The islands run roughly north–south in two parallel chains; the western islands are volcanic and the eastern are coralline encircled by reefs. At 10,800 m (35,400 ft) deep, the Tonga Trench is one of the lowest parts of the ocean floor. The soil on the low-lying coral islands is porous, being a shallow layer of red volcanic ash, devoid of quartz, but containing broken-down limestone particles.
The volcanic islands range in height to a maximum of 1,033 m (3,389 ft) on Kao. Fonuafo'ou (formerly Falcon Island), about 65 km (40 mi) northwest of Nuku'alofa, is famous for its periodic submergences and reappearances, as a result of earthquakes and volcanic action. There are few lakes or streams. Tofua, Vava'u, Nomuka, and Niuafo'ou each have a lake, and there are creeks on 'Eua and one stream on Niuatoputapu. Other islands rely on wells and the storage of rainwater to maintain a water supply.