Question 20·Medium·Central Ideas and Details
Dendrochronologist Lana Ortiz analyzed growth rings from centuries-old pine trees in the Andes to investigate the history of volcanic eruptions in South America. Because volcanic ash in the atmosphere can reduce sunlight, major eruptions usually slow tree growth and leave unusually narrow rings. Ortiz compared the ring patterns with layers of ash preserved in nearby lake sediments and found that several dramatic slow-growth years coincided with thick ash layers, suggesting previously undocumented eruptions. Her work, she says, will help climate historians refine regional climate models by filling gaps in the eruption record.
Which choice best states the main idea of the text?
For main idea questions, first ignore the choices and briefly summarize the passage in your own words (who is involved, what they did, and why it matters). Then test each option against that summary, eliminating any that (1) focus on a minor detail, (2) add new ideas or comparisons not mentioned in the text, or (3) use extreme language like "every" or "more reliable" when the passage is more cautious. Choose the option that best matches both the overall action in the passage and its stated purpose or outcome.
Hints
Focus on the whole passage, not one sentence
Before looking at the choices, ask yourself: if you had to explain this paragraph to a friend in one or two sentences, what would you say it's mainly about?
Notice both the method and the goal
Pay attention to what Ortiz actually did (the kinds of evidence she used) and what she says her work will help other scientists do.
Watch out for extreme or added claims
Look carefully for words like "every" or comparisons between types of evidence. Does the passage really say those things, or are they going beyond what is written?
Step-by-step Explanation
Identify what a “main idea” question wants
The question asks for the main idea of the text, so you should look for a choice that captures:
- who Ortiz is and what kind of evidence she studied, and
- the overall purpose or result of her work. It should cover the whole passage, not just one detail or an extreme claim.
Summarize the passage in your own words
Put the passage into 1–2 simple sentences:
- Ortiz studies tree rings from old pine trees in the Andes to learn about past volcanic eruptions.
- She compares narrow tree rings with thick ash layers in lake sediments and finds several matches, suggesting previously unknown eruptions.
- She believes this will help climate historians improve climate models for the region. Keep this short summary in mind and compare each choice to it.
Eliminate choices that add claims not in the passage
Check each option against the summary:
- One choice talks about comparing pine trees to other species, which the passage never does.
- Another says every narrow tree ring matches an eruption, but the passage only says several slow-growth years coincided with ash layers and calls them "previously undocumented eruptions."
- A third says Ortiz argued that one type of evidence is more reliable than another, but in the passage she uses both together rather than saying one is better. Remove any choice that makes claims the text does not support or that go beyond what is said.
Select the answer that matches both method and purpose
Only one choice correctly combines:
- what Ortiz did: connected tree-ring growth patterns with ash layers in lake sediments to reveal volcanic eruptions that were not documented before, and
- why it matters: her findings, she says, will help climate historians improve regional climate models by filling gaps in the eruption record. The choice that clearly states this link between the evidence and its climate-history purpose is: Ortiz linked tree-ring evidence to ash deposits, revealing volcanic eruptions that can improve regional climate histories.