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- Fri Jun 23, 2017 1:23 pm
#36246
Complete Passage Discussion
The passage compares two methods used by seismologists to date past earthquakes—radiocarbon
dating and lichenometry. After briefly describing each method, the author discusses why
lichenometry is more accurate than radiocarbon dating, but warns that lichenometry is not without
limitations.
Paragraph 1 Overview
This passage begins with a description of radiocarbon dating, an established procedure for detecting
and dating past earthquakes. When fault-line sediments shift during an earthquake, they usually
trap wood or other organic material, which contain naturally occurring radioactive isotope carbon
14. Since carbon 14 decays at a constant rate, seismologists can reconstruct the age of the organic
material trapped inside the sediments by measuring the amount of isotope remaining in it. This, in
turn, shows the location and frequency of past earthquakes.
Paragraph 2 Overview
Whenever a passage begins by describing an established theory or method, you can be almost sure
that the second paragraph will introduce a new theory or method. Indeed, the second paragraph
discusses lichenometry, a new method developed by Bull and Brandon for dating past earthquakes.
Since earthquakes generate rockfalls, and lichens grow at a constant rate on exposed rocks,
measuring the size of the lichens can help determine the age and location of the earthquake. Pay
special attention to the example of North American lichen at the end of this paragraph, which adds to
our understanding of how lichenometry is used to determine and date past earthquakes.
Paragraph 3 Overview
This paragraph compares the advantages and disadvantages of radiocarbon dating and lichenometry.
A savvy test-taker should anticipate that the author would favor the new method (lichenometry) over
its predecessor. Predictably, we are told that lichenometry has “distinct advantages” over radiocarbon
dating, which summarizes the author’s main point in this passage. The problem with radiocarbon
dating has to do with the varying amounts of carbon 14 in the environment. Since the amount of
carbon 14 depends on the intensity of radiation striking Earth’s upper atmosphere, and this intensity
has fluctuated over the past 300 years, radiocarbon dating of any event over that period is unreliable.
Note that lichenometry has its own set of limitations. Because certain factors might impede or
promote lichen growth, using lichenometry requires careful site selection to minimize the influence
of such factors, and is best used for earthquakes that occurred within the last 500 years.
Summary
The passage compares two methods for dating past earthquakes—radiocarbon dating and
lichenometry. The author’s viewpoint is consistent with that of Bull and Brandon, who favor
lichenometry over radiocarbon dating due to the former’s superior ability to accurately date
earthquakes that occurred within the past 500 years. The structure of the passage is as follows:
The passage compares two methods used by seismologists to date past earthquakes—radiocarbon
dating and lichenometry. After briefly describing each method, the author discusses why
lichenometry is more accurate than radiocarbon dating, but warns that lichenometry is not without
limitations.
Paragraph 1 Overview
This passage begins with a description of radiocarbon dating, an established procedure for detecting
and dating past earthquakes. When fault-line sediments shift during an earthquake, they usually
trap wood or other organic material, which contain naturally occurring radioactive isotope carbon
14. Since carbon 14 decays at a constant rate, seismologists can reconstruct the age of the organic
material trapped inside the sediments by measuring the amount of isotope remaining in it. This, in
turn, shows the location and frequency of past earthquakes.
Paragraph 2 Overview
Whenever a passage begins by describing an established theory or method, you can be almost sure
that the second paragraph will introduce a new theory or method. Indeed, the second paragraph
discusses lichenometry, a new method developed by Bull and Brandon for dating past earthquakes.
Since earthquakes generate rockfalls, and lichens grow at a constant rate on exposed rocks,
measuring the size of the lichens can help determine the age and location of the earthquake. Pay
special attention to the example of North American lichen at the end of this paragraph, which adds to
our understanding of how lichenometry is used to determine and date past earthquakes.
Paragraph 3 Overview
This paragraph compares the advantages and disadvantages of radiocarbon dating and lichenometry.
A savvy test-taker should anticipate that the author would favor the new method (lichenometry) over
its predecessor. Predictably, we are told that lichenometry has “distinct advantages” over radiocarbon
dating, which summarizes the author’s main point in this passage. The problem with radiocarbon
dating has to do with the varying amounts of carbon 14 in the environment. Since the amount of
carbon 14 depends on the intensity of radiation striking Earth’s upper atmosphere, and this intensity
has fluctuated over the past 300 years, radiocarbon dating of any event over that period is unreliable.
Note that lichenometry has its own set of limitations. Because certain factors might impede or
promote lichen growth, using lichenometry requires careful site selection to minimize the influence
of such factors, and is best used for earthquakes that occurred within the last 500 years.
Summary
The passage compares two methods for dating past earthquakes—radiocarbon dating and
lichenometry. The author’s viewpoint is consistent with that of Bull and Brandon, who favor
lichenometry over radiocarbon dating due to the former’s superior ability to accurately date
earthquakes that occurred within the past 500 years. The structure of the passage is as follows:
- Paragraph 1: Introduce radiocarbon dating as an established method for dating past
earthquakes.
Paragraph 2: Describe lichenometry, a new method for dating past earthquakes, and provide
an example of how lichenometry has been put into practice.
Paragraph 3: Discuss the advantages of lichenometry over radiocarbon dating, and
introduce the potential limitations of each method. Examples of environmental
conditions that affect lichen growth are given.