GWD-7-23

leeon

Q22 to Q25:

      Earth’s surface consists of rigid

       plates that are constantly shifting and

       jostling one another.  late 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.

Q23:

It can be inferred from the passage that a chain of volcanoes created by a mantle plume would most likely be characterized by

              

1. a curved outline
   
2. constituent volcanoes that differ from each other in age
   
3. occurrence near a plate boundary where one plate bends under another
   
4. appearance near many other volcanic chains
   
5. rocks with a wide range of chemical composition

leeon

看过讨论稿的答案是B，不同意，我认为答案应该是A。B错误因为原文没有提到differ from each other in age ，A正确，因为44行的第二种机理所造成的火山的特征hot spot—— isolated region of volcanoes and uplifted terrain对应A原文改写。

philikittist

你换了蓝颜色那段不是说明，这些火山是在长期形成的吗，所以different ages呀

A里面的curved，火山都是有这个特点吧。

leeon

可是在长期形成的并不能说明differ from each other in age ，这段火山由第二个机理即a mantle plume 形成得特征是hot spot，题干刚好也是问这个特征。所以答案应该是A.

philikittist

我觉得是这样的：不知道你见过缝纫机没有，呵呵，那根针只是上下动，底下的布向前移动，就能缝出一条线。线的前端就是早缝上去的，后端就是晚缝上去的。这个和地下一个固定的熔岩灶上面慢慢移动的板块道理一样。只有一个灶点，不可能在同一个时间形成一条链。顺着板块移动的方向，火山的年龄越来越大。

A：链的轮廓是curved？这个没有道理。

关键什么是chain的outline？指的是你从空中鸟瞰这条链，看到的形状。

链的outline是由板块相对灶点移动的方向决定的。

如果板块是直着移动，outline就是straight；板块是有角度的移动，outline才有可能是curved。文章没有说板块移动的方向，无从判断是不是curved。

leeon

比喻的好。把布当成上面漂浮的板块，hot spot下面的热源保持不动，文章中说了。hot spot的特征是uplifted terrain，所以板块不断移动，板块上面就不断的形成hot spot，有很多的突起。这个就是特征。A说是曲线的轮廓符合这一特征。

philikittist

我觉得你对outline of Chain 的理解不对。是火山都是突起吧。这个outline不是说你站在地上看的山包，是指从空中垂直 航拍，各个火山连成一线，这个线是弯是直。

leeon

这个不一定吧，我们看远处的山，不也是弯弯曲曲的轮廓吗，不一定非得从上往下看。再说也不一定是一条线的，板块的移动不会总是朝着一个方向，所以即使是从空中看，也应该是弯曲的，不是直线的。

leeon

还有就是选B太牵强了，通篇都没有提到过火山的年龄。文章最后都是由火山的特征来推断板块的漂移，说明是研究火山的形状的，弯弯曲曲的刚好说明了板块漂移的路线。

philikittist

咔咔，证据在此：.  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.  

ok，leeon要是非说outline要是指从地面上看的轮廓，偶认了。但是你看提干：most likely be characterized by········

天底下的群山都是弯弯曲曲的，那这个curved outline怎么就最能表现这种特殊火山链的特征呢？

B不牵强呀，这个chain是漫长的岁月中，随板块移动形成的，当然年龄不同。上面那段话就是证据。