Kueishantao Island, the southernmost of five small volcanic islands off the NE coast of Taiwan, is also known as Guieshan Island and "Turtle Mountain Island" due to its profile as seen from some points on Taiwan. It is the southwesternmost volcano along the axis of the Okinawa Trough back-arc basin that extends to the NE from Taiwan to Unzen volcano in Kyushu. The SW tip of the Okinawa Trough extends past Kueishantao onto the Ilao Plain on Taiwan. The island consists of andesitic lava flows and volcaniclastic material. Accounts during the time of King Qianlong of the Qing Dynasty (1775-1795 CE) noted a lava flow on the mountain. Observations from ships in 1853 were likely from this location. Vigorous submarine fumaroles and solfataras discolor seawater in the area.