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Earth's surface consists of rigid plates that are [#permalink]
10 Feb 2005, 18:06
Earthâ€™s surface consists of rigid plates that are constantly shifting and
jostling one another. Plate movements are the surface expressions of motions in the mantleâ€”the thick shell of rock that lies between Earthâ€™s crust and its metallic core. Although the hot rock of the mantle is a solid, under the tremendous pressure of the crust and overlying rock of the mantle, it flows like a viscous liquid. The mantleâ€™s motions, analogous to those in a pot of boiling water, cool the mantle by carrying hot material to the surface and returning cooler material to the depths. When the edge of one plate bends under another and its cooler material is consumed in the mantle, volcanic activity occurs as molten lava rises from the downgoing plate and erupts through the overlying one.
Most volcanoes occur at plate boundaries. However, certain â€œmisplacedâ€
I am by no means an authority is RC, but here is my 2 cents.
only reference to Hawaiian islands we have in the passage is
"The natural history of the Hawaiian island chain clearly shows the movement of the Pacific plate over a fixed plume".
in other words, the Author is using Hawaiian islands to explain a particular phenomenon. No where he mentions that this type of activity doesn't occur else where in the world. Hence choice A is wrong
chunjuwu I got your message, here is what I feel about Q 23
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
A is indeed very tempting. But it is out of context. Passage says
"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 of volcanoes or volcanic islands, a track marking the position of the plume relative to the moving plate."
earlier in the passage it also states
"When the edge of one plate bends under another and its cooler material is consumed in the mantle, volcanic activity occurs as molten lava rises from the downgoing plate and erupts through the overlying one."
that means here emphasis is on the fact that hot spot remains fixed and plates move over it. When the hot spot hits the edges of the plates lava comes out. That means there is a time gap in between volcanos. Hence choice B
Even though the passage states "a track marking the position of the plume relative to the moving plate" it doesn't necessarily mean it is a curved outline because we don't really know how the plates move.
A is irrelevant
You know that it's B because of this part "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 of volcanoes "