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HISTORICAL GEOLOGY

 

GEOLOGIC TIME: CONCEPTS AND PRINCIPLES

 

 

OUTLINE

INTRODUCTION

HOW IS GEOLOGIC TIME MEASURED?

EARLY CONCEPTS OF GEOLOGIC TIME AND EARTH’S AGE

RELATIVE DATING METHODS

ESTABLISHMENT OF GEOLOGY AS A SCIENCE—THE TRIUMPH OF

UNIFORMITARIANISM OVER NEPTUNISM AND CATASTROPHISM

            Neptunism and Catastrophism

            Uniformitarianism

            Modern View of Uniformitarianism

LORD KELVIN AND A CRISIS IN GEOLOGY

ABSOLUTE DATING METHODS

Atoms, Elements, and Isotopes

Radioactive Decay and Half-Lives

Long-Lived Radioactive Isotope Pairs

Fission Track Dating

Radiocarbon and Tree Ring Dating Methods

GEOLOGIC TIME AND CLIMATE CHANGE

¨Perspective Denver’s Weather—280 Million Years Ago!

SUMMARY

 

 

CHAPTER OBJECTIVES

The following content objectives are presented in Chapter 4:

¨     The concept of geologic time and its measurement have changed through human history.

¨     The fundamental principles of relative dating provide a means to interpret geologic history.

¨     The principle of uniformitarianism is fundamental to geology and prevailed over the concepts of neptunism and catastrophism because it provides a better explanation for observed geologic phenomena.

¨     The discovery of radioactivity provided geologists with a clock that could measure Earth’s age and validate that Earth was very old.

¨     Absolute dating methods are used to date geologic events in terms of years before present.

 

¨     The most accurate radiometric dates are obtained from igneous rocks.

¨     Geologic time is an important element in the study of climate change.

 

 

LEARNING OBJECTIVES

To exhibit mastery of this chapter, students should be able to demonstrate comprehension of the following:

¨     the differences between relative and absolute dating methods

¨     the concept of geologic time and its importance to the study of historical geology

¨     the manner in which the concept of geologic time has dramatically changed

¨     the fundamental principles of relative dating and their importance for reconstructing Earth history

¨     the concepts of, and differences between, neptunism, catastrophism, and uniformitarianism

¨     Lord Kelvin’s role in nearly destroying the foundation of uniformitarian geology defined by Hutton and Lyell

¨     radioactive decay, and the principles involved in the use of radiometric dating for absolute age dating of geologic events

¨     the various long-lived radioactive isotope pairs, and their limitations for use in terms of absolute ages

¨     alternative absolute dating methods, including fission track and tree-ring dating methods

¨     the carbon-14 dating method, and its differences from long-lived radioactive isotope dating techniques

¨     methods of determining past climate changes in the geologic record, and the importance of these data in climate change research and predictions

 

CHAPTER SUMMARY

     

1.    Geologists use two frames of reference for geologic time: relative age dating places geological events in a sequential order, while absolute age dating provides specific ages for rock units or events. The geologic time scale uses both frames of reference.

         Figure 4.1       The Geologic Time Scale

 

2.   Early Christian theologians were responsible for formulating the idea that time is linear, and that Earth was very young. James Ussher calculated an age for Earth of approximately 6,000 years based upon his interpretation of scripture.

Other attempts were made to determine Earth’s age on the basis of scientific evidence, but the ages were too young.

 

3.   Relative dating methods place events in sequential order, but do not tell us how long ago the event took place. 

 

4.   Nicholas Steno developed three principles of relative age dating, including the principle of superposition (oldest rock layers are on the bottom), the principle of original horizontality (sediments are originally deposited horizontally), and the principle of later continuity (sediment extends out or terminates against the edge of a depositional basin).

            Figure 4.2       The Principles of Original Horizontality, Superposition, and

                                    Lateral Continuity

 

5.  Hutton is credited for the principle of cross-cutting relationships, which states that igneous intrusions or faults are younger than the rocks they intrude or displace.        

            Figure 4.3       The Principle of Cross-Cutting Relationships

           

   6.    Considering the religious, political, and social climate of the 17^th, 18^th, and 19^th centuries, it is easy to see why concepts such as neptunism, catastrophism, and a very young Earth were eagerly embraced. As geologic data accumulated, it became apparent that these concepts were not supported by evidence and that Earth must be much older than 6,000 years. 

            Table 4.1         Werner’s Subdivision of the Rocks of Earth’s Crust

 

   7.    James Hutton thought present-day processes operating over long periods of time could explain all the geologic features of Earth. He also viewed Earth history as cyclical and through Earth to be very old. Hutton’s observations were instrumental in establishing the principle of uniformitarianism.

            Figure 4.4       Angular Unconformity at Siccar Point, Scotland (Active Figure)

 

   8.   Uniformitarianism, as articulated by Charles Lyell, soon became the guiding principle of geology. According to the principle of uniformitarianism, the laws of nature have been constant through time, and the same processes operating today have also operated in the past, although not necessarily at the same rates.

      

Enrichment Topic 1. Scientific Principles and Scientific Laws

The late Stephen J. Gould argued that a scientific law is not the pinnacle of a hierarchy that begins with observation and ends with a "proven” theory or a mathematical expression of a "truth.” Rather, a scientific law is nothing more than a fundamental assumption used as a basis for the scientific method. The assumption is unquestioned because no unexplained instance of phenomena violating the assumption has ever been documented. As such, the principles of historical geology are actually its scientific laws.

 

 

 

Enrichment Topic 2. The Golden Age of Geology

The “Golden Age of Geology” describes the early years (1798-1840) when the discipline of geology was evolving as a separate science, and the constructs of plutonism and uniformitarianism were being proposed and debated. These proposals weren’t readily accepted, and the discipline only evolved through numerous debates and conflicts. Several articles and books by researchers of the history of science provide some insight into the cultural, political, and social influences of 19^th century upon the evolving science of geology and/or ideas for classroom incorporation of the history of science (Clary, Rudwick, Thackray, and others). The 1830s were particularly important for the development of geology, since it was in the 1830s that a working class emerged in Great Britain, and printing techniques had advanced to allow easier replication and greater affordability of books. The great geologists of the day (De la Beche, Buckland, Lyell, Mantell) wrote geological texts for this working class, and the dissemination of geological thought became more widespread among the lay population.

 

  9.    The modern view of uniformitarianism assumes that the laws of nature are constant,

         but the rates and intensities of change have varied throughout time. Some geologists

         prefer the term “actualism” rather than “uniformitarianism.”

 

 10.   Lord Kelvin precipitated a crisis in geology when he used the heat of Earth, the

         melting temperatures of rocks, and the rate of heat loss to calculate an age for Earth

          of 20-400 million years. The discovery of radioactivity destroyed Kelvin’s

         arguments.

 

11.    Radioactivity was discovered during the late 19^th century, and soon thereafter radiometric dating techniques allowed geologists to determine absolute ages for geologic events. Radioactive decay results in the change of atomic structure; types of radioactive decay include alpha decay, beta decay, and electron capture. Several steps may be involved in an element’s decay.

Figure 4.5       Three Types of Radioactive Decay

 

12.    Absolute-age dates for rock samples are usually obtained by determining how many half-lives of the radioactive parent element have elapsed since the sample originally crystallized. A half-life is the time it takes for one-half of the radioactive parent element to decay to a daughter element.

Figure 4.6       Radioactive Decay Series for Uranium 238 and Lead 206

Figure 4.7       Uniform, Linear Change Compared to Geometric Radioactive

Decay (Active Figure)

 

13.    The most accurate radiometric dates are obtained from long-lived radioactive isotope pairs in igneous rocks. The five common long-lived radioactive isotope pairs are uranium 238-lead 206, uranium 235-lead 207, thorium 232-lead 208, rubidium 87-strontium 87, and potassium 40-argon 40. The most reliable dates are those obtained by using at least two different radioactive decay series in the same rock.

 

Table 4.2           Five of the Principal Long-Lived Radioactive Isotope Pairs Used

in Radiometric Dating

Figure 4.8          Crystallization of Magma Containing Radioactive Parent and

Stable Daughter Atoms

Figure 4.9          Effects of Metamorphism on Radiometric Dating

 

Enrichment Topic 3. The Oldest Known Rocks

Rocks more than 4 billion years old have been found along the Acasta River in Canada’s Northwest Territories. The rock is gneiss, a metamorphic rock that exhibits banding of minerals. It was probably among the first crystal rocks to form on Earth and has been dated radiometrically using the uranium-lead method. In Australia, scientists have dated a 4.4 billion year old zircon crystal inside a rock that is 3.1 billion years old. National Geographic, Sept 2001 v200 no3 p78 (23).

 

14.    Fission-track dating, tree-ring dating, and radiocarbon dating are other absolute age dating methods.Carbon-14 dating is effective back to about 70,000 years ago and can only be used on organic matter such as wood, bones, and shells. Unlike the long-lived isotopic pairs, carbon-14 dating determines age by the ratio of radioactive carbon 14 to stable carbon 12. 

Figure 4.10     Fission-Track Dating

Figure 4.11     Carbon-14 Dating Method (Active Figure)

Figure 4.12     Tree-Ring Dating Method

 

15.  Geologists have reconstructed past climates by analyzing stalagmites from caves.

       Radiometric dating of stalagmite layers using uranium 234 to thorium 230 is reliable

       to about 500,000 years. Precise and accurate geologic calendars allow geologist to

       reconstruct past climate changes and link them to possible causes.

            Figure 4.13     Stalagmites and Climate Change

 

Enrichment Topic 4. Climate Change Assessment in the Geologic Record

In order to assess future climate impacts, paleoclimatologists are investigating the Pliocene as a possible analog for future warmer climate. Occurring approximately 3.3-3.0 million years ago, the middle Pliocene is the most recent period in which global temperatures reached the warmer temperatures projected for the end of this century. The Pliocene is more appropriate analog because the continental positions and CO₂ concentrations more closely match modern levels, as opposed to other warmer periods, such as the Late Cretaceous. However, global warmth of the Pliocene was distributed differently than in modern times. Paleoclimatologists are working on accurate data synthesis and model simulation to “unlock the secret to climate sensitivity” through research into the mid-Pliocene. Robinson, Dowett, & Chandler (2008). “Pliocene Role in Assessing Future Climate Impacts,” EOS, 89 (49), p. 501

 

Enrichment Topic 5. Climate Change in the News

Students can investigate current newspaper and Internet postings on climate change. For example, in December 2008, GermanWatch and the Clean Action Network (CAN)-Europe released the 2009 Climate Change Performance Index at the United Nations Framework Convention on Climate Change in Poland. The top three rankings were left empty because “the lack of will to engage themselves more strongly to avoid dangerous climate change, none of the countries achieved positions one through three.” However, Sweden, Germany, France, India, Brazil, the United Kingdom, and Denmark were ranked 4 through 10. Of the 60 countries ranked, the countries scoring at the bottom were the United States, Canada, and Saudi Arabia. The 60 ranked countries account for 90% of the world’s emissions. Countries are ranked on the estimates from each country’s emissions trends, current emission levels, and current policy.

LECTURE SUGGESTIONS

Catastrophism versus Uniformitarianism

Investigate the differences between these two models with students. Geologists do accept certain catastrophic events from the rock record, including the bollide impact at the end of the Cretaceous Period, thought to be partially responsible for the demise of the dinosaurs.

 

   1.    Do events such as asteroid impacts, which suddenly and globally disrupt ecosystems and the stratigraphic record of a large area, invalidate the principle of uniformitarianism?

 

   2.    Is the principle of uniformitarianism different than our modern assumption that future events will follow the same physical and chemical laws as past events? If not, why is it necessary to stipulate such a principle as a fundamental principle of geology?

 

   3.    Evaluate the assertion that uniformitarianism requires that any geologic or evolutionary phenomenon that is not observable today could not have happened in the past.

 

 

Radioactive Decay and Half-Lives

Some students are confused with the concept of half-lives, and often have the misconception that after 2 half-lives, the original sample has completely decayed to daughter product. A simple demonstration with a piece of paper can help to overcome this misconception.

Start with the original sample that is 100% parent (a sheet of paper). After one half-life, one half of the original sample will decay to daughter product. To illustrate this, fold the paper in half, cut, and remove one half. Note that after two half-lives, one half of the remaining parent will undergo decay.

Again, fold the remaining parent in half, cut, and remove half of the sample. Repeat. Discuss with the class the ratios of remaining parent material relative to the total, original sample. 

 

            Half-lives                                            Ratio of Parent/(Parent + Daughter)

1                                                                                                                    1/2

2                                                                                                                    1/4

3                                                                                                                    1/8

4                                                                                                                    1/16

n                                                                             1/ 2ⁿ

 

Also discuss the decreasing amount of original parent material. Do the students see why absolute age methods can be limited by the number of half-lives that passed, and the amount of remaining parent material?

 

Radiometric Dating

The concept of radioactive decay and its application to dating geologic materials is best illustrated by a specific example. In this activity, students participate in “age dating” a container of pennies.

 

Any unstable isotope can be used, but it is best to use one such as ¹⁰_C, which has a half life of about 20 seconds, so that the example runs in real time. Begin the lecture by explaining that ¹⁰_C is an unstable isotope that undergoes a spontaneous B⁺ decay to ¹⁰_B, a stable isotope. Every 20 seconds on average one half of the ¹⁰_C atoms in a system will decay to ¹⁰_B. Produce a plastic container (the sample we are trying to date), containing 100 pennies with heads up (the ¹⁰_C atoms). (Pennies with tails up will be considered ¹⁰_B atoms; see below.) Four members of the class are designated as follows: two Counters, a Timer, and a Geologist.

 

Begin the demonstration by drawing a graph of “amount of parent” versus “time” on a board or overhead slide. Put the lid on the container and explain the significance of “closing” the system. The demonstration proceeds with the Counters' shaking the container for the first half-life called off by the 'timer'. At the end of this period, time is stopped, the pennies are poured out of the container, and Counters count the number of pennies with heads up. Plot the number on the graph, and the pennies with tails up are returned to the container. This sequence is repeated for several “half lives.” To close, explain how the Geologist makes an age determination for the container by knowing where he/she is on the graph.

 

 

 

 

 

 

 

Geologic Time and Human Induced Climate Change

 

Review how time is measured in the human life (seconds, minutes, hours, days, years), and how time is measured in the geologic past (hundreds of thousands, millions, billions of years). 

 

1.  How long have scientists been measuring data that are used to document climate

    change? If we consider the Industrial Revolution to be the catalyst for global

    warming, how does this time interval compare with any of the geologic time

    intervals (age, epoch, period, era, eon)?

 

2.   If the historical human scale is miniscule on the geologic time scale, does this imply that humans cannot alter Earth systems? Why or why not?

 

3.   What data do scientists currently consider most important when evaluating climate change? Are scientists looking at overall trends, or anomalies within a data set?

 

CONSIDER THIS

1. What component of the rock record was most important in establishing the fact that Earth has a history that extends beyond that of recorded human history?

 

2.  Using a geologic map of a state or the United States, have students consider the relative proportions of rocks of different ages. Where are rocks of Precambrian age preserved? Why do we see rocks as old as the Archean at all? How have geologists determined the ages of rocks shown on these maps?

 

 

IMPORTANT TERMS

absolute dating	principle of cross-cutting	principle of superposition
carbon-14 dating	relationships	principle of
catastrophism	principle of lateral	uniformitarianism
fission track dating	continuity	radioactive decay
half-life	principle of original	relative dating
neptunism	horizontality	tree-ring dating

 

 

 

 

 

SUGGESTED MEDIA

Videos

1.     Geologic Time, Earth Revealed #10, Annenberg/CPB

2.     The Earth Has a History, Geological Society of America

3.     Earth Time: Evolution and Human Memory, BBC

4.     Dating the Earth, BBC

5.     Classroom Encounters® with Global Change Scientists, Climate Change and Our Future

6.     Classroom Encounters® with Global Change Scientists, Thin Ice: Earth in the Time of Climate Change;

7.     Classroom Encounters® with Global Change Scientists,Freeze, Freeze, Fry: Climate Change Past, Present, and Future

8.     Physical Geography II: Fossils, Rocks, and Time, Quantum Leap

9.     Geologic Time, Tell ME Why Sales Co.

10.  Global Warming?, The History Channel

 

Software and Demonstration Aids

1.     Explore Cross Sections CD-ROM, Geological Society of America 

2.     Explore Deep Time, Geological Time and Beyond, Geological Society of America 

 

 

CHAPTER 4 – ANSWERS TO QUESTIONS IN TEXT

Multiple Choice Review Questions

1.    e	5.    e	9.    b
2.    b	6.    d	10.    b
3.    e	7.    a
4.    a	8.    a

 

Short Answer Essay Review Questions

11.     In relative age dating, a rock or a geologic event is put into an order in which it occurred. Therefore, geologists can discuss the “oldest” and “youngest” events, but they do not have dates for the events.  In absolute dating, geologists can assign a date to a rock or geologic event, usually expressed as years before the present.

 

12.    Metamorphism may allow the daughter isotope of a radioactive pair to leak out, causing the calculated age of the mineral to be too young. This is especially a problem with potassium 40–argon 40 dating, in which argon gas must be trapped within a mineral’s structure until it is released into a mass spectrometer for the technique to work. Rubidium 87–strontium 87 dating is more forgiving since strontium is not a gas. Metamorphism can also erase fission tracks, causing the calculated age to be too young in fission track dating.

 

13.     Lyell postulated a “steady-state” Earth system in which the same geologic processes operating today have prevailed at the same rate over geologic time. By accepting this, if we understand Earth’s processes today we can use them to reconstruct Earth’s history.

 

14.     To calculate the absolute age of an intrusive body, you will need to separate minerals that are rich in the isotope pair you will use to measure the amount of time that has passed since the crystallization of the rock. You should ensure that the system has been closed since the rock’s formation, and that daughter product has not escaped the system. Use the ratio of the amounts of parent and daughter isotopes and the half life for the isotope system to calculate the absolute age of the rock.

 

15.     The most likely explanation for the discrepancy between the calculated ages is that the potassium 40-argon 40 involves a gas as a daughter product. It is more likely that a gas escapes from a system than a solid mineral. In order to confirm this hypothesis, the calculated dates using the two isotope systems should be compared. If argon gas has escaped the system since the deposition of the ash, the date for the ash using the potassium 40-argon 40 method should be younger than the date calculated using the rubidium 87-strontium 87 isotope pair.

 

16.    Students may produce a variety of answers for this question; their individual methods should be accompanied by the identified drawbacks. Many of the other absolute dating methods that students may uncover are more useful for younger objects instead of our ancient Earth. For example, other absolute methods include thermoluminescence (for archaeological dating). Chemical methods such as amino-acid racemization and obsidian hydration are more useful for archaeological/younger sites. However, paleomagnetic dating—which uses the changes of the Earth’s magnetic field—can now be extended back for rocks as old as 4 billion years. A drawback of the paleomagnetic method is the oceanic crust is more easily dated through paleomagnetism. Older rocks require refined methods.

 

17.     Kelvin assumed that Earth had begun as a molten mass, and then he calculated the temperature under those conditions. By estimating the Earth’s current temperature and rate of heat loss, Kelvin determined how long it would have taken for Earth to cool to its present temperature. His calculations appeared scientific and convincing, but the calculations produced a date that was far too young. Kelvin’s basic flaw was his failure to account for a heat source within the Earth since its formation. He was unaware that radioactive decay within the Earth generated heat. However, even if radioactivity had not been discovered, there is other scientific evidence for the ancient age of the Earth. Scientists would have accumulated more data, which would lead to eventual acceptance of the ancient age of Earth.

 

18.    Many of the same principles of relative dating can be used on Mars to reconstruct its geologic history. For example, if a dike cuts across a rock feature on Mars, we know that the dike is younger than the rocks it intruded.

         The principle of superposition can be applied, in which the oldest rocks are at the bottom of a sequence.  However, we do not have fossil evidence from Mars, so we could not apply a principle of fossil succession. The organic evolutionary history of Earth is unique to our planet.

 

19.   Igneous rocks yield the most accurate radiometric dates because as magma cools and begins to crystallize, radioactive parent atoms are separated from previously formed daughter atoms. Because they are the right size, some radioactive parent atoms are incorporated into the crystal structure of certain minerals. The stable daughter atoms are a different size and cannot fit into the crystal structure of the same mineral as the parent atoms. Therefore, a mineral crystallizing in a cooling magma will contain radioactive parent atoms but no stable daughter atoms. This allows us to measure the time of crystallization of the mineral. Sedimentary rocks cannot be dated because the minerals within them are not heated to temperatures above their Curie points. (The exception is glauconite.) Minerals within some sedimentary rocks can be dated, but the age determined will be that of the formation of the mineral within the sedimentary rock, and not that of the lithification (formation) of the sedimentary rock.

 

20.     Superposition, cross-cutting relationships, original horizontality, and lateral continuity are some of the fundamental principles used in relative dating. They are important because they are intuitive and can be easily applied in the field by observation. Without these principles, it would be impossible to piece together a geologic history for the Earth.

 

Apply Your Knowledge

 

1.     The fraction of rubidium to the total system is 1,250,000,000/ (1,250,000,000 + 38,750,000,000) = 1/32, or 3.125%. Five half-lives have passed since the crystallization of the mineral.

 

2.     There are a variety of answers for this question. Using the principles of uniformitarianism, we can attempt to extrapolate conditions 10,000 years into the future. Many researchers attempt to do this in an attempt to show human effects on the Earth system. However, the conclusions are not yet definitive, so students may produce a variety of answers based upon their reading and understanding of current science news articles. It would be much more difficult to extrapolate one million years into the future, since student models will probably not include possible catastrophic events that we can not accurately predict.