Tuff Cones Research Articles

GEOLOGY AND GROUND-WATER RESOURCES OF IWO JIMA

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.

GEOLOGY OF THE WALLIS ISLANDS

The Wallis Islands comprise Uvea, the main island, and 22 islets enclosed by or lying upon a nearly circular barrier reef about 200 miles west of the Samoan group in the Southern Pacific Ocean. The highest point in the group, about 470 feet above sea level, and several crater lakes exist on Uvea Island. The high islands are composed of olivine basaltic lavas and pyroclastics, except for one cinder cone and its associated flows of oligoclase andesite on Uvea Island. The low islands either are composed of calcareous sand or are erosional remnants of tuff cones and lava domes. Uvea Island was built by the coalescence of lava flows from 19 volcanic vents. The vents comprise 15 flat shield-shaped lava cones, 3 consolidated ash cones, and 1 cinder cone. Except for two Recent lava cones barely covered with soil, the bulk of the island is composed of deeply weathered middle Pleistocene(?) volcanics. Lavas of intermediate age do not exist. Limestone ejecta in the tuff cones indicate that the whole group is built on a submerged reef, probably about 180 feet below sea level. This reef presumably rests on a Tertiary basaltic volcano. Definite evidence of emerged shore lines at 25 and 5 feet above mean sea level exists, as well as evidence suggestive of higher stands.

PETROGRAPHY OF THE WALLIS ISLANDS

The lavas of Uvea Island, in the Wallis group, include olivine basalts containing colorless monoclinic pyroxene, less abundant olivine basalts containing titaniferous pyroxene, and oligoclase andesite. Palagonite tuff builds cones along the eastern coast. Of the lesser islands in the Wallis group, Nukuatea is built largely of palagonite tuff, with a flow of olivine basalt; Nukufetau is composed of olivine basalt flows and cinders; Luaniva is composed largely of olivine basalt with a little tuff; and Fugalei of olivine basalt which filled the bowl of a tuff cone, most of which has been eroded away. Other islets represent the emergent edge of the barrier reef and are composed of poorly to well-consolidated calcareous sand. The lavas of the Wallis Islands belong to the alkaline suite of the central Pacific volcanoes, rather than to the calc-alkaline suite of the circum-Pacific belt.