Geologists have long known that the
Earth's mantle is heterogeneous, but its
spatial arrangement remains unresolved—
is the mantle essentially layered or
(5) 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
(10) 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.
(15) 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
(20) elements" (elements tending toward
liquid rather than solid state)
percolating upward and transforming
portions of the upper mantle
irregularly, according to the vagaries
(25) 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.
19. It can be inferred from the passage that thesupporters of the “layered-mantle” theory believe which of thefollowing?
I. The volcanicrocks on oceanic islands are composed of material derived from thelower part of the mantle.
II. The materialsof whichvolcanic rocks on oceanic islands and midocean ridges arecomposedare typical of the layersfrom which they are thought tooriginate.
III. The differences in composition betweenvolcanic rocks on oceanic islands and the midocean ridges are aresult 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