Daily RC Article 46

What Happened to the Dinosaurs?

Paragraph 1

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

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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

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Currently available evidence, however, offers more support for a new theory, the volcaniceruption 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 of molten rock, called “diapirs,” that can, under certain circumstances, erupt violently through the Earth’s crust

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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.

Topic and Scope:

Dinosaur extinction; specifically, various different contemporary theories for dinosaurs’ disappearance

Purpose and Main Idea:

The author spends most of the time explaining and touting the newest of three theories (the “volcanic-eruption theory”) as to why the dinosaurs died out. Two earlier theories are described as well, but the author seems to have the most confidence in this new one.

Paragraph Structure:

Each of the first three paragraphs is devoted to a thumbnail description of a theory: the “climatic theory” (paragraph 1) that held sway until the 1980s; the “meteorite-impact theory” (paragraph 2) popular in the ‘80s; and the “volcanic-eruption theory” (paragraph s 3-4), which is supported by newly-discovered evidence in India.

The introductory keyword “Moreover” leads us to expect more information about the new theory, and that’s just what we get: We learn that the evidence for the other two theories (the change in sea level and the iridium deposits) supports, or at least is consistent with, the eruption theory as well.

The Big Picture:

  • Sometimes the CAT writers throw you a real curve when a Reading Comprehension section begins—a really complex passage, or one that’s way beyond most everyone’s ken.  Just remember: At least one of those passages will be of “low difficulty,” like this one, meaning that most people are expected to do well on it. Find it, and attack it early in the section, even if it’s not printed first.
  • Actually, the only examinees who screwed up were those who saw that the topic was science and, knee-jerk, hightailed it to another passage. A word to the wise: Even science passages can be manageable.
  • Whenever the author describes several different points of view, take care to ascertain her views on them: Which, if any, does she especially favor? Here, it’s not too tough to detect the author’s enthusiasm for the volcano theory, but other passages may be more challenging in this regard.

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