To Study Centuries-Old Earthquakes and The Geologic Faults that Caused Them GMAT Reading Comprehension

Reading Passage Question:

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 when they shifted. Since carbon 14 occurs naturally in organic materials and decays at a constant rate, the age of organic materials can be reconstructed from the amount of the isotope remaining in them. These data can show the location 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 involves measuring the size of lichens growing on the rocks exposed by these rockfalls. Lichens—symbiotic organisms consisting of a fungus and an alga—quickly colonize newly exposed rock surfaces in the wake of rockfalls, and once established they grow radially, flat against the rocks, at a slow but constant rate for as long as 1,000 years if left undisturbed. One species of North American lichen, for example, spreads outward by about 9.5 millimeters each century. Hence, the diameter of the largest lichen on a boulder provides direct evidence of when the boulder was dislodged and repositioned. If many rockfalls over a large geographic area occurred simultaneously, that pattern would imply that there had been a strong earthquake. The location of the earthquake’s epicenter can then be determined 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 300 years, causing many radiocarbon datings of events during this period to be of little value. Lichenometry, Bull and Brandon claim, can accurately date an earthquake to within ten years. They note, however, that using lichenometry requires careful site selection and accurate calibration of lichen growth rates, adding that the method is best used for earthquakes that occurred within the last 500 years. Sites must be selected to minimize the influence of snow avalanches and other disturbances that would affect normal lichen growth, and conditions like shade and wind that promote faster lichen growth must be factored in.

‘To study centuries-old earthquakes and the geologic faults that caused them’’ is a GMAT reading comprehension passage with answers. Candidates need a strong knowledge of English GMAT reading comprehension.

This GMAT Reading Comprehension consists of 7 comprehension questions.

The GMAT Reading Comprehension questions are designed for the purpose of testing candidates’ abilities in understanding, analysing, and applying information or concepts. Candidates can actively prepare with the help of GMAT Reading Comprehension Practice Questions.

Solutions and Explanations

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

(A) Lichenometry is a new method for dating past earthquakes that has advantages over radiocarbon dating.
(B) Despite its limitations, lichenometry has been proven to be more accurate than any other method of discerning the dates of past earthquakes.
(C) Most seismologists today have rejected radiocarbon dating and are embracing lichenometry as the most reliable method for studying past earthquakes.
(D) Two geologists have revolutionized the study of past earthquakes by developing lichenometry, an easily applied method of earthquake detection and dating.
(E) Radiocarbon dating, an unreliable test used in dating past earthquakes, can finally be abandoned now that lichenometry has been developed.

2. The passage provides information that most helps to answer which one of the following questions?

(A) How do scientists measure lichen growth rates under the varying conditions that lichens may encounter?
(B) How do scientists determine the intensity of the radiation striking Earth’s upper atmosphere?
(C) What are some of the conditions that encourage lichens to grow at a more rapid rate than usual?
(D) What is the approximate date of the earliest earthquake that lichenometry has been used to identify?
(E) What are some applications of the techniques involved in radiocarbon dating other than their use in studying past earthquakes?

3. What is the author’s primary purpose in referring to the rate of growth of a North American lichen species (Highlighted)?

(A) to emphasize the rapidity with which lichen colonies can establish themselves on newly exposed rock surfaces
(B) to offer an example of a lichen species with one of the slowest known rates of growth
(C) to present additional evidence supporting the claim that environmental conditions can alter lichens’ rate of growth
(D) to explain why lichenometry works best for dating earthquakes that occurred in the last 500 years
(E) to provide a sense of the sort of timescale on which lichen growth occurs

4. Which one of the following statements is most strongly supported by the passage?

(A) Lichenometry is less accurate than radiocarbon dating in predicting the likelihood and location of future earthquakes.
(B) Radiocarbon dating is unlikely to be helpful in dating past earthquakes that have no identifiable fault lines associated with them.
(C) Radiocarbon dating and lichenometry are currently the only viable methods of detecting and dating past earthquakes.
(D) Radiocarbon dating is more accurate than lichenometry in dating earthquakes that occurred approximately 400 years ago.
(E) The usefulness of lichenometry for dating earthquakes is limited to geographic regions where factors that disturb or accelerate lichen growth generally do not occur.

5. The primary purpose of the first paragraph in relation to the rest of the passage is to describe

(A) a well-known procedure that will then be examined on a step-by-step basis
(B) an established procedure to which a new procedure will then be compared
(C) an outdated procedure that will then be shown to be nonetheless useful in some situations
(D) a traditional procedure that will then be contrasted with other traditional procedures
(E) a popular procedure that will then be shown to have resulted in erroneous conclusions about a phenomenon

6. It can be inferred that the statements made by Bull and Brandon and reported (Text in red at the end of the passage) rely on which one of the following assumptions?

(A) While lichenometry is less accurate when it is used to date earthquakes that occurred more than 500 years ago, it is still more accurate than other methods for dating such earthquakes.
(B) There is no reliable method for determining the intensity of the radiation now hitting Earth’s upper atmosphere.
(C) Lichens are able to grow only on the types of rocks that are common in mountainous regions.
(D) The mountain ranges that produce the kinds of rockfalls studied in lichenometry are also subject to more frequent snowfalls and avalanches than other mountain ranges are.
(E) The extent to which conditions like shade and wind have affected the growth of existing lichen colonies can be determined.

7. The passage indicates that using radiocarbon dating to date past earthquakes may be unreliable due to

(A) the multiplicity of the types of organic matter that require analysis
(B) the variable amount of organic materials caught in shifted sediments
(C) the fact that fault lines related to past earthquakes are not always visible
(D) the fluctuations in the amount of the carbon 14 isotope in the environment over time
(E) the possibility that radiation has not always struck the upper atmosphere

8. Given the information in the passage, to which one of the following would lichenometry likely be most applicable?

(A) identifying the number of times a particular river has flooded in the past 1,000 years
(B) identifying the age of a fossilized skeleton of a mammal that lived many thousands of years ago
(C) identifying the age of an ancient beach now underwater approximately 30 kilometers off the present shore
(D) identifying the rate, in kilometers per century, at which a glacier has been receding up a mountain valley
(E) identifying local trends in annual rainfall rates in a particular valley over the past five centuries

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