GEOLOGY AND GROUND-WATER RESOURCES OF IWO JIMA

Abstract

Iwo Jima, in the western Pacific Ocean, consists of Motoyama, a broad volcanic cone, at the north, and Mt. Suribachi at the south, with an undulating isthmus between. Motoyama is largely light-gray-buff tuff. A thick andesitic lava flow under Suribachi, exposed in several places, is overlain by a thick deposit of cinder and scoria. The isthmus (called Tidorigahara by the Japanese) is underlain by more than 200 feet of loose black volcanic ash and fine cinder derived from Suribachi. Several small coral reefs are located about 340 and 110 feet above present sea level. Iwo Jima first came into existence, probably early in Pleistocene time, with the building above sea level of the tuff cone of Motoyama. Quite late in the active life of Motoyama, volcanic activity on the southwestern flank resulted in the formation of Suribachi. This activity may have started with the welling up of the andesitic lava which underlies Suribachi. Following the major eruption of Suribachi, relative sea level changed, and the sea stood about 360 feet higher than at present. The broad cone of Motoyama was beveled; the relative sea level then dropped 240 feet, with minor halts to about 120 feet above present level. As the island rose, Suribachi burst forth in its last stage of explosive activity. Wave erosion cut deeply into the andesite flow of Suribachi, and a prominent bench level was formed 120 feet above present sea level. The Japanese on the island were often faced with serious water shortages. Americans drilled wells and obtained moderately large supplies of usable water. The temperature of the water ranges from 105° to 179° F., and the water is somewhat mineralized. The most favorable area for ground-water development is the isthmus.