iron is abundant on the ancient continents, but not so on island chains formed by volcanoes (japan, hawaii). that reflects the difference between the continental crust (which has many rare and common elements), and the island chains formed by volcanism (oceanic islands). they have basalt, and very little else. islands which were once part of a supercontinent usually carry heavy element ore. the classic example is the british isles, which are a snapped-off part of the large ancient continent, the old red sandstone continent (larussia). consequently, the british isles has a very wide range of metallic ores, whereas hawaii and the japanese archopelago do not.