There are conflicting hypotheses about the heterogeneity of the Earths mantle because few mantle elements have been thoroughly studied.

　　 Further research is needed to resolve the debate among geologists over the composition of the midocean ridge system.

　　 There is clear-cut disagreement within the geological community over the structure of the Earths mantle.

　　 There has recently been a strong and exciting challenge to geologists long-standing belief in the heterogeneity of the Earths mantle.

　　18. According to the passage, it is believed that oceanic islands are formed from

　　 the same material as mantle xenoliths

　　 the same material as the midocean ridge system

　　 volcanic rocks from the upper mantle

　　 incompatible elements percolating up from the lower mantle

　　 mantle plumes arising from the lower mantle

　　19. It can be inferred from the passage that the supporters of the layered-mantle theory believe which of the following?

　　I. The volcanic rocks on oceanic islands are composed of material derived from the lower part of the mantle.

　　II. The materials of which volcanic rocks on oceanic islands and midocean ridges are composed are typical of the layers from which they are thought to originate.

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