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The basic theory of plate tectonics recognizes two ways

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The basic theory of plate tectonics recognizes two ways  [#permalink]

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New post Updated on: 26 Sep 2017, 07:49
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Continental Margins


The basic theory of plate tectonics recognizes two ways continental margins can grow seaward. Where two plates move away from a midocean rift that separates them, the continental margins on those plates are said to be passive. Such continental margins grow slowly from the accumulation of riverborne sediments and of the carbonate skeletons of marine organisms. Since most sequences of such accretions, or miogeoclinal deposits, are unreformed, passive margins are not associated with mountain building.

Along active margins continents tend to grow much faster. At an active margin an oceanic plate plunges under a continental plate, fragments of which then adhere to the continental margin. The process is met with extensive volcanism and mountain building. A classic example is the Andes of the west coast of South America.

In the original plate-tectonic model western North America was described as being initially passive and then active. It was assumed that the continent grew to a limited extent along this margin as oceanic rocks accreted in places such as the Coast Ranges of California. The model was successful in explaining such disparate features as the Franciscan rocks of the California Coast Ranges, created by subduction, and the granite rocks of the Sierra Nevada that originated in volcanoes.

The basic plate-tectonic reconstruction of the geologic history of western North America remains unchanged in the light of microplate tectonics, but the details are radically changed. It is now clear that much more crust was added to North America in the Mesozoic era than can be accounted for by volcanism and by the simple accretion of sediments. Further, some adjacent terranes are not genetically related, as would be expected from simple plate tectonics, but have almost certainly travelled great distances from entirely different parts of the world.

1. Which one of the following best expresses the main idea of the passage?

(A) The margin of the west coast of North America developed through a combination of active and passive mechanisms.
(B) The growth of continental margins is only partially explained by the basic theory of plate tectonics.
(C) Continental margins can grow seaward in two ways, through sedimentation or volcanism.
(D) The introduction of microplate tectonics poses a fundamental challenge to the existing theory of how continental margins are formed.
(E) Continental margins grow more rapidly along active margins than along passive margins.


2. The passage supplies information for answering all of the following questions regarding continental margins EXCEPT:

(A) How have marine organisms contributed to the formation of passive continental margins?
(B) What were some of the processes by which the continental margin of the west coast of North America was formed?
(C) Are miogeoclinal deposits associated with mountain building along continental margins?
(D) How was the continental margin of the west coast of South America formed?
(E) How much crust added to North America in the Mesozoic era can be accounted for by the accretion of sediments from the ocean floor?


3. The author mentions the Franciscan rocks of the California Coast Ranges in order to make which one of the following points?

(A) The basic theory of plate tectonics accounts for a wide variety of geologic features.
(B) The original plate tectonic model falls short of explaining such features.
(C) Subduction processes are responsible for the majority of the geologic features found along the west coast of North America.
(D) Passive margins can take on many geologic forms.
(E) The concept of microplate tectonics was first introduced to account for such phenomena.


4. Which one of the following does the author mention as evidence for the inadequacy of the original plate tectonic model to describe the formation of continental margins?

(A) Nearly flat, unreformed crystal blocks have been found along some continental margins where there are mountains further inland.
(B) Sediments and fragments from the depths of the ocean accumulate along continental margins.
(C) Large pieces of the Earth's crust that appear to be completely unrelated are found in the same area today.
(D) Unreformed miogeoclinal deposits are usually not linked to mountain building.
(E) Oceanic plates drop beneath continental plates along active margins.


Originally posted by Nikkb on 26 Sep 2017, 03:51.
Last edited by broall on 26 Sep 2017, 07:49, edited 1 time in total.
Reformatted question
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Re: The basic theory of plate tectonics recognizes two ways  [#permalink]

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New post 12 Sep 2018, 22:43
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Re: The basic theory of plate tectonics recognizes two ways  [#permalink]

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New post 12 Sep 2018, 23:06
Can anyone help with Question no. 4?

Please give evidences from the passage as well..

Thanks :thumbup:
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New post 13 Sep 2018, 00:34
not sure of the ans for Q4.
can someone please help explain what the last sentence of the passage means?
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Re: The basic theory of plate tectonics recognizes two ways  [#permalink]

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New post 13 Sep 2018, 09:05
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Q1.
A- Partially correct(only final para deals with this).
C- Because its passive/active.
D- The passage does not mention any challenge. It indeed supports the traditional theories.
E- Out of scope.
Correct answer - B

Q2.
A - It is covered. Such continental margins grow slowly from the accumulation of riverborne sediments and of the carbonate skeletons of marine organisms.
B- Para 3 answers this.
C- Last line para 1. Since most sequences of such accretions, or miogeoclinal deposits, are unreformed, passive margins are not associated with mountain building.
D- A classic example is the Andes of the west coast of South America.
Correct Answer - E

Q3.
B- The model was successful in explaining such disparate features. So wrong.
C- Subduction is for one feature. Not main point.
D- It had both active and passive.
E- Out of scope, it is not first.
Correct answer - A

Q4.
Last para:
It is now clear that much more crust was added to North America in the Mesozoic era than can be accounted for by volcanism and by the simple accretion of sediments. Further, some adjacent terranes are not genetically related, as would be expected from simple plate tectonics.
Correct answer: C
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Re: The basic theory of plate tectonics recognizes two ways  [#permalink]

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New post 13 Sep 2018, 09:58
Holy Lord.....9:15 for the entire passage.All correct.

So question 4 is basically asking "What is wrong with the original theory and what does the author give us as proof that it is wrong...?".

As with all RC's(especially when we have all 5 options) find the right answer instead of eliminating the wrong answers.

"Problem with the original theory" is touched upon only in the 4th Para.So go to the 4th para.

the 2nd sentence = " More crust was added than expected by the original theory(active+passive)"
Also 3rd sentence = "Rocks not related(genetically) were also found"

If you read the options you will quite clearly notice that C is a restatement of these 2 sentences.

Any other help with this passage...?
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Re: The basic theory of plate tectonics recognizes two ways  [#permalink]

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New post 13 Sep 2018, 10:13
rahulkashyap wrote:
not sure of the ans for Q4.
can someone please help explain what the last sentence of the passage means?


Further, some adjacent terranes are not genetically related, as would be expected from simple plate tectonics, but have almost certainly travelled great distances from entirely different parts of the world.

There is dissimilarity in the two terrains in spite of being present in the same area.
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New post 13 Sep 2018, 13:19
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Please update the source to "Kaplan 800".
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New post 14 Sep 2018, 00:27
rahulkashyap wrote:
not sure of the ans for Q4.
can someone please help explain what the last sentence of the passage means?



rahulkashyap,

the clue for the answer lies in the last lines of the passage -"Further, some adjacent terranes are not genetically related, as would be expected from simple plate tectonics, but have almost certainly travelled great distances from entirely different parts of the world."

Genetically not related refers to the geographical anomaly of having a piece of crust unrelated to its surroundings and "as would be expected from simple plate tectonics" refers to the inadequacy of the model.

Hope this helps..and if it does, please hit Kudos. workout, await your inputs.
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The basic theory of plate tectonics recognizes two ways  [#permalink]

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New post 24 Sep 2018, 07:44
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Notes:
1) Theory intro.
-2 ways margins grow
-passive

2) Active margins are faster

3) Model and stuff it explains
- model; NA - passive to active

4) Stuff model doesn't explain
-more crust

Prethinking: A theory is talked about and it is implied it doesn't completely explain stuff --> B wins

1. Which one of the following best expresses the main idea of the passage?

(A) The margin of the west coast of North America developed through a combination of active and passive mechanisms. very partial scope
(B) The growth of continental margins is only partially explained by the basic theory of plate tectonics. --> prethought
(C) Continental margins can grow seaward in two ways, through sedimentation or volcanism. -- partial scope
(D) The introduction of microplate tectonics poses a fundamental challenge to the existing theory of how continental margins are formed.
(E) Continental margins grow more rapidly along active margins than along passive margins. --> this is not the main idea though.. partial scope


Can anyone help with Q2 in detail? I couldn't choose b/w A and E. I chose A because of the quoted text below:

2. The passage supplies information for answering all of the following questions regarding continental margins EXCEPT:
B, C, D are eliminated easily

(A) How have marine organisms contributed to the formation of passive continental margins?
Where two plates move away from a midocean rift that separates them, the continental margins on those plates are said to be passive. Such continental margins grow slowly from the accumulation of riverborne sediments and of the carbonate skeletons of marine organisms.

(B) What were some of the processes by which the continental margin of the west coast of North America was formed?
(C) Are miogeoclinal deposits associated with mountain building along continental margins?
(D) How was the continental margin of the west coast of South America formed?


(E) How much crust added to North America in the Mesozoic era can be accounted for by the accretion of sediments from the ocean floor?
It is now clear that much more crust was added to North America in the Mesozoic era than can be accounted for by volcanism and by the simple accretion of sediments.
Clearly, we don't know how much crust was added. The passage literally says that. E wins.


4. Which one of the following does the author mention as evidence for the inadequacy of the original plate tectonic model to describe the formation of continental margins?
author talks about model not always explaining stuff in last para. let's go there and quickly rescan it.

The basic plate-tectonic reconstruction of the geologic history of western North America remains unchanged in the light of microplate tectonics, but the details are radically changed. It is now clear that much more crust was added to North America in the Mesozoic era than can be accounted for by volcanism and by the simple accretion of sediments. Further, some adjacent terranes are not genetically related, as would be expected from simple plate tectonics, but have almost certainly travelled great distances from entirely different parts of the world.

(A) Nearly flat, unreformed crystal blocks have been found along some continental margins where there are mountains further inland.
(B) Sediments and fragments from the depths of the ocean accumulate along continental margins.
(C) Large pieces of the Earth's crust that appear to be completely unrelated are found in the same area today. -->rewording of highlighted part
(D) Unreformed miogeoclinal deposits are usually not linked to mountain building.
(E) Oceanic plates drop beneath continental plates along active margins.

C wins
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Re: The basic theory of plate tectonics recognizes two ways  [#permalink]

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New post 04 Oct 2018, 11:58
workout Still not clear with q 1. Could you please help.


TIA
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Re: The basic theory of plate tectonics recognizes two ways  [#permalink]

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New post 08 Oct 2018, 19:43
workout could you explain this passage as a whole please??
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Re: The basic theory of plate tectonics recognizes two ways &nbs [#permalink] 08 Oct 2018, 19:43
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