Western Caroline Islands - Eastern Caroline Islands

The division between the eastern Caroline Islands and the western Caroline Islands represents a major geological division. The major islands and island groups of the western Caroline Islands are the tops of submerged mountain ranges that rise above the surface of the sea. The Palau group, Yap, Fais, and Sorol, at least, are elevated portions of great ridges that are wrinkles in the continental shelf and are similar in origin to the areas nearer Asia, such as Japan and the Ryukyus. All these islands are west of the sial or andesite line, which marks the edge of the continental shelf. They are recent elevation, and the process of elevation is still going on, as is proved by recently elevated marine terraces and by earthquakes. The islands of the eastern Caroline Islands stand on a great submarine shelf that is fairly stable, although there may be some subsidence in the western portion. Truk shows signs of subsidence whereas Ponape and Kusaie do not.

There are three main types of islands: the high volcanic islands, the low coral atolls, and raised atolls. Each of these types presents its own peculiar setting as a home for man. There are five volcanic islands or island groups: the Palaus, Yap, Truk, Ponape, and Kusaie.

The Palaus and Yap are composed of recent volcanic lavas and ancient metamorphic rock that testify to their continental nature. Truk, Ponape, and Kusaie are composed chiefly of basaltic lavas. Some of these volcanic islands have elevations of more than 2000 feet above sea level. The low coral atolls seldom rise to more than 6 or 8 feet above sea level. The partly raised atolls, such as Fais and Angaur, have elevations up to about 60 feet and usually contain phosphate deposits.