Notes on the geology of Iwo Jima

Abstract

Iwo Jima is near the south end of a chain of volcanic islands extending south from Tokyo Bay. The principal rock on the island is a soft tuff in roughly horizontal layers. Massive trachyandesite, in large part extrusive, is exposed at the north and south ends of the island. Both the lava and plutonic inclusions within the lava have a much higher alkali content than is common in the rocks of Japan proper. Suribachi‐yama, the small volcano at the south end of Iwo, is made up of three roughly horizontal layers, two of tuff and one of lava, topped with a thin veneer of fairly recent pyroclastic debris. Fragments of modern coral at high elevations indicate that all or most of the Island was submerged in the fairly recent geologic past. As the island emerged, terraces were cut in the tuff at the north end and a deposit of loose coarse sand and gravel was formed in the low central part. Numerous solfataras are the only present indication of volcanic activity. The last eruption of Suribachi probably took place several centuries ago, and there is no suggestion in the present activity that further eruptions are imminent.