The islands are volcanic, with coral reefs surrounding most of them. Rugged ranges rise to 1,100 m (3,608 ft) on Upolu and 1,857 m (6,094 ft) on Savai'i. Apolima is a volcanic crater whose wall is pierced by a passage that connects its harbor with the sea. Manono, about 70 m (230 ft) high, consists chiefly of coral sand. The islands are in an area of active volcanism. Severe eruptions occurred in Savai'i during 1905–11.