Sakar is an incised stratovolcano with a summit crater lake. Deep valleys cut the flanks of the volcano, which is partially surrounded by coral reefs. An older volcano that forms much of the island consists mainly of porphyritic basaltic rocks. A younger andesitic cone with a 1.5-km-wide crater has been constructed within the larger old crater, whose rim is exposed on the N and E. No eruptions have been reported, but warm springs are found along the SW coast, and a pyroclastic cone on the southern flank of the 8 x 10 km island may be of Holocene age (Johnson, 1990 pers. comm.). A large submarine debris-avalanche deposit lies north of Sakar.