The small, very low island of Genovesa is the top of a shield volcano. The surface of a summit crater lake is near sea level. A larger, 2-km-wide excentric caldera on the S side forms an embayment that is filled by Darwin Bay. Sediment filling Arctulus Lake, a crater in the center of the island, is less than 6000 years old. Youthful flows erupted from flank fissures cover much of the island and very fresh, glassy spatter has been found (Simkin 1976, pers. comm.). No historical eruptions are known, although the fresh-looking lava flows suggest a very youthful age. Basaltic rocks on the island are noted for their abundant coarse plagioclases.