Reading Comprehension

PT132 · S1 · P1 · Q3 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.

What is the author's primary purpose in referring to the rate of growth of a North American lichen species (fifth sentence of the second paragraph)?

to emphasize the rapidity with
Contradiction
Contradiction The question is about the purpose of referring to the rate of growth. While the sentence before mentions that lichens quickly colonize rockfalls, the rate of growth thereafter is slow.
10% picked this
to offer an example of
Unsupported Comparison
Unsupported Comparison While 9.5 millimeters per century is slow, the passage does not compare this growth rate to other species of lichen.
4% picked this
to present additional evidence supporting
Wrong Purpose
The factors that affect lichen growth rates are discussed in the third paragraph.
4% picked this
to explain why lichenometry works
Wrong Purpose / Too Strong
First, the passage states that lichenometry is best used to date earthquakes that occurred within the past 500 years in the third paragraph, and second, the passage does not say why this is the case.
4% picked this
Correct
to provide a sense of
Why this is right
In the second paragraph the author discusses the growth rate of lichens to demonstrate how fast lichens colonize newly exposed rock surfaces. This is part of a larger explanation of lichenometry that occurs in the second paragraph.
Skill tested: Local Purpose · how this choice captures the passage's function is the move to repeat next time.
78% picked this

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