GWD7-23

xurui0457

   Earth’s surface consists of rigid

      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 surface

plate.  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.

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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

E.   rocks with a wide range of chemical composition

Answer:

请nn们帮我说说这个定位在哪呀?

加拿大帅哥

L48-50

wu41385621

ding

Brilliance

。