The temperature of fumarolic gas ranged from 100 to 128 °C. These gas usually contained few HCl and SO2, and contained a small amount of H2S. These facts indicated that acidic gas components were absorbed into the aquifer during the cause of ascent. Hot springs may be classified into three types according to their chemical composition : 1) volcanic thermal water type ; 2) sea water type ; and 3) type of mixture of 1 and 2.δD and δ18O of waters collected from this island are high. The rocks from Iwo-jima are all trachy andesite with the SiO2 content of 54-58 %, and with the Na2O+K2O content of 9-10 %.
