An archipelago in the Indian Ocean puzzle
Located in the southern hemisphere, between the equator and the Tropic of Capricorn, the island of Mayotte is part of the Comoros archipelago, which is made up of three other islands: Grande Comore, Mohéli and Anjouan. The three islands now form the independent Comoros, united in a state called the Union of the Comoros. Mayotte decided to remain French in 1975, and is now a department of the French Republic. Mayotte lies in the western Indian Ocean between Africa and Madagascar, at the northern entrance to the Mozambique Channel at the 45° meridian and between the 12° and 13° southern parallels. Île Hippocampe is 8,000 km from Paris, 1,500 km from Réunion, 400 km from the east coast of Africa and 300 km from the west coast of Madagascar. Covering an area of 374 km2, Mayotte comprises two main inhabited islands, Petite-Terre and Grande-Terre, plus some thirty scattered islets.
Islands come out of the flames
Like the other islands of the Comoros, the "Mahoran Seahorse" is of volcanic origin. It was formed by the eruption of submarine volcanoes that pierced the water's surface. Explanations: the seafloor is moving, at the rate of a few millimetres a year; underneath, magma forms hot spots that pierce the dotted seafloor. Magma, a mass of molten rock such as basalt that is compact, black and heavy, forms undersea mountains on the floor of abyssal plains, sometimes rising higher than the ocean to create islands. After their violent, eruptive birth, volcanoes die out. Under the weight of age, they begin to subside and sink, carried away by oceanic drift. Wind and sea erosion crush them, and volcanic mountains erode and quickly become covered in vegetation. Depending on the level of volcanic maturity, the island has gone through three types of volcanic activity: Hawaiian-type with fluid basaltic lava, then strombolian with cones and lapillis projections and, finally, explosive activity with crater lakes, known as ultra-vulcanic or phreato-magmatic. Mayotte has a few clearly visible traces of these now extinct volcanoes, such as Lake Dziani in Petite-Terre and Mount Choungui in the south of Grande-Terre.
Mayotte the oldest
This hot spot of plate tectonics first formed Mayotte, the oldest, eight million years ago, then continued to move and create the other Comorian islands. Logically, as the oldest, Mayotte is the most eroded and the least elevated of the archipelago's islands. Its relief is less accentuated than that of its Comorian sisters, and has been subjected to significant sinking as its plateau has progressively collapsed. Mayotte rises from ocean depths of over 3,000 m to a peak of 660 m. It has a lagoon, and what a lagoon it is! At over 1,100 km², it's one of the largest and richest in the world, hemmed in by an almost continuous 160 km-long coral reef, cut by a dozen passes. It can reach depths of up to 70 m. The 374 km2 of land are divided between Petite-Terre (11 km²) and Grande-Terre (363 km²), with a few islets scattered around. Petite-Terre is in the extreme east, very close to the end of the lagoon and therefore to the ocean, while Grande-Terre is right in the middle of the lagoon. At the closest point between the two islands is a 2 km-wide arm of the sea (or rather lagoon).
Mayotte, an atoll in the making
Mayotte features gentle relief, fairly extensive plains and ochre sand beaches, as well as islets of white coral sand. Like all volcanic and tropical islands, Mayotte's history will not exceed 100 million years in all. Yes, these islands are mortal and will not survive, unlike continents. Mayotte, the oldest, is still at an intermediate stage, close to the stage reached by Mauritius. In a few million years, these reefs will be covered with coral sand, like the islet of Sazilé, and will encircle the island like Bora Bora in Polynesia. For the time being, Mayotte has the rare configuration of an island like Mangareva, in Polynesia, with a small landmass in the middle of an immense lagoon with submerged contours. Later still, the central island will disappear beneath the water, leaving only a crown of white sand islets: it will be an atoll, like those seen in the Maldives, the Seychelles and throughout the Pacific..
The archipelago under the tremors
2018 has been a landmark year for Mayotte: on land protests are stirring the population, but underground the ground is just as mysteriously agitated: 1,600 earthquakes were recorded in the space of ten months. An absolute record for the hitherto rather peaceful archipelago. Although most of the tremors were of low intensity, around thirty reached or exceeded 5 on the Richter scale, cracking houses and weakening schools, as was the case at the Dembéni secondary school. At the same time, another phenomenon is gathering pace: the subsidence of the archipelago, mainly on its eastern side. In one year, Petite-Terre is said to have sunk into the sea by 8 to 12 cm! It's a natural phenomenon, but it's disconcertingly rapid: usually, the island loses just 0.19 mm per year. Fortunately, the culprit was soon unmasked: an undersea volcano located 50 km east of Mayotte, 3,400 m below the surface, which gradually formed before erupting. The "Tellus Mayotte" volcanic scientific mission, coordinated by CNRS, was launched in February 2019 to study the phenomenon, revealing that the appearance of the volcano also caused a significant eastward shift of the archipelago. Since then, a succession of scientific missions have been carried out, using state-of-the-art tools to obtain highly accurate images and data on this volcano, now known as France's4th most active volcano. At the end of 2019, the tremors were no longer felt, and it might seem as if the volcano was returning to normal, but scientists warn that it is still active! Major questions remain unanswered, such as the impact of this eruption on marine life. To be continued.
