Madeira Island is the emergent top of a massive shield volcano that rises about 6 km from the floor of the Atlantic Ocean and forms the largest island of the Madeira Archipelago, about 90 km in length.  Construction of the volcano along E-W-trending rift zones from the Miocene to about 700,000 years ago was followed by a period of extensive erosion and possible edifice collapse.  Two steep-walled amphitheaters open to south in the central part of the island.  Late-stage eruptions are scattered throughout the island and lasted until the Holocene, producing scoria cones and intracanyon lava flows mantling rocks of the older eroded edifice.  The youngest activity at Madeira lies in the west-central part of the island, and consists of cinder cones in the upper Sao Vicente valley, a series of intracanyon flows, and a tephra layer on top of the Paul da Serra plateau dated at about 6500 years ago.