Reading comp PrepTest 151 · Section 1 · Question 25

Topic: Science

Paragraph 1

• Paragraph note
  • Background info on "plate tectonics" and central question (why some areas have lots of subduction but no earthquakes)
• Views, minor Meta-Structures, and the author's attitude
  • Definitions:
    • Plate tectonics: The earth's crust features many plates of solid rock that move across the mantle (first sentence)
    • Mantle: The slightly fluid layer of rock between crust and core (first sentence)
    • Subduction: The process in which two plates collide and one plate is forced under the other until it merges with the mantle (third and fourth sentences)
    • Earth's seismic "hot zones": Regions with very high levels of subduction (fifth sentence)
  • Cause-and-effect relationship, according to the theory of plate tectonics:
    • Earthquakes are caused by two plates grinding against each other as they collide, which causes subduction, which causes an enormous build-up of energy abruptly released as an earthquake (second and fifth sentences)
  • Author attitude: "generally accepted" (first sentence); "contrary to expectations" (sixth sentence); "crucial question" (last sentence); "no answer" (last sentence)

Paragraph 2

• Paragraph note
  • Answer: Earthquakes are caused by subduction from plates moving in the opposite direction, not the same direction
• Views, minor Meta-Structures, and the author's attitude
  • Comparisons of two subduction types:
    • When plates move toward each other, the resulting subduction is relatively motionless and shallow, the amount of friction is higher, and the likelihood of earthquakes is increased (second, third, seventh, and last sentences)
      • This amount of resistance is compared to two pieces of sandpaper rubbing against each other (last sentence)
    • On the other hand, when plates move in the same direction, the resulting subduction is high velocity and steep, the amount of friction between places is lower, and there is a lower likelihood of earthquakes (fourth, fifth, and sixth sentences)
      • This type of deduction is compared to an oar being dipped in water from a moving boat (fifth sentence)

Paragraph 3

• Paragraph note
  • Implications: Areas with "low" subduction may be at a greater risk of earthquakes than previously thought
• Views, minor Meta-Structures, and the author's attitude
  • Author's attitude: "provides a warning" (first sentence); "may in fact be at a significant risk of earthquakes" (last sentence)

Main Point: Some scientists believe that the type of subduction caused by two plates moving in the same direction can explain why some regions feature intense subduction and relatively few earthquakes.

Paragraph 1, Sentence 7 (P1, S7) - Question

P2, S8-9 - Answer

P3, S1 - Implications

Question/Answer: We think the Question/Answer Meta-Structure is the closest fit for this passage*. In such a passage, the author will usually pose a question and consider one or more answers for the rest of the passage. And that's the approach that this author uses. After providing some background information on plate tectonics and seismic "hot zones," the author asks a question: "[H]ow can often intense subduction take place at certain locations with little or no seismic activity?" (P1, S7). The author uses the remaining two paragraphs to discuss the answer to this question.

In such a passage, the main point is typically the answer. Here, the author seems to buy the explanation offered by a group of scientists who believe earthquakes are more likely to occur when plates collide when sliding toward each other rather than sliding in the same direction. So, we've summarized that answer as, "Some scientists believe that the type of subduction caused by two plates moving in the same direction can explain why some regions feature intense subduction and relatively few earthquakes."

*Truthfully, about half of the Meta-Structures could describe this passage. Certainly, you could call this a Phenomenon/Explanation, Proposing a Hypothesis, Correcting the Record, Reporting a Viewpoint, or Old Approach/New Approach passage. One could even make a reasonable case that this is a Paradox/Resolution or Problem/Solution passage. Any one of these Meta-Structures could help us understand the passage's structure and main point. Ultimately we went with Question/Answer because the passage explicitly proposes a question and discusses the answer. But if you went with one of the other Meta-Structures, that's OK. Remember, the Meta-Structures are tools that help us think about the overarching structure and ideas in a passage. We don't have to agree on which Meta-Structure a passage has to realize the benefits of the Meta-Structures, as this passage illustrates.

Comparisons: This passage — especially the second paragraph — utilizes the comparison minor Meta-Structure extensively. Most of the comparisons deal with the difference between the two types of subduction zones. We will almost certainly get a few questions that relate to these differences, so let's review the details of each.

• Head-on subduction:
  • More likely to lead to earthquakes.
  • Subduction is relatively motionless.
  • Subduction occurs at a shallower angle of descent.
  • There is a larger plane of contact with the plates and thus more friction and seismic activity.
• Same-direction subduction
  • Less likely to lead to earthquakes.
  • Subduction occurs at a high velocity.
  • Subduction occurs at a steeper angle of descent.
  • There is less contact between the plats and thus less friction and seismic activity.

Let's make sure we understand these two types of subduction for the inevitable Minor Point questions about them. A stumbling block for many test-takers is that the fast-moving, steep-descending plates that experience significant resistance from the mantle are less likely to cause earthquakes. Expect the test-writers to try to trick us by playing up that counterintuitive fact.