Reading Comprehension

PT132 · S1 · P1 · Q1 Lichenometry

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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.

Topic

Lichenometry — a new method for detecting and dating past earthquakes — and how it compares to radiocarbon dating.

Framework

Highlight Noteworthy. The author introduces the new method, explains how it works, and notes its advantages over radiocarbon dating along with its limitations.

Main Point

Lichenometry, developed by Bull and Brandon, is a new method for dating past earthquakes that has distinct advantages over radiocarbon dating, though its use requires careful site selection and accurate calibration.

P1: The traditional method — radiocarbon dating

Seismologists dig trenches along fault lines, looking for shifted sediments. They use radiocarbon dating on organic materials trapped in those sediments to reconstruct the age of past earthquakes — useful for predicting future ones.

P2: The new method — lichenometry

Bull and Brandon developed lichenometry: large earthquakes generate simultaneous rockfalls; lichens colonize newly exposed rock surfaces and grow at a slow but constant rate (e.g., 9.5 mm/century). The diameter of the largest lichen on a boulder dates the rockfall. Many simultaneous rockfalls over a wide area imply a strong earthquake. Mapping rockfall density gives the epicenter.

P3: Lichenometry's advantages — and caveats

Radiocarbon is accurate only to ±40 years (because atmospheric carbon-14 varies); past 300-year fluctuations make many radiocarbon datings of recent events of little value. Lichenometry can date earthquakes to within 10 years. Caveats: requires careful site selection and accurate calibration; best for earthquakes within the last 500 years; sites must minimize avalanche/disturbance; growth-affecting conditions (shade, wind) must be factored in.

1.

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

  1. Correct

    Lichenometry is a new method

    Why this is right

    Whoa. That was spot on with our prediction. It's centered on the main topic, aligned with our Old / New framework, and carrying the author's voice by mentioning the "advantages". What's not to love?

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

    89% picked this

  2. Despite its limitations, lichenometry has

    Too Strong

    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% picked this

  3. Most seismologists today have rejected

    Too Strong

    The passage doesn't say "most seismologists" have rejected radiocarbon and embraced lichenometry or that it's the "most reliable". The passage describes the new method's advantages but doesn't make claims about how widely adopted it is yet.

    0% picked this

  4. Two geologists have revolutionized the

    Too Strong

    "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 goes against the idea that it's an "easily applied" method.

    7% picked this

  5. Radiocarbon dating, an unreliable test

    Too Strong

    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 and says that datings from the past 300 years "of little value", but that is not as strong as "can finally be abandoned". Lichenometry only works for the last 500 years, so radiocarbon would still be needed for older events.

    0% picked this

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