Geologists have long known that the Earth's mantle is heterogeneous, but its spatial arrangement remains unresolved -is the mantle essentially layered or irregularly heterogeneous? The best evidence for the layered mantle thesis is the well-established fact that volcanic rocks found on oceanic islands, islands believed to result from mantle plumes arising from the lower mantle, are composed of material fundamentally different from that of the midocean ridge system, whose source, most geologists contend, is the upper mantle.

Some geologists, however, on the basis of observations concerning mantle xenoliths, argue that the mantle is not layered, but that heterogeneity is created by fluids rich in "incompatible elements" (elements tending toward liquid rather than solid-state) percolatIng upward and transforming portions of .the upper mantle irregularly, according to the vagaries of the fluids' pathways. We believe, perhaps unimaginatively, that this debate can be resolved through further study, and that the underexplored midocean ridge system is the key.

17. Which of the following best expresses the main idea of the. passage?

(A) Current theories regarding the structure of the Earth's mantle cannot account for new discoveries regarding the composition of mantle xenoliths.
(B) There are conflicting hypotheses about the heterogeneity of the Earth's mantle because few mantle elements have been thoroughly studied.
(C) Further research is needed to resolve the debate among geologists over the composition of the midocean ridge system.
(D) There is clear-cut disagreement within the geological community over the structure of the Earth's mantle.
(E) There has recently been a strong and exciting challenge to geologists' long-standing belief in the heterogeneity of the Earth's mantle.



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

(A) the same material as mantle xenoliths
(B) the same material as the mid-ocean ridge system
(C) volcanic rocks from the upper mantle
(D) incompatible elements percolating up from the lower mantle
(E) 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 mid-ocean ridges are composed are typical of the layers from which they are thought to originate.
III. The differences in composition between volcanic rocks on oceanic islands and the midocean ridges are a result of different concentrations of incompatible elements.

(A) I only
(B) III only
(C) I and II only
(D) II and III only
(E) I, II, and III



20. The authors suggest that their proposal for determining the nature of the mantle's heterogeneity might be considered by many to be

(A) pedestrian
(B) controversial
(C) unrealistic
(D) novel
(E) paradoxical