| Abstract: |
Introduction: The lower parts of oceanic high islands are generally composed of tholeiite--a basalt characterized by relatively low potassium and relatively high aluminum content. They are petrographically similar to oceanic floor material and may or may not also contain serpentinites probably formed by hydration of tholeiitic material during tectonic movements. The higher parts of oceanic high islands are more commonly alkali basalts, which are somewhat lower in silica content and appreciably richer in titanium, sodium, and potassium than tholeiites. Often further fractionation of the parent magma has given rise to lavas of even higher alkali content. Such rocks as rhyolites, trachytes, and phonolites are found on high islands as a result.
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