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A volcanic archipelago
The Comoros are volcanic islands born of magma rising through the ocean floor. The exact origin of this volcanism is still debated by geologists: a hotspot, faults linked to the opening of the Mozambique Channel, or a combination of the two. What is certain is the order in which the islands were born: Mayotte is the oldest, followed by Anjouan and Mohéli, and finally Grande Comore, the youngest.
The life cycle of an island
The four islands illustrate the stages of a single process. Grande Comore, young and still active, has regular slopes, recent lava flows and almost no permanent watercourses: the porous lava soaks up the rain. Anjouan and Mohéli, which are older, have been deeply cut by erosion, which has carved out valleys, cirques and waterfalls. Mayotte, the eldest, has slowly subsided: its relief has softened and the corals have had time to build around it a huge lagoon enclosed by a barrier reef.
Karthala, one of the most active volcanoes in the region
Karthala (2,361 m) occupies the whole southern half of Grande Comore. It is a shield volcano, like those of Hawaii or Réunion, crowned by a caldera several kilometres across, one of the largest in the world for an active volcano. Its eruptions, frequent on a geological scale, have sometimes thrown ash as far as Moroni, as in the mid-2000s. In the north, the older massif of La Grille is dotted with scoria cones.
Crater lakes and curiosities
- Lac Salé (Grande Comore): a crater close to the sea, filled with brackish water.
- Lake Dzialandzé (Anjouan): a high-altitude lake in a forested crater, at around 900 m.
- Lake Dziani Boundouni (Mohéli): a crater lake visited by water birds.
- Lake Dziani (Mayotte): a maar on Petite-Terre with emerald-green alkaline water.
Fani Maoré, the volcano born in 2018
In May 2018, a series of unusual earthquakes shook Mayotte. The scientific campaigns that followed revealed the birth of a new underwater volcano, named Fani Maoré, about fifty kilometres east of Petite-Terre, at a depth of more than 3,000 m. It is one of the largest underwater eruptions ever observed. The phenomenon was accompanied by a slight subsidence of the island, which is now continuously monitored by a surveillance network.
