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.

Caroline Islands Climate - Temperatures

Although there are slight climatic variations over this large area, the low latitude and oceanic location combine to produce uniform conditions at any one station during the entire year. Although the climate is oceanic in character, it is dominated by the northeast trades except in the area west of about 145° east longitude, where the effect of the Asiatic monsoon is felt during the summer months. From November to April the prevailing winds over the whole area are from the northeast or east. In the northern summer, the southeast trades are dominant over the southern Carolines and occasionally reach as far northward as the Marianas.

Typhoons, or tropical cyclones, do considerable damage in the Carolines. They can occur in any month, but they are most common from July through November, with the highest frequency in September. There is an average of 25 typhoons each year. Originating in the Carolines and Marianas, they may occur anywhere west of 160° east longitude and 5° north latitude. Although they are commonest between Yap and Guam, at rare intervals one strikes as far east as the Gilberts.

In general the rainfall is heaviest in a belt between 1° 30′ and 8° 30′ north latitude, which marks the zone where the northeast and southeast trades meet. The rainfall is usually over 120 inches in this belt. However, there is orographic influence on the high islands that results in heavier rainfall on the windward side. The 2000-foot mountains of Kusaie produce as much as 255 inches on their windward (west) side whereas on the leeward side the annual rainfall is only 177 inches. The low coral atolls do not produce this orographic effect and are much drier.

Temperatures are uniform throughout the area. The mean monthly temperatures are seldom less than 79° F or more than 83° F. The difference between the means of the summer and winter months is rarely more than 2°. The daily range usually amounts to about 9° or 10° F. Day temperatures vary from 83° to 89° F, and night temperatures from 74° to 77° F. The mean relative humidity is high throughout the year. The early morning relative humidity varies from 85 to 95 per cent; the early afternoon readings may vary from 70 to 80 per cent. There is little variation in the humidity from month to month.





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SOILS
Great contrast exists between the soils of the high volcanic islands and those of the low coral islands. On the coral isles the soils consist of group-up coral and shells with an accumulation of humus near the surface. This is seldom very thick and is speedily lost under cultivation. These soils do not retain moisture well as the rainfall quickly soaks through the sand and is lost in the porous coral beneath.

There is greater variation on the volcanic islands. The parent material is more varied, and there is an accumulation of fairly fertile alluvium on the coastal plains and in the stream valleys. However, the frostless year with high rainfall is favorable to leaching and alluviation, with the result that tropical laterites or lateritic soils with little fertility are common.