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March 29, 2001

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.

Reunion island: sister island on the other side of the world

Nearly on the other side of the planet, the Big Island has a sister, older by at least a couple of million years: Reunion Island, built in the same way as ours.

Reunion is a French island in the Western Indian Ocean, about 700 km (440 miles) east of Madagascar and 200 km (125 miles) west of Mauritius. With Mauritius and Rodrigues Islands, they form the Mascarene Archipelago. The island is entirely volcanic and originates on the ocean floor at a depth of 4,000 meters (13,000 feet).

The origin of the island is commonly attributed to a hot spot. According to certain scientists, this hot spot first created the Deccan Traps, a large basalt province in India, about 65 million years ago and may have caused the extinction of the dinosaurs. Its trace corresponds to Chagos-Lacadive Ridge, Mascarene Plateau, and Mauritius Island (created between 18 and 28 million years ago). Reunion became active about 5 million years ago, reaching the surface about 2 million years ago, and is the youngest island originating from this hot spot.

The island is elliptical (50 x 70 km) (31 x 44 miles), with a northwest-southeast elongation. It is composed of two Hawaiian-type shield volcanoes: Piton des Neiges (3,069 m) (10,069 ft) and Piton de la Fournaise (2,631 m) (8,632 ft). Piton des Neiges emerged from the sea about 2 million years ago and has been inactive for 20,000 years. Deeply eroded, it occupies the northwestern two thirds of the island. Piton de la Fournaise, on the southeastern part of the island, became active more than 500,000 years ago on the flanks of Piton des Neiges. Like Hawaiian volcanoes, it is composed mainly of pahoehoe and `a`a flows, with traces of explosive activity.

Piton de la Fournaise summit cone, inside Caldera de l'Enclos Fouque, is characterized by two craters, Bory and Dolomieu. Bory Crater, west of Dolomieu, is smaller and inactive. Dolomieu Crater is the starting point of numerous recent eruptions. Two main axes of radial fracturing extend southeast and northeast of the summit. These alignments are recognized outside the Enclos by numerous eruptive vents and are interpreted as active rift zones. Unlike Hawaiian rift zones, which form narrow ridges extending tens of kilometers offshore, Piton de la Fournaise rift zones widen downslope.

Piton de la Fournaise, one of the most active volcanoes in the world, has a mean time between eruptions of 10 months for the last two centuries and at least 125 recorded eruptions during the last century. Seventy-five percent of the eruptions have lasted less than one month, and 39 percent, less than one week. The lava output is generally 10 times lower than Kilauea's and two times lower than Etna's.

Most recent eruptions occurred within the Enclos caldera and presented no danger. About twice per century, eruptions occur outside the Enclos. Two such eruptions occurred in 1977 on the northeast rift zone and in 1986 on the southeast rift zone.

The people of Reunion still remember the spectacular March 1986 eruption, when two lava flows crossed the main coast road, while a third flowed into the sea at two points, increasing the area of the island by 25 hectares (62 acres). Even more spectacular, the April 1977 eruption flowed through the village of Piton Sainte Rose and destroyed numerous houses and fields. There were no casualties. The church of Notre Dame des Laves is witness to this eruption: its steps were cut out of solidified lava.

This type of event is unusual. In Reunion, the main risks are indirectly linked to its volcanic nature. The fragile nature of the land, combined with mountainous terrain and high rainfall, often produces landslides and catastrophic rockfalls.

The partial destruction of the village of Sainte Rose by the 1977 eruption led to the construction of an observatory for monitoring the frequent eruptions of Piton de la Fournaise. This observatory, directed by the IPGP (Institut Physique du Globe de Paris), is situated about 15 km (9 miles) in a straight line from the summit of the volcano. The permanent network includes seismic, deformation, magnetic and radon measurements.

Eruption Update

The eruption of Kilauea Volcano continued unabated at the Pu`u `O`o vent during the past week and provided visitors with an occasional glimpse of surface flow activity on Pulama pali and on the coastal flats. Activity remains robust on the coastal flat near the truncated road that formerly accessed Royal Gardens. A lobe of the flow is 500 m (1,650 ft) from the sea coast in the old Kupapa`u area near the eastern boundary of Hawai`i Volcanoes National Park.

One earthquake was reported felt during the week ending on March 29. A resident of Leilani Estates subdivision felt an earthquake at 12:19 p.m. on Friday, March 23. The magnitude-2.0 earthquake was located 2.5 km (1.5 mi) southwest of Pu`ulena Crater at a depth of 2.0 km (1.2 mi).

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Updated: April 2, 2001