, India, Madagascar and China helped to diversify the agriculture into sugar cane, vanilla, cotton, rice, perfume plants and commercial trade. The current population is 1.1 million.

A famous son of the island was Roland Garros, a legendary fighter pilot, inventor and later tennis player after whom the stadium in Paris, where the French Open tennis tournament is played, was named. His statue, holding an aircraft propeller is to be seen in downtown St Dennis.

Of historical and geographical interest are the people living in the isolated erosional remnant plateaux (the 'Islets') within the Cirques of Cilaos, Salazie and Mafate, who are termed 'le petits blancs des hauts' (the lesser whites of the highlands). They descended from poor white people (usually the youngest son of a family) who refused to work together with freed slaves and left the coastal plains for the remote and inaccessible islets within the Cirques away from the fast developing modern life of the coast. With their isolation (there is no road into the Cirque Mafate) in-breeding has become a major concern.

Réunion Island is a place of beauty and breathtaking landscape with plentiful tropical plants, fruits and birds. The first settlers described the island as a paradise on Earth with abundant bird life, including ducks, geese and white flamingos and possibly the famous dodo. The bird population has unfortunately decreased significantly, as have other species such as the aye-aye, a lemurian. All the participants owe a debt of gratitude to the organisers for assembling such a superb tour at such a reasonable price. We were all hooked on Réunion and I would not be surprised if another volcanic eruption would not be an ideal excuse to board an aircraft (Air Austral has a regular service) and pay a return visit to this magnificent island.

As a post script it is of interest to add that, based on the evidence we were shown at the volcanic observatory, many of the party were predicting an imminent volcanic event (and hoping it might occur whilst we were still on the island). Our prediction appears to have been correct as a recent message from Luc informed us that an eruption took place 10 days after we left the island, a new vent opening just below the eruption of July 2000 on the northern flank of Piton de la Fournaise.

Volcanology excursion to Réunion Island