A land magnetic survey on the island, a three component aeromagnetic survey and geological/geophysical investigation by a survey vessel around the island were conducted in 1980 and 1981. These geomagnetic studies suggested that Iwo-zima volcano is composed of a non-magnetic central portion and the other highly magnetized majority. The computed intensity of magnetization of the latter is roughly 1 × 10-2 emu/cc. Chemical and mineralogical studies on the dredged rock samples in the sea around Iwo-zima judged them as trachy andesites. Most of these samples show weak NRM, while the other few exceptions show relatively strong NRM according to the result of magnetic study.
