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 composed of three other islands: Grande Comore, Moheli 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 is located in the western part of the 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. Seahorse Island is 8,000 km from Paris, 1,500 km from Reunion Island, 400 km from the east coast of Africa and 300 km from the west coast of Madagascar. With a surface area of 374 km2, Mayotte comprises two main inhabited islands, Petite-Terre and Grande-Terre, as well as about 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 broke through the surface of the water. Explanations: the ocean floor moves, at the rate of a few millimetres per year; underneath, the magma forms hot spots which pierce the submarine floor in dotted lines. The magma, a mass of molten rock like basalt that is compact, black and heavy, forms seamounts on the bottom of the abyssal plains, which sometimes rise higher than the ocean to create islands. After their violent, eruptive birth, volcanoes die out and then die. Under the weight of the ages, they begin to subside, to sink, the oceanic drift carrying them away. The erosion of the wind and the sea collapses them, the volcanic mountains erode and are quickly covered with vegetation. According to the level of maturity of volcanism, 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 some visible traces of these extinct volcanoes, such as Lake Dziani in Petite-Terre or 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. Mayotte, logically as the oldest, is the most eroded and the least elevated of the islands of the archipelago. Its relief is less accentuated than that of its Comorian sisters and has been subjected to a significant sinking, its plateau having gradually collapsed. Mayotte rises from the ocean depths of more than 3,000 m to a height of 660 m. It has a lagoon, and what a lagoon! With more than 1,100 km², it is one of the largest and richest lagoons in the world. It is surrounded by an almost continuous 160 km long coral reef, cut by a dozen passes. It can reach a depth of 70 m. The 374 km2 of land are divided between Petite-Terre (11 km²) and Grande-Terre (363 km²), and 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 in the middle of the lagoon. At the closest point of these two islands, an arm of the sea (or rather of the lagoon) is 2 km wide.
Mayotte, an atoll in the making
Mayotte has gentle relief, fairly large plains and ochre sand beaches, as well as islets of white coral sand. Like all volcanic and tropical islands, Mayotte has a history that will not exceed 100 million years in all. Yes, these islands are mortal and will not survive, unlike the continents. Mayotte, which is 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 go all the way around the island like Bora Bora in Polynesia. For the moment, Mayotte has the rare configuration of an island like Mangareva in Polynesia, with a small land mass in the middle of an immense lagoon with submerged contours. Later still, the central island will disappear under the water, leaving only a crown of white sandy islets: it will be an atoll, as can be seen in the Maldives, the Seychelles, and the whole Pacific..
The archipelago under the tremors
The year 2018 will have been a landmark year for Mayotte: on land protests agitate the population, but underground the ground is also mysteriously agitated: 1,600 earthquakes were recorded in the space of ten months. An absolute record for the previously rather peaceful archipelago. If most of the tremors were of low intensity, about thirty of them reached or exceeded 5 on the Richter scale, cracking some houses and weakening schools, as was the case at the Dembéni school. At the same time, another phenomenon is suddenly accelerating: the subsidence of the archipelago, mainly on its eastern side. In one year, Petite-Terre would have sunk into the sea by 8 to 12 cm! This is a natural phenomenon, but one that is disconcertingly fast: usually the island loses only 0.19 mm per year. Fortunately, the culprit was soon unmasked: it was an underwater volcano located 50 km east of Mayotte, 3,400 m below the surface, which formed little by little before erupting. The "Tellus Mayotte" volcanic scientific mission, coordinated by the 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. Scientific missions have followed one another since then, with state-of-the-art tools making it possible to obtain very precise images and data on this volcano, now known as the4th active volcano in France. At the end of 2019 the tremors are no longer felt, one could believe in a return to normal but scientists warn: the volcano is still active! Big questions remain, like the impact of this eruption on marine life. To be continued.
