This is the last in a series about difficult, highly technical science passages in the Reading Comprehension section on the GRE.
Previous posts in this series went over basic strategies for tackling this type of passage, and applying them to an actual example passage. This post offers practice questions and explanations for the same passage, for some actual hands-on practice. Good luck!
In the mid-1970’s, Walter Alvarez, a geologist, was studying Earth’s polarity. It had recently been learned that the orientation of the planet’s magnetic field reverses, so that every so often, in effect, south becomes north and vice versa. Alvarez and some colleagues had found that a certain formation of pinkish limestone in Italy, known as the scaglia rossa, recorded these occasional reversals. The limestone also contained the fossilized remains of millions of tiny sea creatures called foraminifera. Alvarez became interested in a thin layer of clay in the limestone that seemed to have been laid down around the end of the Cretaceous Period. Below the layer, certain species of foraminifera—or forams, for short—were preserved. In the clay layer, there were no forams. Above the layer, the earlier species disappeared and new forams appeared. Having been taught the uniformitarian view, which held that any apparent extinctions throughout geological time resulted from ‘the incompleteness of the fossil record’ rather than an actual extinction, Alvarez was not sure what to make of the lacuna in geological time corresponding to the missing foraminifera, because the change looked very abrupt.
Had Walter Alvarez not asked his father, the Nobel Prize-winning physicist Luis Alvarez, how long the clay had taken to deposit, the younger Alvarez may not have thought to use iridium, an element rarely found on earth but more plentiful in meteorites, to answer this question. Iridium, in the form of microscopic grains of cosmic dust, is constantly raining down on the planet. The Alvarezes reasoned that if the clay layer had taken a significant amount of time to deposit, it would contain detectable levels of iridium. The results were startling – far too much iridium had shown up. The Alvarez hypothesis, as it became known, was that everything – not just the clay layer, could be explained by a single event: a six-mile-wide asteroid had slammed into Earth—killing off not only the forams but the dinosaurs, and all the other organisms that went extinct at the end of the Cretaceous period.
1. It can be inferred from the passage that had the scaglia rosa not exhibited a certain geological property then which of the following would most likely have been true?
(A) Walter Alvarez would not have been able to use the forams as the basis for a conjecture
(B) Scientists would have unlikely observed distinct features in limestone
(C) The rocks would not have been of any immediate utility to scientists
(D) Iridium would not have been used to substantiate a hypothesis
(E) The gap in foraminfera fossil record would have served no purpose
2. Which of the following can be most reasonably inferred from the first paragraph?
(A) The forams in the clay were driven to extinction by the impact of a large meteorite
(B) Walter Alvarez was aware of the shortcomings of the uniformitarian view
(C) Iridium is more likely to be found on meteorites than on the surface of the earth
(D) The observation of the absence of forams in the clay layer was in some ways serendipitous
(E) The earth’s magnetic poles were shifted by a catastrophic occurrence
3. Proponents of the uniformitarian view would most likely argue that
(A) the clay layer actually contained foram too small for Alvarez to detect
(B) the absence of foram suggested a mass extinction
(C) iridium could likely be found in the clay layer
(D) that the lack of forams in the clay reflected a gap in the fossil record
(E) the orientation of the earth’s magnetic field is capable of reversing
4. According to the passage, it can most likely be inferred that iridium
(A) can be used by scientists to determine the duration of an event
(B) causes mass extinctions in a short period
(C) gathers excessively in clay deposits
(D) helps scientists determine the orientation of the magnetic field
(E) leads to inaccurate conclusions regarding the chronology of an event
5. The Alvarezes concluded that a meteorite was responsible for a mass extinction because
(A) dinosaurs were prone to the effects of iridium
(B) it could account for the heightened presence of an element
(C) cosmic dust in the form of iridium constantly makes its way to the Earth’s surface
(D) the scaglia rosa had a sudden gap in traceable iridium
(E) the scientific community was unable to provide a more valid hypothesis
1. Answer: (A)
We learn from the passage that Alvarez had been using scaglia rosa as part of his studies on Earth’s polarity. Had he not have been, he would not have encountered the foraminifera, which led to his theory on dinosaur extinction. This supports answer (A).
(B) is tempting, but nowhere in the passage does it mention that the only way to learn about shifts in Earth’s magnetic fields is through the use of scaglia rosa.
(C) is similar to (B) in that we do not have enough information. That is nowhere in the passage does it say scientists have no other uses for scaglia rosa.
(D) Had Alvarez not noted the gap in the foraminfera, he would not have arrived at a theory on dinosaur extinction. Thus he would not have had any use for iridium.
(E) Had Alvarez not been studying Earth’s polarity, he would not have studied scaglia rosa and thus not noticed the gap in foraminfera.
2. Answer: (D)
(A) is a little bit vague about which forams in the clay. While the meteorite was responsible for killing off one group of forams new ones did appear.
(B) Alvarez had been taught the uniformitarian view. The passage does not say he was aware of any shortcomings.
(C) Notice it says first paragraph.
(D) This answer choice relates to the fact that had Alvarez not been studying magnetic poles, he would not have come across the scaglia rosa. ‘Serendipity’ means to an unexpected discovery of something promising.
(E) Nowhere does the passage state that the earth’s magnetic poles were effected by any catastrophic occurrence.
3. Answer: (D)
The uniformitarian view is “any apparent extinctions” were a gap in the fossil record. This matches up best with (D).
(A) is wrong because a gap is not the same thing as tiny forams.
(B) is the opposite of what they would think
(C) is incorrect since uniformitarian view does not relate to iridium
(E) is incorrect because the uniformitarian view relates to fossil record
4. Answer: (A)
The passage says, “how long…iridium…answer this question.” This best supports (A).
(B) is wrong because iridium does not cause extinctions. (C) is incorrect because the forma, not the iridium, form in clay deposits. (D) is wrong because iridium does not relate to magnetic fields. (E) is incorrect because the passage suggests that iridium provides accurate, not inaccurate, conclusions about chronology.
5. Answer: (B)
Based on the startling amount of iridium that the Alvarezes found they concluded that only a meteorite could have caused the extinction of the dinosaurs. This matches up best with (B).
(A) is wrong because dinosaurs were not effected by the iridium itself but by the meteorite.
(C), while mentioned in the passage, does not answer the question.
(D) is wrong because the scaglia rosa showed a gap in foraminifera, not iridium.
(E) is incorrect because the passage does not mention the scientific community, nor does it imply that the Alvarez’s explanation was a default one.