The only known historical volcanic activity at Biliran was a phreatic eruption and possible debris avalanche at a thermal area on September 26, 1939. Ashfalls were reported at Caibiran and adjoining areas with maximum deposits at thick. There are five active solfatara fields on the island. The solfatara on the west side of Mt. Giron contained more than 400 tones of sulfur in 1880.