Geology
The Comoro Archipelago consists of volcanic islands. These volcanic islands like some of the high mountains in the north of Madagascar were formed in the Tertiary and Quaternary Eras. The island of Mayotte is the oldest one still above sea level and underwent three volcanic phases between 15 million and 500,000 years ago. The ages progressively decrease from east to west. The most recent island is that of Grande Comore, and its volcano, Karthala, is still active. The summit caldera is quite large, being approximately 3 x 4 km (1.9 x 2.5 miles) in size at the rim (2007).
Read more about this topic: Comoros Islands