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

Question

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.   1 
 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

2 
 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

3 
 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

swapnak · Answer

Official Explanation

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:  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 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 . 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 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

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.

Harsh9676 · Answer

Hi

I got 2 correct. Can some one explain the reason to eliminate Answer choice E in Question 2. (Changes to bodies of water).

Thanks in advance.
Harsh

bM22 · Answer

Hi  Harsh9676 , 
 
The following lines from the 2nd paragraph : "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" ,

and the last lines of the passage: "It is unlikely that this amazing geological record will change substantially until major shifts in the river volume and/or direction take place.", help eliminate Option E.

Let me know if this helps.
Thanks.

MohdSaqib123 · Answer

I still can't understand Q.3 ...how 'B' can't be inferred. It is stated that VE played an important role(which can be a +ve role, a -ve role.... it can be any kind of role) but that role needs to be important.. which it is. Then why 'B' can't be inferred.. Can anyone explain?

elavendan1 · Answer

The result lies in your response. When answering Inference questions, always look for unstated ideas. In a way, both A and B are already mentioned in the passage. So the correct option is C, which is not stated in the passage, and we do have to infer the idea.

Hope this helps!

MPRS22 · Answer

Experts, the passage only refers to the depth of lakes not their sizes. How can it not be D?

Thanks

AndrewN   VeritasKarishma

AndrewN · Answer

I think you are talking about Question 1,  MPRS22 . A close look at the second and third paragraphs of the passage reveals that both lake depth and size are mentioned, even if the latter is more indirect, without numbers attached:

We are to understand from the end of the second paragraph that the flow of the Colorado River system, the one that combined  the new  large ] river  with  older drainage , was powerful enough to start  to form the Grand Canyon . I suppose the question takes some liberties in that nowhere does it mention the size of the canyon itself; however, these 13 lava dams were apparently large enough to block the flow of this canyon-forming river, so a large volume of water must have been confined to these lakes. The references to  river volume  in the first and last sentences of the final paragraph only help us to appreciate the relationship between the large volume of water in the river and the large volume of the water that would have been deposited  by  the river into the lakes at one time. That takes a lot of work, but anyway, (A) is way easier to disprove:

The specificity of the answer choice works against it. There is a reference at the end of the first paragraph to  at least 14 known unconformities (breaks indicating different eras of sedimentary deposits) in the geologic record found in the Grand Canyon area , but we cannot say that  each  of these 14 layers is discussed anywhere in the passage, not to mention other layers that could be included within that  at least  part. So, rather than get caught up in an answer you know is at least half correct in (D), you will find an easier approach in going with something you  know  is wrong, through and through. (A) fits our EXCEPT condition.

I hope that helps. It is a tricky question, but one that can be broken down, nonetheless.

- Andrew

stutim96 · Answer

GMATNinja, e-gmat Can you please help with Q2 and Q3?

MBAB123 · Answer

stutim96 , See if this helps - Q2 - Volcanic activity actually slowed the excavation of the grand canyon. I'm quoting an extract of the last passage where the answer lies - "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. " Q3 - "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." A series of events, including seismic activity, led to the formation of the grand canyon. Hence, option C! I think I've understood the passage well so let me know if you'd like to discuss any other option!.

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

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