The volcanic island of Biliran, located across the narrow Biliran Strait from the northern tip of Leyte Island, contains many prominent andesitic lava domes, the highest of which is 1301 m Surio. Several Pleistocene K-Ar dates have been obtained from volcanic centers on northern Biliran Island. Fumarole fields are scattered throughout the 20 x 35 km wide island. The only known historical activity was a phreatic explosive eruption and possible debris avalanche at a thermal area in 1939.