The Age of the Earth

Author: G. Brent Dalrymple
Date: 1991
Publisher: Stanford University Press, Stanford, CA
ISBN: 0804723311
Reviewed by: Allan H. Harvey, steamdoc@aol.com

The Author

G. Brent Dalrymple is a Research Geologist at the United States Geological Survey in Menlo Park, California.

The Reviewer

My specialty is thermodynamics in the context of Chemical Engineering and Chemical Physics. I have never taken a college or even a high school course in geology. Given this deficient background, one might ask why I am reviewing this book. The reason is the hope that it will be useful for people with as little knowledge of geology as I. On the Science and Christianity mailing list and elsewhere, one runs into the usual young-Earth claims about the age of the Earth, often repeated by people ignorant of the relevant science. Geologists often exclaim in exasperation that they wish these people would read real scientific work explaining why scientists believe the Earth to be billions of years old, rather than relying on the misrepresentations of that work in the creationist literature. In particular, the moderator of SCICHR-LIST has suggested this book. For his suggestion to work, the book would have to be written so that a person of reasonable intelligence and scientific literacy but little or no background in geology could understand it. Since I fit those criteria, I decided to test how well this book could reach a non-geologist reader.

Preliminary Comments

The author was inspired to write this book primarily because of his involvement in defending the scientific age of the Earth against young-Earth creationists. However, this is a "positive" book; it does not devote any attention to direct refutation of creationist arguments (a few sources of such refutations are listed; I wish they had included some more recent work and more easily accessible references). As the author puts it:
The creationists' "scientific" arguments for a young Earth are absurd, I and other authors have dealt with them at length elsewhere, and they do not merit further attention here [Preface, p.x].
One should not, however, get the idea that the author has succumbed to "Carl Sagan's Disease," where the findings of science are distorted to conclude that the universe is Godless and without purpose. He states the limitations of what he presents:
Let us be clear about one thing, however: this information provides us no answers to the larger question of whether we are ultimately the result of a grand and purposeful design, or merely an accident of past and current physical processes. Science can attempt to determine how and when the Earth and its surroundings were created, but the question of why it all exists is not one that science can speak to [p.3].
Finally, a word on notation. In keeping with geologic convention, I have used Ga (Giga-annum) to represent a billion years and Ma (Mega-annum) to represent a million years.

Introduction and Historical Background

The book begins with a brief introduction outlining the current best understanding of the chronology of the physical universe. This material was useful in setting the stage for the rest of the book.

The second chapter concerns early attempts to estimate the Earth's age. I was unfamiliar with much of this history, and found it fascinating. A few pages are even devoted to the Biblical chronologies devised by Bishop Ussher and others. Much of the chapter focuses on four early methods; all are well explained and each is interesting enough to mention briefly here.

The first method was "cooling" calculations based on assumptions of initial conditions and subsequent processes on the Sun and the Earth. These were performed by many people in the 19th century, of whom William Thomson (Lord Kelvin) was by far the most prominent. The calculations typically gave ages of tens or hundreds of Ma. Because Lord Kelvin commanded such respect, his results carried authority for many years. We now know that the Sun burns via nuclear fusion rather than the gravitational contraction assumed by Kelvin. As for the Earth, perhaps the largest flaw in his approach is that he did not know about heat from the Earth's natural radioactivity.

The second method was that of George Darwin, son of Charles Darwin. He performed calculations based on tidal forces between the Earth and Moon using a model in which the Moon was "spun off" from a rapidly rotating molten Earth. He came up with a minimum age of 56 Ma, and was careful to state that it depended on a "wild speculation" on the origin of the Moon. It has since become clear that this model for the Moon's origin is physically unreasonable.

The third method used the amount of salt in the oceans. Estimates were made of the inflow of sodium, and it was assumed that the sodium never left the oceans. A simple calculation then produced an age on the order of 100 Ma. It is now recognized that this method is worthless in determining absolute age, because the assumption that sodium never leaves the oceans is incorrect. In fact, sodium and most other elements are at an equilibrium concentration, where the inflow is equal to the amount being removed by evaporation and other natural processes. These calculations only produce a residence time, which has no bearing on age. (Sadly, though the difference has been known for many years, some creationist literature still misrepresents residence time as age.)

The fourth method used the thickness of sedimentary rock formations and assumptions of the deposition rates of such sediments. These calculations varied wildly, but most were from 100 Ma to 1 Ga. Even then it was recognized that the uncertainties were large; it has since become clearer that variations in rates of deposition and erosion render these methods next to worthless.

Dalrymple also mentions some early (1905-1931) estimates of rock ages based on radioactive decay. While far less precise than modern methods, these were the first clear indications that the age of the Earth was on the order of a few Ga.

While reading this chapter, I could not help but be humbled. We often state our results with certainty, but even great scientists like Kelvin could proceed in good faith and end up at wrong conclusions. In retrospect, it is easy to see where these methods went wrong. We are even tempted to laugh at Kelvin for assuming that the radiation of heat from the sun was like that from burning coal. I wonder whether any of my work will be a source of laughter in 100 years.

However, it is important to recognize that past wrong estimates for the age of the Earth do not automatically make the current estimates suspect. One occasionally hears comments like "Last century scientists thought the Earth was 100 million years old. Now they think it is 4.5 billion years old. Who knows what the next century will bring?" The implication is that a major change in the current number is possible and even likely as science advances. I think this is misleading, because the nature of science is that it usually "homes in" on the truth. In any field of inquiry, one working at the fringes of knowledge stands a good chance of drawing some wrong conclusions (Kelvin and others of that era usually recognized the tentative nature of their age calculations). But as fields mature (and our knowledge in this area is much more complete than Kelvin's), answers become established with more certainty. While it is of course possible that some scientific revolution could drastically change the 4.5 Ga estimate, the overturning of such a mature result would be a much bigger surprise than it was when Lord Kelvin's calculations were invalidated.

Radiometric Methods

The third chapter explains the radiometric dating of rocks. It starts with an explanation of radioactive decay, requiring only a high-school understanding of the composition of nuclei in terms of protons and neutrons. There is then a brief section on the constancy of decay, an assumption on which these methods rely. The insensitivity of radioactive decay to changes in temperature and pressure is explained. I was disappointed that the author did not address a common creationist objection, which is that the decay rates might have changed with time. Perhaps he puts this in the "absurd" category; all he says is:
...unless there has been some undiscovered change in the fundamental nature of matter and energy since the universe formed, the presumption of constancy for radioactive decay is, for all practical purposes, eminently reasonable [p.87].
It would have been nice if he had cited some literature on the enormous violence one would have to do to the laws of physics to achieve a change in radioactive decay rates with time.

He then describes the various parent/daughter isotope pairs used to date rocks. It is explained that naive use of these decays faces two problems. First, it requires knowledge of the initial concentration of the daughter isotope in the rock, which is usually not known. Second, an incorrect age (usually too young) can result if the composition has been disturbed (perhaps by melting or some chemical process) subsequent to the rock's formation. This leads to an explanation of various age-diagnostic diagrams in which multiple samples from a rock are used in a way that avoids the need for the initial daughter isotope concentration. Significantly, these methods are self-checking in that disturbances in composition will show up in the diagrams.

While some effort was required to keep track of what was happening on various diagrams, for the most part these methods were explained clearly. There are several helpful examples, and the methods are presented without forgetting to mention their limitations.

Earth's Oldest Rocks

The next chapter discusses specific rocks on Earth that have been dated to great ages. The rocks discussed, many of which have been dated by multiple independent techniques, range in age from 2.5 Ga to almost 4.0 Ga. Some discussion is given of the hope for finding even older rocks on Earth, but it appears that almost all of the rock formed in the first 0.5 Ga of Earth history has since been "recycled" through geologic activity.

This chapter was the most geology-intensive in the book, and was therefore the most difficult for this non-geologist. My eyes glazed when I saw a figure caption about "Strongly deformed Ameralik mafic dike (now amphibolite) cutting Amitsoq gneiss..." Fortunately, there is an extensive glossary, so with sufficient effort (and a finger kept at Table 2.2 which shows the various geologic periods) the careful reader could follow the explanations. Or, one could skim past the geology and simply recognize that, in various places on Earth, rocks have been found that are of these great ages.

The Moon

Study of the Moon can help us understand the Earth's geology because, while the mechanism of the Moon's origin is not settled, the two bodies are probably closely related. Since the Moon lost its internal heat much sooner than the larger Earth, its rocks have not been subjected to nearly as much metamorphosis. Lunar geology is also simplified by the lack of wind and water erosion. On the other hand, lunar geology is complicated by the effects of meteoric bombardment and by the small number of samples geologists have to work with.

Chapter five begins with a discussion of the hypotheses for the origin of the Moon. The author seems to favor the theory in which it was formed from a collision between the Earth and another body. There is then an informative summary of the Moon's geography and geology. Finally, there is a discussion of the radiometric dating of various lunar samples. Several samples from the lunar highlands have been dated to 4.4-4.5 Ga, which is taken to be the approximate time of formation of the Moon's crust.

While there was again some geologic jargon that was difficult to follow, for the most part I found this chapter to be well-written and informative. While the lack of samples hinders detailed understanding of the Moon's geology, the basic outlines seem to be well established and to point to a lunar age of at least 4.5 Ga.

Meteorites

Meteorites are in a sense ideal for age studies since there is essentially no way for their compositions to be disturbed after their formation. While meteorite ages have no direct bearing on the Earth's age, they say something about the age of the Solar System which is thought to be closely related.

Chapter six begins with a history of the study of meteorites, a discussion of the various types of meteorites, and a discussion of their sources (most apparently originated in the Asteroid belt, though a few are thought to be debris from large meteor impacts on the Moon or Mars). Then there is a section on the ages of meteorites. Most have ages in the range of 4.4-4.6 Ga. The chapter is clearly written and shows that the vast age of these particular pieces of the Solar system is well established.

Lead Isotopes

Chapter seven describes a model using lead isotopes which most geologists accept as giving the most accurate value for the age of the Earth. The method makes use of the fact that uranium-235 and uranium-238 decay independently into two different isotopes of lead. It depends on assumptions about the "primordial" isotopic composition of lead and about the development of certain lead ores. Most of the chapter is devoted to explaining evidence from meteorites and terrestrial lead sources which has produced consistent results. It is these results which yield the "best" age of 4.52-4.56 Ga.

After reading this chapter twice, I must confess that, perhaps because I missed some things, my confidence in the lead-based methods does not match that of the author. The "best" result is based on just a few lead ores, and even these are not thought to satisfy fully the "single-stage" history assumed in the model. There seem to be uncertainties at several points in the calculation, but fortunately the final number appears to be fairly insensitive to the shaky assumptions. Overall, these results struck me as less (or at least less obviously) clear cut than most of the other material in the book. If I looked at this work in isolation, my skeptical confidence limits might be 4.4-4.7 Ga rather than the author's 4.52-4.56. However, the fact that 4.54 Ga so closely matches precisely determined meteorite ages (along with evidence discussed in this chapter that indicates common origin of the lead in meteorites and the Earth) gives it additional credence.

Cosmic Considerations

Chapter eight first discusses evidence regarding the age of the universe. While this is not the age of the Earth, it does provide an upper bound and is interesting in its own right. A few pages are devoted to estimates from the expansion of the universe. While the section is well-written and accessible to us non-astronomers, I believe recent research has rendered parts of it somewhat dated. However, the basic answer of 10-20 Ga has not changed much. A rather lengthy section is then devoted to the oldest stars which are thought to be those in globular clusters and are dated at 14-18 Ga.

A final section gives a convincing (and new to this reader) argument for the antiquity of the Earth. Among the known radioactive isotopes, there is a clear division at a half-life of about 80 Ma between those that are and are not found naturally on Earth. Since there is no reason that the stellar processes producing the elements would favor isotopes with long half-lives, the clear indication is that the shorter-lived isotopes are not found because enough time has passed for them to decay away beyond the limits of our detection methods. Consideration of the number of half-lives needed for an element to decay beyond detection limits indicates that billions (but not trillions, because then more isotopes would be missing) of years have passed since these elements were formed. In some cases theory can estimate the isotopic ratios of elements as produced in stars; for those elements the present isotopic abundances permit an estimate of the time since the element was produced. These calculations yield numbers around 10 Ga.

Closure

A final short chapter provides a concise summary of the material. It also lists some gaps in our knowledge and suggests research to help fill some of them. None of the gaps seem likely to affect the age of the Earth substantially, but there are still many unknown details about the formation of the Earth and the Solar System.

Evaluation

The author's stated intention was to write a book "for people with some modest background in science ... [but] without a deep knowledge of geology or physics." In large measure he has succeeded in this endeavor. I certainly found it both accessible and enjoyable. The writing is clear, and even those sections where one might get bogged down in geology or geochronological diagrams will communicate their main messages (even if some details slip by) with a moderate amount of effort on the part of the reader. For those who want to go deeper into specific areas, ample references are provided.

Will this book convince the curious creationist who wants to know why scientists think the Earth is billions of years old? I can't say, since I was an old-Earth believer when I started the book. The evidence presented is, however, so overwhelming that it seems to me that an open-minded reader would be forced to conclude that, from a scientific standpoint, the Earth is clearly billions of years old. The only alternative is that God has at some point since creation changed the laws of physics in some bizarre way and/or carefully rigged the evidence to give us a false history and false age for the Earth. That is, of course, not impossible for God, but such theories are outside the realm of science.

Whatever one believes about the age of the Earth, I can recommend this book to anyone (at least anyone with a minimal background such as a science or engineering major would get early in college) wanting a clear exposition of how that age is determined scientifically.

Disclaimer: The views expressed in this review are the opinion of the author of this review alone and should not be taken to represent the views of any other person or organization.

Review originally written January 1996.
Page last modified September 2, 2000