nn请进:一篇有点难的文章

prfdu2005

    

好像是关于火山的两种形成方式。我分析了好久，还是不行：（  哪位好心nn 能花点时间看一下这篇文章，指点我一下其大意呢？小妹将不胜感激，先谢谢了

 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.

prfdu2005

有看了一遍，还是不懂。。。。nn快现身啊！