plates that are constantly shifting and
jostling one another. Plate movements Line are the surface expressions of motions (5) in the mantle—the thick shell of rock
that lies between Earth’s crust and its
metallic core. Although the hot rock of
the mantle is a solid, under the tre-
mendous pressure of the crust and (10) overlying rock of the mantle, it flows like
a viscous liquid. The mantle’s motions,
analogous to those in a pot of boiling
water, cool the mantle by carrying hot
material to the surface and returning (15) cooler material to the depths. When
the edge of one plate bends under
another and its cooler material is con-
sumed in the mantle, volcanic activity
occurs as molten lava rises from the (20) downgoing plate and erupts through the
overlying one.
Most volcanoes occur at plate
boundaries. However, certain “mis-
placed” volcanoes far from plate
(25) edges result from a second, indepen-
dent mechanism that cools the deep
interior of Earth. Because of its prox-
imity to Earth’s core, the rock at the
base of the mantle is much hotter than (30) rock in the upper mantle. The hotter the
mantle rock is, the less it resists flow-
ing. Reservoirs of this hot rock collect
in the base of the mantle. When a
reservoir is sufficiently large, a sphere (35) of this hot rock forces its way up
through the upper mantle to Earth’s
surface, creating a broad bulge in the
topography. The “mantle plume” thus
formed, once established, continues to (40) channel hot material from the mantle
base until the reservoir is emptied.
The surface mark of an established plume is a hot spot—an isolated
region of volcanoes and uplifted terrain (45) located far from the edge of a surfaceplate. Because the source of a hot
spot remains fixed while a surface
plate moves over it, over a long period
of time an active plume creates a chain
(50) of volcanoes or volcanic islands, a
track marking the position of the plume
relative to the moving plate. The natural
history of the Hawaiian island chain
clearly shows the movement of the
Pacific plate over a fixed plume.
Q23:
It can be inferred from the passage that a chain of volcanoes created by a mantle plume would most likely be characterized by
A a curved outline
B constituent volcanoes that differ from each other in age
C occurrence near a plate boundary where one plate bends under another
D appearance near many other volcanic chains
