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# This topic has 27 member replies

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rahul.s Master | Next Rank: 500 Posts
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#### Primary purpose, again

Thu Jan 21, 2010 7:31 am
Earth's surface consists of rigid plates that are constantly shifting and jostling one another. Plate movements are the surface expressions of motions 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 tremendous pressure of the crust and 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 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 down going plate and erupts through the overlying one. Most volcanoes occur at plate boundaries. However, certain 'misplaced' volcanoes far from plate edges result from a second, independent mechanism that cools the deep interior of Earth. Because of its proximity to Earth's core, the rock at the base of the mantle is much hotter than rock in the upper mantle. The hotter the mantle rock is, the less it resists flowing. Reservoirs of this hot rock collect in the base of the mantle. When a reservoir is sufficiently large, a sphere 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 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 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 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.

The passage is primarily concerned with discussing

A) the composition of Earth's mantle
B) how the Hawaiian Islands were created
C) what causes Earth's surface plates to move
D) two different mechanisms by which volcanoes are formed
E) why most volcanoes occur at plate boundaries

Last edited by rahul.s on Thu Jan 21, 2010 10:59 am; edited 2 times in total

sreak1089 Master | Next Rank: 500 Posts
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Thu Jan 21, 2010 7:54 am
Is it C?

rahul.s Master | Next Rank: 500 Posts
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Thu Jan 21, 2010 10:57 am
sreak1089 wrote:
Is it C?
Nope, the OA is D

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Thu Jan 21, 2010 12:08 pm
The answer here is D, because the text has two distinctive sections, discussing two different processes:
- volcanic activity molten lava rises from the down going plate and erupts through the overlying one
- volcanoes that appear far away from plate intersections, in the "mantle plume" situation

A is incorrect because the composition of the mantle is not discussed, rather its movements. B is also incorrect because the islands are only mentioned once as an example C is wrong because these surface plates are only discussed in the first part of the text. The second concerns that mantle plume situation, which is not a part of the "surface plates" topic. The last answer choice is similarly wrong, because this is only mentioned in the first part of the text.

rahul.s Master | Next Rank: 500 Posts
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Fri Jan 22, 2010 7:29 am
Thanks Dana.

It's a difficult one though

bhumika.k.shah Legendary Member
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Sat Jan 23, 2010 6:22 am
D it is.

tanviet Legendary Member
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Sun Jan 24, 2010 4:02 am
first time reading for 1,2 minute gives us structure of passage if you read the details just to understand PURPOSE OF DETAILS. we can not understand details, here, a complicated scientific process, for 1, or 2 minutes of reading.

gmatmachoman Legendary Member
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Tue Jan 26, 2010 1:46 am
DanaJ wrote:
The answer here is D, because the text has two distinctive sections, discussing two different processes:
- volcanic activity molten lava rises from the down going plate and erupts through the overlying one
- volcanoes that appear far away from plate intersections, in the "mantle plume" situation

A is incorrect because the composition of the mantle is not discussed, rather its movements. B is also incorrect because the islands are only mentioned once as an example C is wrong because these surface plates are only discussed in the first part of the text. The second concerns that mantle plume situation, which is not a part of the "surface plates" topic. The last answer choice is similarly wrong, because this is only mentioned in the first part of the text.

1. volcanic activity occurs as molten lava rises from the down going plate and erupts through the overlying one. Most volcanoes occur at plate boundaries. 2. However, certain 'misplaced' volcanoes far from plate edges result from a second, independent mechanism that cools the deep interior of Earth.

IMO D

guga3008 Newbie | Next Rank: 10 Posts
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Empirestateofmind Senior | Next Rank: 100 Posts
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Thu Feb 17, 2011 1:46 am
Passage is quite difficult. My answer is D

Warlock007 Senior | Next Rank: 100 Posts
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Sun Jul 03, 2011 11:20 am
D Seems to be most convincing answer as it covers the passage as a whole

primary purpose questions mostly needs skimming
the whole passage talks about the mechanisms of volcanoes...

smackmartine Legendary Member
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Sun Jul 03, 2011 12:46 pm
IMO D
For guys who think that this passage is tough,I have one suggestion..
Details fill our mind with garbage and blocks our mind from thinking the overall idea(why the passage is written??) TO AVOID THIS SITUATION, AVOID THESE DETAILS. Just know WHERE they are.

While you are looking for primary concern / purpose, don't get caught in details. Concentrate on buzz words rather.Of coz 1st 2-3 lines are very important to understand what the author is talking about. IN MIND ,put a spoiler over rest of the details !!!. lets see what I mean.

Earth's surface consists of rigid plates that are constantly shifting and jostling one another. Plate movements are the surface expressions of motions 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 tremendous pressure of the crust and 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 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 down going plate and erupts through the overlying one. Most volcanoes occur at plate boundaries. However, certain 'misplaced' volcanoes far from plate edges result from a second, independent mechanism that cools the deep interior of Earth. Because of its proximity to Earth's core, the rock at the base of the mantle is much hotter than rock in the upper mantle. The hotter the mantle rock is, the less it resists flowing. Reservoirs of this hot rock collect in the base of the mantle. When a reservoir is sufficiently large, a sphere 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 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 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 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.

The bold part indicates that 1st part of the passage is talking about Another type of Mechanism. After the bold part the passage just explains the other mechanism(which we don't want for main purpose question).

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immaculatesahai Senior | Next Rank: 100 Posts
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Thu Dec 15, 2011 1:53 am
Nice passage. Very much like GMAT passages that we are likely to see in the exam.

D wins. Only choice that caused a bit of confusion was B, because the author refers to Hawaiian islands towards the end of the passage. But I reminded myself, that bulk of the passage is not talking about the formation of the hawaiian islands.

D represents the overall theme of the passage.

mourinhogmat1 Senior | Next Rank: 100 Posts
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Sat Dec 31, 2011 3:01 am
I was stuck between D and E. But the differentiating point is the word MOST VOLCANOES. That gives the difference away.

happymanocha Senior | Next Rank: 100 Posts
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Sat Dec 31, 2011 2:58 pm
Answer is D. Passage is primarily divided into two sections. Second section explains the volcano activity which happens far from the plate edges and describes how it happens. First section explains a similar activity in reverse order. Describes the method first and then mention it as a volcano activity.

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