Until the 1980s, most scientists believed that noncatastrophic

Question

Until the 1980s, most scientists believed that noncatastrophic geological processes caused the extinction of dinosaurs that occurred approximately 66 million years ago, at the end of the Cretaceous period. Geologists argued that a dramatic drop in sea level coincided with the extinction of the dinosaurs and could have caused the climatic changes that resulted in this extinction as well as the extinction of many ocean species.

This view was seriously challenged in the 1980s by the discovery of large amounts of iridium in a layer of clay deposited at the end of the Cretaceous period. Because iridium is extremely rare in rocks on the Earth’s surface but common in meteorites, researchers theorized that it was the impact of a large meteorite that dramatically changed the earth’s climate and thus triggered the extinction of the dinosaurs.

Currently available evidence, however, offers more support for a new theory, the volcanic-eruption theory. A vast eruption of lava in India coincided with the extinctions that occurred at the end of the Cretaceous period, and the release of carbon dioxide from this episode of volcanism could have caused the climatic change responsible for the demise of the dinosaurs. Such outpourings of lava are caused by instability in the lowest layer of the Earth’s mantle, located just above the Earth’s core. As the rock that constitutes this layer is heated by the Earth’s core, it becomes less dense and portions of it eventually escape upward as blobs or molten rock, called “diapirs,” that can, under certain circumstances, erupt violently through the Earth’s crust.

Moreover, the volcanic-eruption theory, like the impact theory, accounts for the presence of iridium in sedimentary deposits; it also explains matters that the meteorite-impact theory does not. Although iridium is extremely rare on the Earth’s surface, the lower regions of the Earth’s mantle have roughly the same composition as meteorites and contain large amounts of iridium, which in the case of a diapir eruption would probably be emitted as iridium hexafluoride, a gas that would disperse more uniformly in the atmosphere than the iridium-containing matter thrown out from a meteorite impact. In addition, the volcanic-eruption theory may explain why the end of the Cretaceous period was marked by a gradual change in sea level. Fossil records indicate that for several hundred thousand years prior to the relatively sudden disappearance of the dinosaurs, the level of the sea gradually fell, causing many marine organisms to die out. This change in sea level might well have been the result of a distortion in the Earth’s surface that resulted from the movement of diapirs upward toward the Earth’s crust, and the more cataclysmic extinction of the dinosaurs could have resulted from the explosive volcanism that occurred as material from the diapirs erupted onto the Earth’s surface.  
 1 
 1. The passage suggests that during the 1980s researchers found meteorite impact a convincing explanation for the extinction of dinosaurs, in part because

(A) earlier theories had failed to account for the gradual extinction of many ocean species at the end of the Cretaceous period
(B) geologists had, up until that time, underestimated the amount of carbon dioxide that would be released during an episode of explosive volcanism
(C) a meteorite could have served as a source of the iridium found in a layer of clay deposited at the end of the Cretaceous period
(D) no theory relying on purely geological processes had, up until that time, explained the cause of the precipitous drop in sea level that occurred at the end of the Cretaceous period
(E) the impact of a large meteorite could have resulted in the release of enough carbon dioxide to cause global climatic change

2 
 2. According to the passage, the lower regions of the Earth’s mantle are characterized by

(A) a composition similar to that of meteorites
(B) the absence of elements found in rocks on the Earth’s crust
(C) a greater stability than that of the upper regions
(D) the presence of large amounts of carbon dioxide
(E) a uniformly lower density than that of the upper regions

3 
 3. It can be inferred from the passage that which one of the following was true of the lava that erupted in India at the end of the Cretaceous period?

(A) It contained less carbon dioxide than did the meteorites that were striking the Earth’s surface during that period.
(B) It was more dense than the molten rock, located just above the Earth’s core.
(C) It released enough iridium hexafluoride into the atmosphere to change the Earth’s climate dramatically.
(D) It was richer in iridium than rocks usually found on the Earth’s surface.
(E) It was richer in iridium than were the meteorites that were striking the Earth’s surface during that period.

4 
 4. In the passage, the author is primarily concerned with doing which one of the following?

(A) describing three theories and explaining why the latest of these appears to be the best of the three
(B) attacking the assumptions inherent in theories that until the 1980s had been largely accepted by geologists
(C) outlining the inadequacies of three different explanations of the same phenomenon
(D) providing concrete examples in support of the more general assertion that theories must often be revised in light of new evidence
(E) citing evidence that appears to confirm the skepticism of geologists regarding a view held prior to the 1980s

5 
 5. The author implies that if the theory described in the third paragraph is true, which one of the following would have been true of iridium in the atmosphere at the end of the Cretaceous period?

(A) Its level of concentration in the Earth’s atmosphere would have been high due to a slow but steady increase in the atmospheric iridium that began in the early Cretaceous period.
(B) Its concentration in the Earth’s atmosphere would have increased due to the dramatic decrease in sea level that occurred during the Cretaceous period.
(C) It would have been directly responsible for the extinction of many ocean species.
(D) It would have been more uniformly dispersed than iridium whose source had been the impact of a meteorite on the Earth’s surface.
(E) It would have been more uniformly dispersed than indium released into the atmosphere as a result of normal geological processes that occur on Earth.

6 
 6. The passage supports which one of the following claims about the volcanic-eruption theory?

(A) It does not rely on assumptions concerning the temperature of molten rock at the lowest pan of the Earth’s mantle.
(B) It may explain what caused the gradual fall in sea level that occurred for hundreds of thousands of years prior to the more sudden disappearance of the dinosaurs.
(C) It bases its explanation on the occurrence of periods of increased volcanic activity similar to those shown to have caused earlier mass extinctions.
(D) It may explain the relative scarcity of iridium in rocks on the Earth’s surface compared to its abundance in meteorites.
(E) It accounts for the relatively uneven distribution of iridium in the layer of clay deposited at the end of the Cretaceous period.

7 
 7. Which one of the following, if true, would cast the most doubt on the theory described in the last paragraph of the passage?

(A) Fragments of meteorites that have struck the Earth are examined and found to have only minuscule amounts of iridium hexafluoride trapped inside of them.
(B) Most diapir eruptions in the geological history of the Earth have been similar in size to the one that occurred in India at the end of the Cretaceous period and have not been succeeded by periods of climatic change.
(C) There have been several periods in the geological history of the Earth, before and after the Cretaceous period, during which large numbers of marine species have perished.
(D) The frequency with which meteorites struck the Earth was higher at the end of the Cretaceous period than at the beginning of the period.
(E) Marine species tend to be much more vulnerable to extinction when exposed to a dramatic and relatively sudden change in sea level than when they are exposed to a gradual change in sea level similar to the one that preceded the extinction of the dinosaurs.

auradediligodo · Accepted Answer

Hi everyone,
Got 5/6 correct in 15 minutes.

----------------------------------------------------------------------------

P1

Paragraph one gives us the point of view of scientists before the 1980s. They did not believe in the cause of a catastrophic geological event as the explanation for the extinction of dinosaurs and they are sure that such extinction is related to a decrease in the ocean level and to a drastic change in the climatic behavior of the planet.

Brief summary : Scientists' point of view before 1980s

P2

After the 1980s scientists discovered a significant amount of iridium that was deposited at the end of the Cretaceous period. The fact that iridium is rarely present o the earth surface but it is largely preset in meteorites, scientists hypothesized that a meteorite could have been responsible for the extinction of dinosaurs.

Brief summary : Meteorite theory

P3

Another theory is the volcanic theory. Scientists observed that at the end of the cretaceous period there were volcanic eruptions in India that released carbon dioxide, an agent considered responsible for the dinosaurs' demise. We are also given that there were molten rocks or diapirs that came out from the earth mantle as the core of the earth got warmer, heated the material inside the mantle and made such material less dense.

Brief summary : the volcanic theory

P4

Paragraph 4 continues to support the volcanic theory. Like the impact theory, the volcanic theory accounts for the iridium aspect since the mantle is rich of such element. In addition, the kind of iridium present in the mantle would expand more uniformly in the atmosphere than the iridium print in meteorites would. Furthermore the gradual change in the sea level might as well be explained by the volcanic theory since the diapirs or molten rocks could have moved from the mantle towards the crust for a thousand years before the disappearance of dinosaurs.

Brief summary : why the volcanic theory is better than the impact theory

Main point

The main point is to support the volcanic theory against all the previous theories

----------------------------------------------------------------------------

1. The passage suggests that during the 1980s researchers found meteorite impact a convincing explanation for the extinction of dinosaurs, in part because

Pre-thinking

Inference question

Let's refer to P2: "   Because iridium is extremely rare in rocks on the Earth’s surface but common in meteorites,  "
Inference: Scientists found the impact theory convincing because the iridium present in meteorites could account for the iridium found at the end of the cretaceous period

(A) earlier theories had failed to account for the gradual extinction of many ocean species at the end of the Cretaceous period
 Not in line with pre-thinking

(B) geologists had, up until that time, underestimated the amount of carbon dioxide that would be released during an episode of explosive volcanism
 Not in line with pre-thinking

(C)   a meteorite could have served as a source of the iridium found in a layer of clay deposited at the end of the Cretaceous period
 in line with pre-thinking

(D) no theory relying on purely geological processes had, up until that time, explained the cause of the precipitous drop in sea level that occurred at the end of the Cretaceous period
 Not in line with pre-thinking

(E) the impact of a large meteorite could have resulted in the release of enough carbon dioxide to cause global climatic change
 Not in line with pre-thinking

----------------------------------------------------------------------------

2. According to the passage, the lower regions of the Earth’s mantle are characterized by

Pre-thinking

Detail question

Let's refer to the last paragraph:"  the lower regions of the Earth’s mantle have roughly the same composition as meteorites and contain large amounts of iridium, which in the case of a diapir eruption would probably be emitted as iridium hexafluoride,  "

(A)   a composition similar to that of meteorites
 in line with pre-thinking

(B) the absence of elements found in rocks on the Earth’s crust
 Not in line with pre-thinking

(C) a greater stability than that of the upper regions
 Not in line with pre-thinking

(D) the presence of large amounts of carbon dioxide
 Not in line with pre-thinking

(E) a uniformly lower density than that of the upper regions
 Not in line with pre-thinking

----------------------------------------------------------------------------

3. It can be inferred from the passage that which one of the following was true of the lava that erupted in India at the end of the Cretaceous period?

Pre-thinking

Inference question

Let's refer to P3 and let's analyze the options

(A) It contained less carbon dioxide than did the meteorites that were striking the Earth’s surface during that period.
 Cannot be inferred

(B) It was more dense than the molten rock, located just above the Earth’s core.
 Cannot be inferred

(C) It released enough   iridium hexafluoride   into the atmosphere to change the Earth’s climate dramatically.
  Trap answer  since per the passage: "the release of  carbon dioxide  from this episode of volcanism could have caused the climatic change responsible for the demise of the dinosaurs."

(D)   It was richer in iridium than rocks usually found on the Earth’s surface.
 This can be inferred from: " Although iridium is extremely rare on the Earth’s surface, the lower regions of the Earth’s mantle have roughly the same composition as meteorites and contain large amounts of iridium, "

(E) It was richer in iridium than were the meteorites that were striking the Earth’s surface during that period.
 The problem with this answer is that we cannot infer whether there was more iridium in the lava than in the meteorites. We just now that the iridium content in the lava can propagate more uniformly than the iridium present in meteorites

----------------------------------------------------------------------------

4. In the passage, the author is primarily concerned with doing which one of the following?

Pre-thinking

Main point question

Refer to main point and summaries above

(A)   describing three theories and explaining why the latest of these appears to be the best of the three
 In line with pre-thinking

(B) attacking the assumptions inherent in theories that until the 1980s had been largely accepted by geologists
 The main point is not to attack the other theories but to explain why the V theory is the best one

(C) outlining the  inadequacies  of three different explanations of the same phenomenon
 Just 2 theories are inadequate

(D) providing concrete examples in support of the more general assertion that theories must often be revised in light of new evidence
 out of scope

(E) citing evidence that appears to confirm the  skepticism  of geologists regarding a view held prior to the 1980s
 out of scope

----------------------------------------------------------------------------

5. The author implies that if the theory described in the third paragraph is true, which one of the following would have been true of iridium in the atmosphere at the end of the Cretaceous period?

Pre-thinking

assumption question

Assumption: the iridium present in the air was uniformly propagated

(A) Its level of concentration in the Earth’s atmosphere would have been high due to a slow but steady increase in the atmospheric iridium that began in the early Cretaceous period.
 out of scope

(B) Its concentration in the Earth’s atmosphere would have increased due to the dramatic decrease in sea level that occurred during the Cretaceous period.
 no correlation is given between the two phenomenons

(C) It would have been directly responsible for the extinction of many ocean species.
 The explanation for the extinction of such species is not related to the iridium per se but to the movement of the molten rocks from the mantle towards the crust

(D)   It would have been more uniformly dispersed than iridium whose source had been the impact of a meteorite on the Earth’s surface.
 In line with pre-thinking

(E) It would have been more uniformly dispersed than irium released into the atmosphere as a result of normal geological processes that occur on Earth.
 normal geological processes are not even discussed

----------------------------------------------------------------------------

6. The passage supports which one of the following claims about the volcanic-eruption theory?

Pre-thinking

Strengthen question

Let's analyze the answer choices

(A) It does not rely on assumptions concerning the temperature of molten rock at the lowest pan of the Earth’s mantle.
 This statement is quite vague. However the temperature component is discussed in P3

(B)   It may explain what caused the gradual fall in sea level that occurred for hundreds of thousands of years prior to the more sudden disappearance of the dinosaurs.
 " This change in sea level might well have been the result of a distortion in the Earth’s surface that resulted from the movement of diapirs upward toward the Earth’s crust,

(C) It bases its explanation on the occurrence of periods of increased volcanic activity similar to those shown to have caused earlier mass extinctions.
 earlier mass extinctions are not even discussed

(D) It may explain the relative scarcity of iridium in rocks on the Earth’s surface compared to its abundance in meteorites.
 no such explanation is given

(E) It accounts for the relatively uneven distribution of iridium in the layer of clay deposited at the end of the Cretaceous period.
 the passage does not discuss the distribution component

----------------------------------------------------------------------------

7. Which one of the following, if true, would cast the most doubt on the theory described in the last paragraph of the passage?

Pre-thinking

Weaken question

Arguments to be weakened:

#1:" Moreover, the volcanic-eruption theory, like the impact theory, accounts for the presence of iridium in sedimentary deposits; "

#2:" In addition, the volcanic-eruption theory may explain why the end of the Cretaceous period was marked by a gradual change in sea level.  "

(A) Fragments of meteorites that have struck the Earth are examined and found to have only minuscule amounts of iridium hexafluoride trapped inside of them.
 This claim would strengthen the volcanic theory by eliminating the impact theory

(B)   Most diapir eruptions in the geological history of the Earth have been similar in size to the one that occurred in India at the end of the Cretaceous period and have not been succeeded by periods of climatic change.
 Argument #2 is supported by the evidence related to diapirs. This statement clearly undermines the connection between diapers and climate change. Hence this is our answer

(C) There have been several periods in the geological history of the Earth, before and after the Cretaceous period, during which large numbers of marine species have perished.
 irrelevant

(D) The frequency with which meteorites struck the Earth was higher at the end of the Cretaceous period than at the beginning of the period.
  irrelevant

(E) Marine species tend to be much more vulnerable to extinction when exposed to a dramatic and relatively sudden change in sea level than when they are exposed to a gradual change in sea level similar to the one that preceded the extinction of the dinosaurs. 
 this answer tells us that marine species are MORE vulnerable when the change is sudden than when it is gradual. However it does not say that marine species are not vulnerable at all when the change is gradual.

----------------------------------------------------------------------------

It is a good day to be alive!

Skywalker18 · Answer

All correct in 12 mins 20 seconds, including almost 4 mins to read

Para 1- Theory 1- noncatastrophic geological processes caused the extinction of dinosaurs
Para 2- meteorite impact theory- iridium presence
Para 3- volcanic-eruption theory
Para 4- Why volcanic-eruption theory is the best among the three?

1. The passage suggests that during the 1980s researchers found meteorite impact a convincing explanation for the extinction of dinosaurs, in part because
(C) a meteorite could have served as a source of the iridium found in a layer of clay deposited at the end of the Cretaceous period - Correct, refer to 2nd para

2. According to the passage, the lower regions of the Earth’s mantle are characterized by
(A) a composition similar to that of meteorites

Although iridium is extremely rare on the Earth’s surface, the lower regions of the Earth’s mantle have roughly the same composition as meteorites and contain large amounts of iridium

3. It can be inferred from the passage that which one of the following was true of the lava that erupted in India at the end of the Cretaceous period?
(C) It released enough iridium hexafluoride into the atmosphere to change the Earth’s climate dramatically.- incorrect, iridium hexafluoride did not cause climate change but carbon dioxide did
(D) It was richer in iridium than rocks usually found on the Earth’s surface.- Correct

Although iridium is extremely rare on the Earth’s surface, the lower regions of the Earth’s mantle have roughly the same composition as meteorites and contain large amounts of iridium

4. In the passage, the author is primarily concerned with doing which one of the following?
(A) describing three theories and explaining why the latest of these appears to be the best of the three- Correct,

Moreover, the volcanic-eruption theory, like the impact theory, accounts for the presence of iridium in sedimentary deposits

In addition, the volcanic-eruption theory may explain why the end of the Cretaceous period was marked by a gradual change in sea level.

5. The author implies that if the theory described in the third paragraph is true, which one of the following would have been true of iridium in the atmosphere at the end of the Cretaceous period?
(D) It would have been more uniformly dispersed than iridium whose source had been the impact of a meteorite on the Earth’s surface.

Although iridium is extremely rare on the Earth’s surface, the lower regions of the Earth’s mantle have roughly the same composition as meteorites and contain large amounts of iridium, which in the case of a diapir eruption would probably be emitted as iridium hexafluoride, a gas that would disperse more uniformly in the atmosphere than the iridium-containing matter thrown out from a meteorite impact.

6. The passage supports which one of the following claims about the volcanic-eruption theory?
(B) It may explain what caused the gradual fall in sea level that occurred for hundreds of thousands of years prior to the more sudden disappearance of the dinosaurs

In addition, the volcanic-eruption theory may explain why the end of the Cretaceous period was marked by a gradual change in sea level. Fossil records indicate that for several hundred thousand years prior to the relatively sudden disappearance of the dinosaurs, the level of the sea gradually fell, causing many marine organisms to die out. This change in sea level might well have been the result of a distortion in the Earth’s surface that resulted from the movement of diapirs upward toward the Earth’s crust

7. Which one of the following, if true, would cast the most doubt on the theory described in the last paragraph of the passage?
(B) Most diapir eruptions in the geological history of the Earth have been similar in size to the one that occurred in India at the end of the Cretaceous period and have not been succeeded by periods of climatic change.-- Correct, cause(diapir eruptions) is present but effect(periods of climatic change) is not

redskull1 · Answer

All correct...8 mins 37secs(including reading which took about 2mins, maybe because I am a science student)

For Q3)

Iri Hex is mentioned only in the last para.Going there we get that iri hex was released and it acted as a source of iridium.Climate change is not touched upon AT ALL in this para.

as  skywalker  rightly said it is carbon dioxide, but I dint use that information to eliminate C...

D though is clearly mentioned from the third line onwards..

For Q5)

D is very clearly mentioned in the same 4th para.There is no other competition for option D) I believe..

Cheerio..

VerbalBot · Answer

Automated notice from GMAT Club VerbalBot:

A member just gave Kudos to this thread, showing it’s still useful. I’ve bumped it to the top so more people can benefit. Feel free to add your own questions or solutions.

This post was generated automatically.

Until the 1980s, most scientists believed that noncatastrophic

Prep Toolkit

Top 5 GMAT Debrief Videos

615 to 715 in 15 Days (Ria's Strategies)

More than Quant/Verbal Abilities: Speed vs. Accuracy

745 with Only Self-Prep and GMAT Club

From 555 to 765: 210-point Improvement

Perfect 805 Score Debrief by Julia

My Rewards

GMAT Reading Comprehension (RC) Questions

Until the 1980s, most scientists believed that noncatastrophic

Prep Toolkit

615 to 715 in 15 Days (Ria's Strategies)

More than Quant/Verbal Abilities: Speed vs. Accuracy

745 with Only Self-Prep and GMAT Club

From 555 to 765: 210-point Improvement

Perfect 805 Score Debrief by Julia

My Rewards