Lichenometry

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

TopicsLocal PurposeScience

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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

Local Purpose

Topic

The author is introducing a clever new way to figure out when past earthquakes happened — using the size of lichens growing on rocks.

Framework

Highlight Noteworthy.

Main Point

The simpler version: scientists used to date past earthquakes by digging trenches and using carbon-14 dating on organic material in shifted dirt. Two geologists (Bull and Brandon) came up with a new approach: when earthquakes happen, rocks fall, and lichens — which grow slowly but steadily — start growing on the newly exposed rock. Measure the biggest lichen on a boulder, and you know roughly when the rock fell. The new method is more accurate than carbon dating (within 10 years vs. 40), though it has its own quirks.

P1: The old way

Dig along faults; carbon-date the organic material trapped in shifted sediments.

P2: The new way

Earthquakes shake rocks loose. Lichens then start growing on those rocks at a known rate. Measure the biggest lichen and you've dated the earthquake. Find lots of same-age rockfalls in one region and you've found an earthquake; map them and you find the epicenter.

P3: Why it's better — but not perfect

Carbon dating is only accurate to ±40 years, and the last 300 years are especially noisy. Lichenometry can hit ±10 years. Catch: it works best within the last 500 years, you have to pick the right sites, and you have to factor in things that affect lichen growth.

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

The question

What is the author’s primary purpose in referring to the rate of growth of a North American lichen species

Answer choices

Contradiction10% picked this
to emphasize the rapidity with which lichen colonies can establish themselves on newly
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%
Unsupported Comparison4% picked this
to offer an example of a lichen species with one of the slowest known
Unsupported Comparison While 9.5 millimeters per century is slow, the passage does not compare this growth rate to other species of lichen.
4%
Wrong Purpose4% picked this
to present additional evidence supporting the claim that environmental conditions can alter lichens'
The factors that affect lichen growth rates are discussed in the third paragraph.
4%
Wrong Purpose / Too Strong4% picked this
to explain why lichenometry works best for dating earthquakes that occurred in the
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%
Correct78% picked this
to provide a sense of the sort of timescale on which
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%

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