Passage 3 | Geology
Relative and Absolute Geological Dating
A geologist’s natural inclination on viewing exposed rock layers is to attempt to determine their ages. Since the rocks are most likely millions, or even billions, of years old, this represents a difficult task. To accomplish it, geologists have come up with two different measurement methods: relative and absolute dating. Each can be applied when studying a cross-section containing diverse layers of ) rock, such as the wall of a canyon or a cliff, in order to date the individual layers, called strata. Relative dating focuses on the chronological order in which the strata of the cross-section formed, whereas absolute dating endeavors to ^ label each stratum with a numerical age.
The term “relative” refers to the fact that this method defines a stratum’s age only in relation to the others around it. In other words, relative dating can ascertain that stratum A is o older than stratum B and younger than stratum C. What it cannot do is address how much older or younger stratum A is, or specify its exact age in years. The most significant advantage of relative dating is that geologists can usually s draw their conclusions simply by looking at the cross-section.
To accurately employ the relative dating method, geologists use several established guidelines. The most fundamental is the o principle of superposition. It holds that, in undisturbed stratified formations, the bottom layers are the oldest and those at the top are the youngest. This is because most rock strata develop from the accumulation of sediments, deposited in flat layers one on top of the other. Naturally, the layer buried the deepest would be the oldest. Most formations, however, are not so simplistic. While underground, rocks are often exposed to extreme pressures that can fold and shift the originally horizontal strata. Sometimes magma is ejected up through the layers to create its own rock deposit. And erosion experienced by layers that once were on the surface can distort the borderlines between strata. Such factors drastically complicate the relative dating procedure. Yet, with their extensive knowledge of the processes involved, geologists can usually work out the relative order of these events and a ages of the strata.
Another concept used to gain hints about relative age is correlation—the identification of fossils. These remains of ancient organisms are common in sedimentary rocks. Since certain > species were alive only at certain times, their fossils appear only in strata of specific ages. Correlation allows geologists to compare the relative ages of rock layers uncovered at different locations. If stratum A contains fossils known to be older than the fossils in stratum B, stratum A must be older than stratum B.
[A] For years, relative dating was the only tool for age measurements available to geologists. [B] While it revealed some very important information, it was limited. [C] This situation changed in the early twentieth century with the advent of absolute dating. [D] It was discovered that certain substances within rocks undergo steady, gradual transformations from one state into another over millions of years. Therefore, by analyzing the extent of these changes in a particular rock sample, the actual age of the rock can be calculated. Of course, the term “absolute” is somewhat misleading because a rock’s age is only estimated to within a range of several million years, but this dating method has enabled geologists to learn fascinating things about the age of the Earth and events that took place in its distant past.
Despite the vast amounts of new knowledge provided by absolute dating, drawbacks exist. The procedure for determining absolute age requires advanced equipment and must be performed carefully. Even slight flaws in the process can lead to a loss of accuracy when age is calculated. The absolute ages for certain strata are constantly being revised as improved technologies develop, allowing more exact measurements.
Ultimately, the choice of which dating method to use depends on the goals of the geologist, as well as the resources available. If the focus is on the relationship between the various strata of a cross-section, relative » age may be most important and can be easily determined. If, on the other hand, the geologist needs to place the creation of a stratum within a particular time frame, its absolute age must be ascertained.
27. The word inclination in the passage is closest in meaning to
(A) problem
(B) desire
(C) study
(D) system
28. What is true about the relationship between relative and absolute dating according to paragraph 1?
(A) They utilize different methods to obtain the same information.
(B) They were used in separate periods in the history of geology.
(C) They are designed to measure two different types of geological age.
(D) They each work best on different kinds of exposed cross-sections.
29. How does the author explain the focus of relative dating in paragraph 2?
(A) By discussing a specific cross-section recently examined by geologists
(B) By providing a standard definition for the word “relative”
(C) By describing how the age differences between strata are calculated
(D) By illustrating a potential relationship between three generic strata
30. The word it in the passage refers to
{A) term
(B) stratum
(C) age
(D) relative dating
31. According to paragraph 3, geologists cannot rely solely on the principle of superposition to determine the relative ages of strata because
(A) many external forces disrupt the natural layering process
(B) they do not know enough about the processes described by the principle
(C) it is only valid for strata created within a certain time frame
(D) many rocks do not form from the buildup of sediments
32. What can be inferred from paragraph 4 about ancient species?
(A) Some have survived to the present day.
(B) Their fossils appear only in lower strata.
(C) Most found fossilized in rocks have not been identified.
(D) Scientists are aware of their relative ages.
33. Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information.
(A) Events that happened millions of years ago can be studied indirectly by determining the absolute ages of rocks found on Earth today.
(B) Absolute dating helps geologists understand the Earth’s age and history despite the fact that it does not actually provide the precise ages of rocks.
(C) Even though some geologists use the term “absolute” to label this dating method, it cannot establish the exact year in which a rock was formed.
(D) The absolute dating of rocks is only one method by which geologists can learn what happened within the ancient Earth.
34. The word advent in the passage is closest in meaning to
(A) definition
(B) effort
(C) combination
(D) arrival
35. Based on the information in paragraph 5 and paragraph 6, it can be inferred that absolute dating was not possible until
(A) the necessary technology was invented
(B) estimates for the ages of rocks became available
(C) relative dating techniques were improved
(D) enough rock samples had been collected
36. The word flaws in the passage is closest in meaning to
(A) errors
(B) steps
(C) data
(D) delays
37. The word revised in the passage is closest in meaning to
(A) stored
(B) corrected
(C) divided
(D) cited
38. According to paragraph 7, the decision of which dating method to use is in part based on
(A) what the researcher is trying to accomplish
(B) which time frame the rocks appear to be from
(C) how many strata are contained in the cross-section
(D) what kind of training the geologist has
39. Look at the four squares m that indicate where the following sentence could be added to the passage.
Relative dating cannot pinpoint a precise age for any single stratum and so cannot provide the specific differences between the ages of various rock layers.
Where would the sentence best fit?
40. Directions: Complete the table by matching the phrases below.
Select the appropriate phrases from the answer choices and match them to the type of dating method to which they relate. TWO of the answer choices will NOT be used. This question is worth 4 points.
| Answer Choices
(A) Can be performed without any complicated equipment (B) Concentrates more on the order of strata formation (C) Includes the analysis of hardened animal and plant remains (D) Requires measuring the changes of certain elements within rocks (E) Must be carried out with extreme care to ensure reliable results (F) Determines the exact number of years a rock has aged (G) Is continuously being improved for better accuracy (H) Is used by geologists to learn what materials rocks are made of (I) Can be difficult due to changes inflicted on strata overtime |
Relative dating method |
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| Absolute dating method | |
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