Isla Floreana is a shield volcano, which has erupted alkaline basalts since 1.5 Ma. It is the southernmost island in the Galapagos Archipelago, and a 3,400 m submarine escarpment 10 km south of the island forms the southern boundary of the Galapagos Platform. There are over 50 scoria cones onshore and 6 tuff cones offshore. Mostly composed of tephra, these cones are the origin of the A'a lava flows. The oldest flows are on the northern end of the island, while the youngest (26 ka) are on the southern end. Cerro Pajas, the tallest inactive volcano on the island, is the origin of the largest lava flow (272 ka).