The four islands in the Comoros archipelago were formed by a ‘hotspot trace’ of volcanic activity caused by tectonic movements starting almost eight million years ago. This is very recent in geological timescales; for comparison, the Comoros’ older and larger neighbour Madagascar used to be part of a continental landmass and is between 70 and 120 million years old.
The islands are arranged roughly in order of age from East to West and their differences in age are reflected in their geography today. Mayotte (Maoré in Comorian) is the oldest of the four islands, formed around 7.7 million years ago. Its age means there has been more time for the elements to erode the rocks, making the island relatively flat, and covered with a thick soil layer and abundant fresh water. Well-developed coral reefs encircle the island forming a lagoon. In contrast, the youngest island, Grande Comore (Ngazidja), formed only 10,000 years ago, is still being reshaped by the active volcano, Mount Karthala, which last erupted in 2007 and reaches 2361m, with the largest caldera of an active volcano in the world. As you travel around the island you can see landscapes of black rock formed from recent lava flows. Due to its youth, the island has a very shallow soil layer, so there is little chance for rainfall to permeate into the water table and so there are no permanent rivers. Only a narrow band of coral reef fringes the island.
Of the two remaining islands, Mohéli (Mwali) is 5 million years old and formed of a low central ridge reaching 790m, while Anjouan is younger (3.9 million years) with steep sided mountains reaching to 1595m at the summit of Mt. Ntringui. Both have permanent rivers.
At only 12 degrees south of the equator in latitude, the islands have a tropical climate. There are two main seasons: Kashkazi is from November to April and is hot and humid with temperatures between 28 and 32 degrees Celsius. This is also the cyclone season in the Indian Ocean, although Madagascar often blocks the islands from their direct paths, reducing the impact of violent weather in the Comoros. Kusi is the dry and cool season between May and October, with temperatures between 24 and 27 degrees.