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Since 1980, the notion that mass extinctions at the end of the

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Since 1980, the notion that mass extinctions at the end of the  [#permalink]

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Since 1980, the notion that mass extinctions at the end of the Cretaceous period 65 million years ago resulted from a sudden event has slowly gathered support,although even today there is no scientific consensus. In the Alvarez scenario, anasteroid struck the earth, creating a gigantic crater. Beyond the immediate effects of fire, flood, and storm, dust darkened the atmosphere, cutting off plant life. Many animal species disappeared as the food chain was snapped at its base. Alvarez’s main evidence is an abundance of iridium in the KT boundary, a thin stratum dividing Cretaceous rocks from rocks of the Tertiary period. Iridium normally accompanies the slow fall of interplanetary debris, but in KT boundary strata, iridium is 10–100 times more abundant, suggesting a rapid, massive deposition. Coincident with the boundary, whole species of small organisms vanish from the fossil record. Boundary samples also yield osmium isotopes, basaltic sphericles, and deformed quartz grains, all of which could have resulted from high-velocity impact.

Paleontologists initially dismissed the theory, arguing that existing dinosaur records showed a decline lasting millions of years. But recent studies in North America, aimed at a comprehensive collection of fossil remnants rather than rare or well-preserved specimens, indicate large dinosaur populations existing immediately prior to the KT boundary. Since these discoveries, doubts about theories of mass extinction have lessened significantly. Given the lack of a known impact crater of the necessary age and size to fit the Alvarez scenario, some scientists have proposed alternatives. Courtillot, citing huge volcanic flows in India coincident with the KT boundary, speculates that eruptions lasting many thousands of years produced enough atmospheric debris to cause global devastation. His analyses also conclude that iridium in the KT boundary was deposited over a period of 10,000–100,000 years. Alvarez and Asaro reply that the shock of an asteroidal impact could conceivably have triggered extensive volcanic activity. Meanwhile, exploration at a large geologic formation in Yucatan, found in 1978 but unstudied until 1990, has shown a composition consistent with extraterrestrial impact. But evidence that the formation is indeed the hypothesized impact site remains inconclusive.

1. It can be inferred from the passage that supporters of the Alvarez and Courtillot theories would hold which of the following views in common?

A. The KT boundary was formed over many thousands of years.
B. Large animals such as the dinosaurs died out gradually over millions of years.
C. Mass extinction occurred as an indirect result of debris saturating the atmosphere.
D. It is unlikely that the specific cause of the Cretaceous extinctions will ever be determined.
E. Volcanic activity may have been triggered by shock waves from the impact of an asteroid.


2. The author mentions “recent studies in North America” (lines 16–17) primarily in order to

A. point out the benefits of using field research to validate scientific theories.
B. suggest that the asteroid impact theory is not consistent with fossil evidence.
C. describe alternative methods of collecting and interpreting fossils.
D. summarize the evidence that led to wider acceptance of catastrophic scenarios of mass extinction.
E. show that dinosaurs survived until the end of the Cretaceous period.


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Since 1980, the notion that mass extinctions at the end of the  [#permalink]

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New post 20 Feb 2018, 17:18
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Quote:
Since 1980, the notion that mass extinctions at the end of the Cretaceous period 65 million years ago resulted from a sudden event has slowly gathered support,although even today there is no scientific consensus. In the Alvarez scenario, anasteroid struck the earth, creating a gigantic crater. Beyond the immediate effects of fire, flood, and storm, dust darkened the atmosphere, cutting off plant life. Many animal species disappeared as the food chain was snapped at its base. Alvarez’s main evidence is an abundance of iridium in the KT boundary, a thin stratum dividing Cretaceous rocks from rocks of the Tertiary period. Iridium normally accompanies the slow fall of interplanetary debris, but in KT boundary strata, iridium is 10–100 times more abundant, suggesting a rapid, massive deposition. Coincident with the boundary, whole species of small organisms vanish from the fossil record. Boundary samples also yield osmium isotopes, basaltic sphericles, and deformed quartz grains, all of which could have resulted from high-velocity impact.

Pare summary : Some asteroid struck, impact on food chain, evidence given


Paleontologists initially dismissed the theory, arguing that existing dinosaur records showed a decline lasting millions of years. But recent studies in North America, aimed at a comprehensive collection of fossil remnants rather than rare or well-preserved specimens, indicate large dinosaur populations existing immediately prior to the KT boundary. Since these discoveries, doubts about theories of mass extinction have lessened significantly. Given the lack of a known impact crater of the necessary age and size to fit the Alvarez scenario, some scientists have proposed alternatives. Courtillot, citing huge volcanic flows in India coincident with the KT boundary, speculates that eruptions lasting many thousands of years produced enough atmospheric debris to cause global devastation. His analyses also conclude that iridium in the KT boundary was deposited over a period of 10,000–100,000 years. Alvarez and Asaro reply that the shock of an asteroidal impact could conceivably have triggered extensive volcanic activity. Meanwhile, exploration at a large geologic formation in Yucatan, found in 1978 but unstudied until 1990, has shown a composition consistent with extraterrestrial impact. But evidence that the formation is indeed the hypothesized impact site remains inconclusive.

Para summary : there were doubts in previous theory. evidence found, doubts faded but still remain



While reading I have attempted the paragraph this way. Read only the green part while skim the rest.

Passage summary : two theories about mass extinction , but doubt as evidence remain inconclusive.
Scope : theories about cause of mass extinction
Tone : Doubtful , Inconclusive, Ambiguous, Skeptical

Question 1:
Quote:
1. It can be inferred from the passage that supporters of the Alvarez and Courtillot theories would hold which of the following views in common?


its an inference question, means according to the passage it must be true. Something is common in between these two theories. Now here is a trick. if Common or opposite is asked between two theories, what is best place to look for ? Post second theory ! here in this case 2nd theory is mentioned in mid of 2nd para. look for answer there and match it with what you know about the first theory. How about these lines. first theory also something has to do with volcano.

Quote:
speculates that eruptions lasting ... period of 10,000–100,000 years.

now use POE and eliminate answer choices. its very easy to find it now.
Quote:
C. Mass extinction occurred as an indirect result of debris saturating the atmosphere.


Question 2:
Quote:
2. The author mentions “recent studies in North America” (lines 16–17) primarily in order to


for such questions, immediately start reading one sentence prior to this part. mind the "BUT" mentioned just before the bold part, meaning that something opposite to that is coming up. its an evidence in support. Think on these line and you find the answer.
Quote:
D. summarize the evidence that led to wider acceptance of catastrophic scenarios of mass extinction.


Note : I have done this exercise to concretize my concept and strategy for RC. In case of any doubts or suggestions , you are most welcome.
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Simple strategy:
“Once you’ve eliminated the impossible, whatever remains, however improbable, must be the truth.”

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Re: Since 1980, the notion that mass extinctions at the end of the  [#permalink]

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New post 22 Feb 2018, 00:51
Q2. How is the answer D? Nowhere the author summarizes the evidence in the passage.
Option E is very much inferred from the passage....so why can't it be the answer?
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Re: Since 1980, the notion that mass extinctions at the end of the &nbs [#permalink] 22 Feb 2018, 00:51
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Since 1980, the notion that mass extinctions at the end of the

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