Magnetic anomalies identified in the vicinity of the archipelago indicate that the structures forming the islands date back 125-150 million years: the islands themselves date from 8 million (in the west) to 20 million years (in the east). The oldest exposed rocks occurred on Maio and northern peninsula of Santiago and are 128-131 million year old pillow lavas. The first stage of volcanism in the islands began in the early Miocene, and reached its peak at the end of this period, when the islands reached their maximum sizes. Historical volcanism (within human settlement) has been restricted to the island of Fogo.The origin of the islands' volcanism has been attributed to a hotspot, associated with bathymetric swell that formed the Cape Verde Rise.[12] The Rise is one of the largest protuberances in the world's oceans, rising 2.2 kilometers in a semi-circular region of 1200 km², associated with a rise of the geoid and elevated surface heat flow.
Though Cape Verde's islands are all volcanic in origin, they vary widely in terrain.
Most recently erupting in 1995, Pico do Fogo is the largest active volcano in the region. It has a 8 km (5 mi) diameter caldera, whose rim is 1,600 m (5,249 ft) altitude and an interior cone that rises to 2,829 m (9,281 ft) above sea level. The caldera resulted from subsidence, following the partial evacuation (eruption) of the magma chamber, along a cylindrical column from within magma chamber (at a depth of 8 km (5 mi)).Geologically, the islands are principally composed of igneous rocks, with volcanic structures and pyroclastic debris comprising the majority of the archipelago's total volume. The volcanic and plutonic rocks are distinctly basic; the archipelago is a soda-alkaline petrographic province, with a petrologic succession which is similar to that found in other Macaronesian islands.
Extensive salt flats are found on Sal and Maio. On Santiago, Santo Antão, and São Nicolau, arid slopes give way in places to sugarcane fields or banana plantations spread along the base of towering mountains.
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