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.