GRE Reading Comprehension
Home > GRE Test > GRE Reading Comprehension Questions
Next steps
- Use your browser's back button to return to your test results.
- Do more GRE Reading Comprehension Questions.
Source: NO8
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.
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.