Question 75·Hard·Text Structure and Purpose
Geologist Melissa Kipp’s 2008 core samples from Lake Viel revealed an unexpectedly dense band of zircons, apparently corroborating the prevailing view that a nearby volcanic eruption roughly 10,000 years ago had sprinkled mineral-rich ash across the basin. Subsequent isotopic work by Raul Peres, however, dated the zircons far earlier than the eruption, undermining that interpretation. Peres suggested that retreating glaciers might instead have conveyed the minerals, but that hypothesis fails to account for the abruptness of the zircon layer’s lower boundary. Kipp now advances a third possibility: a series of small, quake-triggered landslides from the surrounding cliffs, occurring in quick succession, could have concentrated the zircons in a single, geologically rapid episode.
Which choice best describes the overall structure of the text?
For overall-structure questions, label what each sentence is doing (supporting an idea, undermining it, offering an alternative, critiquing an alternative, proposing something new). Then pick the choice that matches both the sequence of moves and how the author evaluates each explanation.
Hints
Break the paragraph into parts
Read the passage one sentence at a time. For each sentence, ask: is it presenting evidence, supporting an idea, questioning an idea, or offering a new idea?
Notice changes in interpretation
Pay attention to transition words like "however" and "but." How do these words signal shifts from one explanation or viewpoint to another?
Compare how each explanation is treated
Does the passage treat all explanations as equally strong, or does it show that some are undermined or incomplete while another is put forward afterward?
Avoid adding conclusions
Don’t pick an option that adds a conclusion like "no explanation can be supported" or "more research is needed" unless the passage explicitly says so.
Step-by-step Explanation
Map each sentence to its role
Sentence 1: Evidence seems to support the prevailing volcanic-eruption explanation.
Sentence 2: Isotopic dating undermines that volcanic interpretation.
Sentence 3: Glacial transport is suggested but said to fail to explain the abrupt lower boundary.
Sentence 4: A third explanation (quake-triggered landslides in quick succession) is proposed.
Match that sequence to a choice
The only option that matches support → undermining evidence → critique of a second explanation → proposal of a third is:
"It begins with evidence that seems to support a volcanic explanation, then presents findings that undermine it, critiques a glacial alternative, and proposes landslides as a third possibility."