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# The geology of the Grand Canyon area exposes one of the most complete

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The geology of the Grand Canyon area exposes one of the most complete  [#permalink]

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Updated on: 16 Sep 2018, 17:25
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Question 1
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based on 224 sessions

42% (03:13) correct 58% (03:05) wrong

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Question 2
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43% (01:04) correct 57% (01:18) wrong

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Question 3
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45% (01:47) correct 55% (01:48) wrong

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The geology of the Grand Canyon area exposes one of the most complete and studied sequences of rock on Earth. The nearly 40 major sedimentary rock layers exposed in the Grand Canyon and in the Grand Canyon National Park area range in age from about 200 million to nearly 2 billion years old. Most were deposited in warm, shallow seas and near ancient, long-gone seashores in western North America. Both marine and terrestrial sediments are represented, including fossilized sand dunes from an extinct desert. There are at least 14 known unconformities (breaks indicating different eras of sedimentary deposits) in the geologic record found in the Grand Canyon area.

It is believed that uplift of the region that eventually led to this dramatic exposure of rock started about 75 million years ago during the Laramide orogeny, a mountain-building event that is largely responsible for creating the Rocky Mountains. In total, the Colorado Plateau, where the Grand Canyon is located, was uplifted an estimated 2 miles (3.2 km). The adjacent Basin and Range province to the west started to form about 18 million years ago as the result of crustal stretching. A drainage system that flowed through what is today the eastern Grand Canyon emptied into the now lower Basin and Range province. The opening of the Gulf of California because of seismic activity around 6 million years ago enabled a large river to cut its way northeast from the gulf. The new river captured the older drainage to form the ancestral Colorado River, which in turn started to form the Grand Canyon.

Wetter climates brought upon by ice ages starting 2 million years ago greatly increased excavation of the Grand Canyon, which was nearly as deep as it is now by 1.2 million years ago, when the river volume decreased at the end of that climatic period. Volcanic activity deposited lava over the area 1.8 million to 500,000 years ago. At least 13 lava dams blocked the Colorado River over that time period, forming lakes that were up to 2,000 feet (610 m) deep and slowing excavation of the canyon. The end of the last ice age and subsequent human activity has further reduced the ability of the Colorado River to deepen the canyon. It is unlikely that this amazing geological record will change substantially until major shifts in the river volume and/or direction take place.

Solution: A. Nearly everything in the answer choices is mentioned in the passage, but choice A goes a bit too extreme by saying "each layer" of the Grand Canyon. The passage does mentions layers but does not go into detail on each individual layer, so choice A is correct mainly because it's too emphatic a statement to prove to be mentioned.

Question 1. All of the following are discussed in the passage EXCEPT:

(A) the geological history of each of the exposed layers in the Grand Canyon
(B) unconformities in the geological record of the Grand Canyon
(C) important historical events in the formation of the Grand Canyon
(D) the depth and the size of lakes formed in the Grand Canyon
(E) events that affected the speed with which the Grand Canyon was formed

Solution: C. While volcanic activity is certainly mentioned as a factor in the development of the Grand Canyon, it is actually a factor that slowed the development of the canyon (see the middle of the third paragraph "slowing the excavation of..."), so it did not play a role in increasing the speed of the canyon's formation.

Question 2. According to the passage, all of the following played a role in increasing the speed with which the Grand Canyon was formed EXCEPT:

(A) the climate
(B) seismic activity
(C) volcanic activity
(D) river volume
(E) changes to bodies of water

Solution: C. The evidence for choice C can be found in the last few sentences of the second paragraph, with the sequence:

seismic activity --> large river formed --> older drainage formed the Colorado River --> which in turn formed the Grand Canyon.

Choice A can be tricky; the passage does state that the excavation rate was quite high 6 million years ago and is low today, but that leaves a million year gap from 6 million years ago to 5 million, so we cannot guarantee that 5 million years ago was also a period of any excavation. Just as today is a relatively dormant period, so too 5 million years ago could have been.

Choice D is also a bit tricky, but be careful with precision-in-language. The third paragraph states that the Grand Canyon was "nearly" as deep 1.2 million years ago as it is today, meaning that "some" excavation must have (or at least very well could have) happened in that time.

Choice B can also be tricky because of precise language in the question. We're told in paragraph three that volcanic activity did occur and deposited lava that created lakes in the region, but those lava flows actually slowed the rate of excavation, so the lava wasn't important in the "excavation" of the canyon even if it played an important role in the canyon's overall history.

Question 3. It can be inferred from the passage that:

(A) the excavation rate of the Grand Canyon by the Colorado River is lower today than it was 5 million years ago.
(B) volcanic activity played an important role in the excavation of the Grand Canyon.
(C) the Grand Canyon was formed in part as a result of seismic activity
(D) there has been no excavation of the Grand Canyon by the Colorado River over the past 1.2 million years
(E) there are more marine layers than terrestrial layers exposed in the sediments of the Grand Canyon

Originally posted by pulkitaggi on 29 Jun 2017, 01:12.
Last edited by hazelnut on 16 Sep 2018, 17:25, edited 4 times in total.
Reformatted question
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Re: The geology of the Grand Canyon area exposes one of the most complete  [#permalink]

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29 Jun 2017, 15:09

The third sentence in the last paragraph says that 'at least 13 lava dams blocked the Colorado River... slowing excavation of the canyon'. So it makes sense to say that while most of the other factors helped or had a positive effect on the formation or excavation of Grand Canyon, volcanic activity did not. This is also what makes Choice (C) right for Q2.

Why is it then wrong to infer that volcanic activity played an important role in the excavation of the Grand Canyon? I'm confused why Choice (C) wins over Choice (B) in Q3. What am I missing in my approach, and how do I refrain from making the same error again? Thanks in advance for your guidance.
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Re: The geology of the Grand Canyon area exposes one of the most complete  [#permalink]

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01 Jul 2017, 00:40
1
This passage contains so much of data that it is very difficult to complete it in short span of time.
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Re: The geology of the Grand Canyon area exposes one of the most complete  [#permalink]

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29 Aug 2017, 09:28
swapnak

If you read the passage carefully, it actually suggests that volcanic activity slowed the formation of the canyon, rather than play a major role in excavating it. This is also why the answer to Q2 is C.

"Volcanic activity deposited lava over the area 1.8 million to 500,000 years ago. At least 13 lava dams blocked the Colorado River over that time period, forming lakes that were up to 2,000 feet (610 m) deep and slowing excavation of the canyon."
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Re: The geology of the Grand Canyon area exposes one of the most complete  [#permalink]

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29 Aug 2017, 10:10
Question 3) It can be inferred from the passage that:

A) the excavation rate of the Grand Canyon by the Colorado River is lower today than it was 5 million years ago.
B) volcanic activity played an important role in the excavation of the Grand Canyon.
C) the Grand Canyon was formed in part as a result of seismic activity
D) there has been no excavation of the Grand Canyon by the Colorado River over the past 1.2 million years
E) there are more marine layers than terrestrial layers exposed in the sediments of the Grand Canyon

I was able to eliminate B,E easily.
Please suggest the excerpts from passage which helped arrived at C.
I marked A
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Re: The geology of the Grand Canyon area exposes one of the most complete  [#permalink]

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29 Aug 2017, 23:40
GmatDM2016

For Q3, C stood out to me the most, while the rest felt a little iffy. Here's what led me to think C is the best answer: "The opening of the Gulf of California because of seismic activity around 6 million years ago enabled a large river to cut its way northeast from the gulf" from P2.

How long did everyone else take to complete this passage - from reading the passage to answering all the questions?
Took me just a little under 8 min... And only got 2/3.
This passage was dense.
I was left with nothing when I initially skimmed the passage.
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Re: The geology of the Grand Canyon area exposes one of the most complete  [#permalink]

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30 Aug 2017, 11:08
rahulagrawal51 wrote:
ACC

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Can some one help me to eliminate option D in the first question.
Option D gives information on the depth but not about the size ,why can't this be the answer.
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Re: The geology of the Grand Canyon area exposes one of the most complete  [#permalink]

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30 Aug 2017, 13:32
sandeep211986

Question 1 is an EXCEPT question :

Since the passage talks about the depth and size of lakes in last paragraph - hence we can eliminate D
"At least 13 lava dams blocked the Colorado River over that time period, forming lakes that were up to 2,000 feet (610 m) deep and slowing excavation of the canyon."

Hope this helps !
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Re: The geology of the Grand Canyon area exposes one of the most complete  [#permalink]

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30 Aug 2017, 13:53
HappyQuakka Thanks for your response. I did this question again today un-timed and was able to get to C.
Initially, I took around 7 mins and I agree the passage was really dense. But Gmat passages are never simple !

Below is my analysis for question 3 , it would be great if an expert can affirm-

Question 3. It can be inferred from the passage that:

A) the excavation rate of the Grand Canyon by the Colorado River is lower today than it was 5 million years ago.
We don't know. From passage we just know that GC was formed 6 million years ago and then increased excavation 2 mn and 1.2 mn years ago.
We cant say anything about the excavation rate is lower today.

B) volcanic activity played an important role in the excavation of the Grand Canyon.
Its opposite actually according to the passage - "Volcanic activity deposited lava over the area 1.8 million to 500,000 years ago. At least 13 lava dams blocked the Colorado River over that time period, forming lakes that were up to 2,000 feet (610 m) deep and slowing excavation of the canyon"

C) the Grand Canyon was formed in part as a result of seismic activity
YES -
The opening of the Gulf of California because of seismic activity ...........................................gulf. The new river captured the older drainage .................. which in turn started to form the Grand Canyon.

D) there has been no excavation of the Grand Canyon by the Colorado River over the past 1.2 million years
No data available to support this

E) there are more marine layers than terrestrial layers exposed in the sediments of the Grand Canyon
No data available to support this

Hence C
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Re: The geology of the Grand Canyon area exposes one of the most complete  [#permalink]

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01 Oct 2018, 04:04
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Re: The geology of the Grand Canyon area exposes one of the most complete   [#permalink] 01 Oct 2018, 04:04
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