GRE Reading Comprehension
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 mid-ocean 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 mid-ocean ridge system is the key.
Question List: 2
It can be inferred from the passage that the supporters of the layered-mantle theory believe which of the following?
- A The volcanic rocks on oceanic islands are composed of material derived from the lower part of the mantle.
- B 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.
- C The differences in composition between volcanic rocks on oceanic islands and the mid-ocean ridges are a result of different concentrations of incompatible elements.
Correct Answer: AB