Reading Comprehension

PT15 · S1 · P1 · Q7 Volcanic-Eruption Theory

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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.

Topic

Why did the dinosaurs go extinct at the end of the Cretaceous period? Competing scientific theories about the cause.

Framework

Old / New (with an element of Present Debate between theories)

Main Point

Currently, the volcanic-eruption theory best explains dinosaur extinction and related evidence, providing a better account than either older sea-level-drop explanations or the meteorite-impact theory.

The "Most Valuable Sentence" is at the start of paragraph 3:

P1: The Original View – Sea-Level Drop Theory

Most scientists used to believe dinosaurs died out due to noncatastrophic events such as a big drop in sea level, which changed the climate and caused extinctions.

P2: Meteorite-Impact Theory Emerges

This long-held view was challenged in the 1980s after scientists found lots of iridium in a clay layer from the time of the extinction. Since iridium is rare on Earth but common in meteorites, some researchers proposed that a massive meteorite impact changed the climate and wiped out the dinosaurs.

P3: Newest Theory – Volcanic Eruption

More recent evidence points instead to a huge volcanic eruption in India. The eruption released lots of carbon dioxide, possibly driving the climatic changes that killed the dinosaurs. The science behind it involves "diapirs" (blobs of hot rock from deep in the Earth) erupting as lava.

7.

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

  1. Fragments of meteorites that have

    Opposite (if anything)

    If this answer helped us to argue that a meteorite impact is what killed the dinos, we'd consider it. But this answer does nothing like that. Meteorites have iridium in mostly solid form. Volcanos erupt iridium in a gaseous form (iridium hexafluoride). The gas form is more likely to explain the worldwide later of iridium we see from that era than is the solid form, so being told that meteorites don't have much of the gas form makes meteorites seem less likely to be the cause of the global iridium layer that coincides with the dinos' demise.

    4% picked this

  2. Correct

    Most diapir eruptions in the

    Why this is right

    This hurts the plausibility of the volcano theory, which held that "diapir eruptions led to releases of CO2 which led to poisonous climate change for the dinos". This is doing a classic Cause w/o Effect style weakener, providing us with tons of data points in which diapir eruptions happened (the supposed Cause), but they didn't cause a period of climate change (the supposed Effect).

    Skill tested: Weaken · how this choice captures the passage's function is the move to repeat next time.

    65% picked this

  3. There have been several periods

    No Impact

    This just tells us that other extinctions have happened, but it isn't giving us any clue about what caused these extinctions (and thus isn't helping us assess what caused the extinctions of the dinos and lots of marine species 66 million years ago).

    4% picked this

  4. The frequency with which meteorites

    Too Weak

    This is telling us that closer to the end of the Cretaceous period (which is when the dinosaur extinctions occurred, 66 million years ago), there was an uptick in Earth getting hit by meteorites. That would start to seem like support for the Alternate Explanation that the dino extinctions were caused by a large meteorite, not by volcanic eruptions. But the Earth is always getting hit by meteorites (we have yearly meteor showers that are harmless displays of "shooting stars"). The meteorite theory is specifically about one huge mamma-jamma that hit the earth. It doesn't really matter whether that gigantic meteorite happens during a period of higher or lower meteorite impacts in general. In other words, saying that the Earth was getting hit by more meteorites than in a previous era isn't suggestive enough that ONE of those meteorites was a huge one that killed the dinos.

    7% picked this

  5. Marine species tend to be

    No Impact

    This answer makes it seem surprising that there were lots of marine extinctions that coincided with the dinos' demise, since the sea level had changed pretty gradually. But we already know there WERE marine extinctions. We're trying to assess whether they were caused by a meteorite impact, bad eruptions, or something else.

    19% picked this

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