The island is the top of a massive submerged shield volcano that rises about 6 km (3.7 mi) from the floor of the Atlantic Ocean. The volcano formed atop an east-west rift[1][2] in the oceanic crust along the African Plate, beginning during the Miocene epoch over 5 million years ago, continuing into the Pleistocene until about 700,000 years ago.[3] This was followed by extensive erosion, producing two large amphitheatres open to south in the central part of the island.