Lichenometry

A free, expert breakdown of this official LSAT Reading Comprehension question.

TopicsMain PointScience

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Passage

To study centuries-old earthquakes and the geologic faults that caused them, seismologists usually dig trenches along visible fault lines, looking for sediments that show evidence of having shifted. Using radiocarbon dating, they measure the quantity of the radioactive isotope carbon 14 present in wood or other organic material trapped in the sediments and frequency of past earthquakes and provide hints about the likelihood and location of future earthquakes.

Geologists William Bull and Mark Brandon have recently developed a new method, called lichenometry, for detecting and dating past earthquakes. Bull and Brandon developed the method based on the fact that large earthquakes generate numerous simultaneous rockfalls in mountain ranges that are sensitive to seismic shaking. Instead of dating fault-line sediments, lichenometry by mapping these rockfalls, since they decrease in abundance as the distance from the epicenter increases.

Lichenometry has distinct advantages over radiocarbon dating. Radiocarbon dating is accurate only to within plus or minus 40 years, because the amount of the carbon 14 isotope varies naturally in the environment depending on the intensity of the radiation striking Earth’s upper atmosphere. Additionally, this intensity has fluctuated greatly during the past growth, and conditions like shade and wind that promote faster lichen growth must be factored in.

What this question is testing

Main Point

Anticipate

This is a Main Point question. Step back and ask: across all three paragraphs, what is the author actually telling me about?

P1 sets up the old method (radiocarbon). P2 explains the new method (lichenometry). P3 explicitly says lichenometry has "distinct advantages" over radiocarbon, then gives caveats. So the main point is: there's a new method called lichenometry that has advantages over the old radiocarbon approach.

Goal

Look for an answer that captures the new method + the advantages framing. Common traps:

Answers that overstate — "proven more accurate" than any method, "most reliable," "revolutionized"

Answers that have seismologists "rejecting" or "abandoning" radiocarbon — too strong; the passage says lichenometry has advantages, not that it replaces

Answers that frame radiocarbon as "unreliable" overall — the passage says it's less accurate, not unreliable

Reading along? Open the full official question in LawHub — we show a fragment here and keep the reasoning in our own words.

The question

Which one of the following most accurately expresses the main idea of

Answer choices

Correct89% picked this
Lichenometry is a new method for dating past earthquakes that has advantages
Why this is right
The passage introduces lichenometry as a new method developed by Bull and Brandon (P2), and P3 explicitly states it has "distinct advantages over radiocarbon dating." (A) captures both halves: a new method with advantages over radiocarbon. The caveats are subordinate.
Skill tested: Main Point · how this choice captures the passage's function is the move to repeat next time.
89%
Too Strong4% picked this
Despite its limitations, lichenometry has been proven to be more accurate than any other method of discerning the
The passage says lichenometry has advantages over radiocarbon dating — not that it has been "proven more accurate than any other method." That's a much broader claim. The passage compares only these two methods, not all dating methods.
4%
Too Strong0% picked this
Most seismologists today have rejected radiocarbon dating and are embracing lichenometry as the most reliable method
The passage doesn't say "most seismologists" have rejected radiocarbon and embraced lichenometry. The passage describes the new method's advantages but doesn't make claims about field-wide adoption.
0%
Too Strong7% picked this
Two geologists have revolutionized the study of past earthquakes by developing lichenometry, an easily applied method of
"Revolutionized" overstates the passage. The author presents lichenometry as a new method with advantages and caveats — not as a revolution. Also, the passage emphasizes site-selection care and limitations, which contradicts the "easily applied" framing.
7%
Too Strong0% picked this
Radiocarbon dating, an unreliable test used in dating past earthquakes, can finally be abandoned now that
The passage doesn't describe radiocarbon as "unreliable" or say it can be abandoned. It says radiocarbon is "accurate only to within plus or minus 40 years" and the past-300-year radiocarbon datings are "of little value" — which is more limited. Lichenometry only works for the last 500 years, so radiocarbon would still be needed for older events.
0%

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