Comoros

The Comoros archipelago consists of four main volcanic islands — Grande Comore, Moheli, Anjouan, and Mayotte — located in the northern Mozambique Channel between Madagascar and the East African coast. Like Cape Verde, the Comoros owe their existence to a mantle hotspot that has been generating volcanic activity as the African plate slowly migrates over it. The archipelago displays a classic hotspot age progression, with the oldest island (Mayotte) to the east and the youngest and most active (Grande Comore) to the west.

Grande Comore hosts Karthala, one of the largest and most active shield volcanoes in the world measured by crater volume. Karthala has erupted more than 20 times since the mid-19th century, with significant eruptions in 2005 and 2007 that forced the evacuation of thousands of people. Each eruption is accompanied by intense [[seismic-wave]] swarms as magma rises through the volcanic system. The 2007 eruption involved a phreato-magmatic explosion that deposited ash across the island and contaminated drinking water supplies for much of the population.

In 2018 to 2019, an extraordinary seismic-volcanic crisis occurred near the island of Mayotte (now a French overseas territory). Beginning in May 2018, Mayotte experienced the largest seismic swarm ever recorded in the Indian Ocean region, with thousands of events including a [[magnitude]] 5.8 earthquake that was the largest in the archipelago's instrumental history. The swarm was accompanied by the birth of a new submarine volcano approximately 50 kilometres east of Mayotte — a rare and scientifically remarkable event that demonstrated active hotspot volcanism in real time.

The tectonic framework of the Comoros involves the eastward movement of the African plate over the presumed Comoros hotspot, situated in thinned continental crust that transitions to oceanic crust along the Mozambique Channel. Regional [[fault]]s along the channel contribute occasional tectonic events in addition to the dominant volcanic seismicity. [[tsunami]] risk from submarine eruptions or slope failures on the volcanic flanks exists but is difficult to quantify.