GeoClassroom Physical Geology Historical Geology Structure Lab

Review Questions and Answers; Geologic Time


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1. Distinguish between absolute and relative dating.

Absolute dating involves a numerical age measurement in actual time units, like thousands or millions of years. Relative dating involves placing sequences of rocks, geological features, and events in the correct order in which they occurred, without necessarily knowing their absolute ages.

2. Describe two early methods for dating Earth. How old was Earth thought to be according to these estimates? List some weaknesses of each method. (Box 8.1)

Geologists such as Hutton and Lyell, on a knowledgeable, intuitive basis, argued that the Earth was very old. With the general acceptance of Darwin's ideas of biological evolution in the late nineteenth century, naturalists and biologists had an increased intellectual stake in an "old Earth". Early nineteenth century catastrophists believed in a "young Earth", and their intellectual descendants, the "latter-day creationists", still have a strong bias against an old Earth. On a scientific basis, these long standing arguments were settled once and for all early in this century when the first, chemical (isotopes were not considered), uranium-lead age determinations showed that the analyzed samples of Precambrian rocks were in excess of one billion years old. Elucidation of the uranium-lead and thorium-lead decay schemes (Table 8.1) and subsequent, technological and instrumental advances (the mass spectrometer, in particular) led the way to the 4.6 billion-year age for Earth that is widely accepted today.

Prior to the discovery of radioactivity and its exploitation for dating, substantive methods for "guessing" the Earth's age fell into three categories: accumulation rates of sedimentary rocks; rates of salt accumulation in the oceans; and Lord Kelvin's highly authoritative, but incorrect, calculation based on thermal considerations. His age (<= 100 million years) was much too young because radioactivity, which supplies much of the heat flow observed through the continents, was ignored. It had yet to be discovered! Lord Kelvin assumed an initially molten Earth, heat losses by conduction only and no internal sources of heat. His calculation of the Sun's age, made about the same time, was based on similar considerations. Since nuclear reactions were unknown at that time, the Sun's main, energy-producing process was ignored in calculating its energy budget! Unfortunately, Lord Kelvin's cooling-interval ages for the Earth and Sun came out to be about the same, leading him to believe that both bodies had formed simultaneously and providing further justification to argue that his ages were correct.

Salt-content-of-the-ocean calculations were based on the following assumptions: all water on Earth was initially fresh; dissolved salts were added to the oceans only by streams and rivers; and the amounts of dissolved salts added on a yearly basis could be computed by multiplying dissolved salt concentrations in streams and rivers by the volumes of freshwater flowing into the oceans each year. Thus theoretically, one could compute how many years were required for the oceans to acquire their present-day content of dissolved salts. Many important aspects of the ocean's salt budget were unknown or ignored. These included the deposition of bedded salts and their "storage" in sedimentary rocks, transfer of sea salts to the atmosphere and their return to the ocean via rivers and streams, and addition of salts through volcanism and submarine, hot spring emissions. Although computed ages (about 90 million years) were much too young, the results supported the concept of an "old Earth".

Ages based on rates of sedimentary rock accumulation were also subject to many incorrect assumptions and unknown aspects of the sedimentary cycle. Rates of

deposition and erosion were highly variable in space and time, and many sedimentary rocks were derived from sedimentary source rocks. An "average" rate of sedimentary rock accumulation had to be assumed. Despite these numerous problems, oldest estimates approached one billion years and gave further credence to the idea that the Earth was indeed "old". Even the youngest estimates (about three million years) gave scant comfort to those who supported Bishop Ussher's old testament, genealogical analysis purporting to show that the Earth was born in the year 4004 B. C.

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3. What is the law of superposition? How are cross-cutting relationships used in relative dating?

The law of superposition is the idea or notion that beds in a sequence of horizontal, sedimentary strata become younger upward in the sequence. In other words, younger strata are deposited over older strata. A feature that truncates or cuts across another geologic feature is the younger of the two. For example, a dike of basalt injected into a crack in sedimentary strata is younger than the strata.

4. Refer to Figure 8.4 and answer the following questions:

(a) Is fault A older or younger than the sandstone layer? Fault A cuts the sandstone layer so the fault is younger.

(b) Is dike A older or younger than the sandstone layer? Dike A also crosscuts the sandstone layer so the dike is younger.

(c) Was the conglomerate deposited before or after fault A? Fault A stops at the base of the conglomerate; thus the conglomerate layer truncates the fault and is younger than the fault.

(d) Was the conglomerate deposited before or after fault B? The conglomerate is cut and displaced by fault B; thus fault B is younger.

(e) Which fault is older, A or B? The faults do not cross, but the relationship between the faults and the conglomerate proves that fault A is older than fault B.

(f) Is dike A older or younger than the batholith? Dike A does not cut the batholith so other relationships must be used. Dike B clearly cuts the batholith; the sill fed by dike B is crosscut by dike A, proving that dike A is younger than dike B and younger than the batholith.

5. When you observe an outcrop of steeply inclined sedimentary layers, what principle allows you to assume that the beds were tilted after they were deposited?

The principle of original horizontality states that, in general, stratification in sedimentary beds was horizontal when the beds were deposited.

6. A mass of granite is in contact with a layer of sandstone, but does not cut across it. How might you determine whether the sandstone was deposited on top of the granite, or whether the granite was intruded from below after the sandstone was deposited?

A depositional contact or unconformity would be proven if detrital rock and mineral grains from the granite (Fig. 8.5) were found in the sandstone. Also the granite just below the contact might show reddish discoloration or other evidences of having been weathered before the sandstone was deposited. Bedding in the sandstone will be parallel or nearly parallel to the contact; there will be no evidence for contact metamorphism in the sandstone; and the sandstone will not be cut by granite dikes.

If the contact is intrusive, the sandstone may be cut by granite dikes and may show contact metamorphism. Rock and mineral grains in the sandstone will not show any direct correlation to the granite, and bedding in the sandstone will probably not be parallel to the contact.

7. Distinguish among angular unconformity, disconformity, and nonconformity.

These are all erosion surfaces buried beneath younger strata. The older strata below an angular unconformity were tilted before the younger strata were deposited; thus the older and younger strata exhibit a sharp, angular, erosional discordance (Figs. 8.6, 8.7 & 8.8). Strata above and below a disconformity exhibit parallel stratification or bedding orientations, indicating that the underlying, older strata were not tilted or deformed before the younger strata were deposited. Younger, sedimentary beds deposited on an eroded mass of older, igneous rock comprise a nonconformity (Fig. 8.5).

8. What is meant by the term correlation?

Correlation is the process of establishing equivalency of rock units, ages, depositional environments, and events in geologic history (faults, tectonic events, unconformities, etc.) in different areas. Correlation can be local (between rocks intersected in neighboring drill holes) or world-wide (continent to continent).

9. Describe William Smith's important contribution to the science of geology.

Smith was an English naturalist who first convinced other geologic thinkers of his day that strata containing the same assemblages of fossils were correlatable from place to place. Thus Smith can be thought of as the founder of the study of stratigraphy and as a leading advocate of using fossil assemblages to correlate equivalent-aged strata (the principle of faunal succession).

10. Why are fossils such useful tools in correlation?

Fossil organisms have great diversity, and certain individual organisms and/or assemblages of organisms are characteristic of beds deposited during specific periods of geologic time. Thus fossils are useful for correlating the same bed or same sequence of beds among different localities and for determining the geologic ages of the beds.

11. Figure 8.17 is a block diagram of a hypothetical area in the American Southwest. Place the lettered features in the proper sequence, from oldest to youngest. Identify an angular unconformity and a nonconformity.

The contact between sedimentary beds I (younger and horizontal) and sedimentary beds A (older and tilted) is an angular unconformity. The contact between igneous rock D (older) and the sedimentary beds I is a nonconformity.

Younger

(10) alluvial fan E; dike, cinder cone, and lava flow, F

(9) fault G

(8) igneous rock, dike and sill, C

(7) igneous intrusion K

(6) sedimentary beds J

(5) sedimentary beds I

(4) intrusive igneous rock D (a batholith)

(3) dike of igneous rock B

(2) sedimentary strata A

(1) metamorphic rock mass H

Older

12. If a radioactive isotope of thorium (atomic number 90, mass number 232) emits 6 alpha particles and 4 beta particles during the course of radioactive decay, what are the atomic number and mass number of the stable daughter product?

Each beta decay raises the atomic number by one and does not affect the mass number. Each alpha decay decreases the atomic number by 2 and the mass number by 4. Thus, for 6 alpha decays and 4 betas, the atomic number of the daughter would be (90 - 6X2 + 4) = 82, which is the atomic number of lead. The mass number of the daughter would be (232 - 6X4) = 208. The stable daughter is lead-208.

13. Why is radiometric dating the most reliable method of dating the geologic past?

With careful sample collection and laboratory procedures, the radiometric methods consistently give accurate, reliable, absolute ages. No other method can be applied to all of geologic time. Fossils are accurate and reliable for Phanerozoic sedimentary rocks but are not found in most igneous and metamorphic rocks and are very rare in Precambrian rocks. The Phanerozoic time scale has been accurately calibrated with radiometric ages, and Proterozoic and Archean chronologies are based entirely on radiometric dates.

14. A hypothetical radioactive isotope has a half-life of 10,000 years. If the ratio of radioactive parent to stable daughter product is 1 : 3, how old is the rock containing the radioactive material?

A ratio of 1 : 1 would be produced in 10,000 years (one half-life). After two half-lives, 25 percent of the original parent would be left and 75 percent of the daughter would have formed. The ratio (25 : 75) is 1 : 3, so the sample is 20,000 years old.

15. Why is potassium-40 used more frequently in radiometric dating than other isotopes?

The daughter isotope, argon-40, is a nonreactive gas that is relatively easy to extract from minerals and purify. Recent advances in laboratory techniques and instruments have allowed the K-Ar method to be used on very young samples, even those of Pleistocene age. Thus K-Ar is the only radiometric technique that is suitable for rocks of all ages. Potassium-bearing minerals, such as feldspars and micas, are common in many, different, kinds of rocks and they are relatively easy to separate; thus the K-Ar method has wide applicability in age-dating studies. However, the method is known to give unreliable, low ages for very old rocks that were deeply buried and warmed for long periods of time.

16. Why is the ratio between potassium-40 and calcium-40 not used for radiometric dating?

Potassium-40 undergoes a branching decay; some atoms decay to calcium-40 by beta emission and the others decay to argon-40 by electron capture. The K-Ar method is widely used for radiometric age determinations, especially for samples of Phanerozoic age (see Review Question 15). Recent, technological advances have extended the reliability of K-Ar dating to very young samples.

In contrast, the K-40/Ca-40 decay is rarely used for age determinations. Calcium is an abundant element and Ca-40 atoms comprise 96 percent of all calcium atoms. Thus radiogenic Ca-40 comprises just a small proportion of the Ca-40 present in most K-bearing rocks and minerals, making the measurement of the ratio K-40/*Ca-40 (*Ca-40 means radiogenic calcium-40 atoms) difficult and relatively imprecise. In contrast, most of the argon-40 in rocks and minerals is radiogenic, so the ratio K-40/*Ar-40 can be measured with high precision in most cases.

Early attempts to use the K-40/Ca-40 method focused on dating sylvite (KCl). However, its high water solubility led to "closed system" behavior problems (see Review Question 17) with respect to parent and daughter elements. The method might eventually be revived and improved to date potassium-bearing, low-Ca, Precambrian samples; but at present, the Rb/Sr method and the U/Pb methods applied to zircons give good results on these samples.

17. In order to provide a reliable radiometric date, a mineral must remain a closed system from the time of its formation until the present. Why is this true?

If the abundances of the parent or daughter isotopes in a mineral or rock sample have been changed by any process other than radioactive decay, the parent to daughter ratio will not be a true measure of the age of the sample.

18. What precautions are taken to insure reliable radiometric dates?

The work must be done carefully, and the laboratory environment must be free of materials that might contaminate the sample and produce a change in the measured, parent to daughter isotopic ratio. Other precautions include careful sample collection, good mineral separations, repeated analyses of the same samples to establish precision limits, and age determinations by other methods to check for consistency and accuracy.

Finally, careful attention to geologic relationships will reduce the chances of misinterpreting the results.

19. To make calculations easier, let us round the age of Earth to 5 billion years.

(a) What fraction of geologic time is represented by recorded history (assume 5000 years for the length of recorded history)? The percentage is 5 X 10^3 yrs divided by 5 X 10^9 yrs X 10^2 % which equals 1 X 10^-4 % or 0.0001 %.

(b) The first abundant fossil evidence does not appear until the beginning of the Cambrian period (570 million years ago). What percentage of geologic time is represented by abundant fossil evidence? The percentage is 6 X 10^8 yrs divided by 5 X 10^9 yrs X 10^2 % = 1.2 X 10^1 % or 12 %.

20. What subdivisions make up the geologic time scale?

The following are the various divisions listed from longest to shortest time intervals: eons, eras, periods, and epochs.

21. Explain the lack of a detailed time scale for the vast span known as the Precambrian.

Three main factors are involved. Fossils are lacking or very difficult (single-celled organisms) to use for age determinations; many Precambrian rocks formed deep in the ancient crust and can be dated only by radiometric methods; and Precambrian rocks are deeply eroded and/or buried by Phanerozoic rocks. Thus compared to younger strata, Precambrian rocks are less accessible and their geologic record is much less detailed.

22. Briefly describe the difficulties in assigning absolute dates to layers of sedimentary rock.

In general, sedimentary rocks do not contain minerals that are both suitable for dating and that crystallized when the bed was deposited. One exception would be feldspar or mica grains in volcanic ash deposited at the time of the eruption. Minerals such as glauconite crystallize as sedimentary grains but contain large quantities of nonradiogenic daughter element, making an age determination imprecise.

In recent years, advances in instrumentation and the application of new geochronological methods have led to much more precision and accuracy in dating of sedimentary rocks. For example, extensive, detailed micropaleontological data and very precise Sr-87/Sr-86 measurements in Mesozoic and Cenozoic marine limestones have been correlated, resulting in very precise age assignments for many marine carbonate strata. In addition, paleomagnetic measurements combined with the known geomagnetic time scale and paleontological data can often result in very precise age assignments for some strata.


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