How Old is the Earth

A Response to “Scientific” Creationism

by G. Brent Dalrymple

Copyright © 1984-2006

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SCIENTIFIC AGE OF THE EARTH

efore analyzing the arguments advanced by creation “scientists” for a very young Earth, I here summarize briefly the evidence that has convinced scientists that the Earth is 4.5 to 4.6 billion years old.

There can be no doubt about the Earth’s antiquity; the evidence is abundant, conclusive, and readily available to all who care to examine it. The best evidence is contained in the Earth’s incomplete and complex but accurate stratigraphic record — a record that has been the subject of nearly two centuries of study. Slowly and painstakingly, geologists have assembled this record into the generalized geologic time scale shown in Figure 1. This was done by observing the relative age sequence of rock units in a given area and determining, from stratigraphic relations, which rock units are younger, which are older, and what assemblages of fossils are contained in each unit. Using fossils to correlate from area to area, geologists have been able to work out a relative worldwide order of rock formations and to divide the rock record and geologic time into the eras, periods, and epochs shown in Figure 1. The last modification to the geologic time scale of Figure 1 was in the 1930s, before radiometric dating was fully developed, when the Oligocene Epoch was inserted between the Eocene and the Miocene.

Although early stratigraphers could determine the relative order of rock units and fossils, they could only estimate the lengths of time involved by observing the rates of present geologic processes and comparing the rocks produced by those processes with those preserved in the stratigraphic record. With the development of modern radiometric dating methods in the late 1940s and 1950s, it was possible for the first time not only to measure the lengths of the eras, periods, and epochs but also to check the relative order of these geologic time units. Radiometric dating verified that the relative time scale determined by stratigraphers and paleontologists (Figure 1) is absolutely correct, a result that could only have been obtained if both the relative time scale and radiometric dating methods were correct.

The abundance and variety of fossils in Phanerozoic rocks have allowed geologists to decipher in considerable detail the past 600 million years or so of the Earth’s history. In Precambrian rocks, however, fossils are rare; thus, the geologic record of this important part of the Earth’s history has been especially difficult to decipher. Nonetheless, stratigraphy and radiometric dating of Precambrian rocks have clearly demonstrated that the history of the Earth extends billions of years into the past.

Radiometric dating has not been applied to just a few selected rocks from the geologic record. Literally many tens of thousands of radiometric age measurements are documented in the scientific literature. Since beginning operation in the early 1960s, the Geochronology laboratories of the U. S. Geological Survey in Menlo Park, California, have alone produced more than 20,000 K-Ar, Rb-Sr, and ¹⁴C ages. Add to this number the age measurements made by from 50 to 100 other laboratories worldwide, and it is easy to see that the number of radiometric ages produced over the past two to three decades and published in the scientific literature must easily exceed 100,000. Taken as a whole, these data clearly prove that the Earth’s history extends backward from the present to at least 3.8 billion years into the past.

A particularly fascinating question about the history of the Earth is “When did the Earth begin?” The answer to this question was provided by radiometric dating and is now known to within a few percent.

Three basic approaches are used to determine the age of the Earth. The first is to search for and date the oldest rocks exposed on the surface of the Earth. These oldest rocks are metamorphic rocks with earlier but now erased histories, so the ages obtained in this way are minimum ages for the Earth. Because the Earth formed as part of the Solar System, a second approach is to date extraterrestrial objects, i.e., meteorites and samples from the Moon. Many of these samples have not had so intense nor so complex histories as the oldest Earth rocks, and they commonly record events nearer or equal to the time of formation of the planets. The third approach, and the one that scientists think gives the most accurate age for the Earth, the other planets, and the Solar System, is to determine model lead ages for the Earth, the Moon, and meteorites. This method is thought to represent the time when lead isotopes were last homogeneously distributed throughout the Solar System and, thus, the time that the planetary bodies were segregated into discrete chemical systems. The results from these methods indicate that the Earth, meteorites, the Moon, and, by inference, the entire Solar System are 4.5 to 4.6 billion years old.

Before reviewing briefly the evidence for the age of the Earth, I emphasize that the formation of the Solar System and the Earth was not an instantaneous event but occurred over a finite period as a result of processes set in motion when the universe formed. It is, therefore, more correct to talk about formational intervals rather than discrete ages for the Solar System and the Earth. Present evidence indicates, however, that these intervals were rather short (100-200 million years) in comparison with the length of time that has elapsed since the Solar System formed some 4 to 5 billion years ago. Thus, the ages of the Earth, the Moon, and meteorites as measured by different methods represent slightly different events, although the differences in these ages are generally slight, and so, for the purposes of this chapter they are here treated as a single event.

THE EARTH’S OLDEST ROCKS

All the major continents contain a core of very old rocks fringed by younger rocks. These cores, called Precambrian shields, are all that remain of the Earth’s oldest crust. The rocks in these shields are mostly metamorphic, meaning they have been changed from other rocks into their present form by great heat and pressure beneath the surface; most have been through more than one metamorphism and have had very complex histories. A metamorphic event may change the apparent radiometric age of a rock. Most commonly, the event causes partial or total loss of the radiogenic daughter isotope, resulting in a reduced age. Not all metamorphisms completely erase the radiometric record of a rock’s age, although many do. Thus, the radiometric ages obtained from these oldest rocks are not necessarily the age of the first event in the history of the rock. Moreover, many of the oldest dated rocks intrude still older but undatable rocks. In all cases, the measured ages provide only a minimum age for the Earth.

So far, rocks older than 3.0 billion years have been found in North America, India, Russia, Greenland, Australia, and Africa. The oldest rocks in North America, found in Minnesota, give a U-Pb discordia age of 3.56 billion years (Figure 5). The oldest rocks yet found on the Earth are in Greenland, South Africa, and India. The Greenland samples have been especially well studied. The Amitsoq Gneisses in western Greenland, for example, have been dated by five different methods (Table 6); within the analytical uncertainties, the ages are the same and indicate that these rocks are about 3.7 billion years old.

Whole-rock samples from the Sand River Gneisses in the Limpopo Valley, South Africa, have been dated by the Rb-Sr isochron method at 3.79 ± 0.06 billion years (15). These samples are from rocks that contain inclusions of still older but as yet undatable rocks. Recently, Basu and others (16) reported a nine-sample Sm-Nd isochron age of 3.78 ± 0.11 billion years for rocks in eastern India.

Studies of the oldest rocks from the Precambrian shields show that the Earth is older than 3.8 billion years. The geology of these oldest rocks also indicates that there was a substantial period of history of the Earth before 3.8 billion years ago for which no datable geological record now exists. There are several possible reasons for the apparent absence of this earliest record. One reason is that during that period of Earth’s history not only was the first continental crust forming, but it was also being vigorously recycled and regenerated. A second reason is that the Moon and, by inference, the Earth, were subjected to intense bombardment by large meteorites from the time of their initial formation to about 3.8 billion years ago; this bombardment occurred because the Earth was still sweeping up material in its orbital path. A third reason may be that the record of the Earth’s early history exists somewhere but simply has not yet been found. The correct reason for the absence of data may well be some combination of the above. Whatever the reasons, if we are to learn more about the Earth’s history before 3.8 billion years ago, we must examine the evidence obtained from other, older sources, particularly meteorites and the Moon.

AGES OF METEORITES

There are two basic types of meteorites, stone and iron; other types are intermediate in composition between these two. Stone meteorites are composed primarily of the silicate minerals olivine and pyroxene, whereas iron meteorites consist primarily of nickel-iron alloy. Stone meteorites commonly contain small amounts of nickel-iron, and many iron meteorites include small amounts of silicate minerals. Once thought to be the remains of a shattered planet, meteorites probably originated from some 20 to 70 different parent bodies the size of large asteroids. Some meteorites are samples of the parent bodies that apparently were large enough to undergo partial melting and differentiation to produce different rock types. Others, primarily the stone meteorites called chondrites, seem to represent rocks essentially unchanged since condensation from the solar nebula. The orbits of meteorites indicate that they are parts of the Solar System, probably samples of the asteroids, and thus that their age is relevant to the age of the Earth.

Like most things in nature, meteorites are not simple objects. This is especially true of those that have undergone differentiation, heating, and collisions with other bodies in space. To determine the age of the Solar System and the Earth, we must search for the oldest, least disturbed meteorites.

K-Ar ages on stone meteorites range from about 400 million years to nearly 5 billion years, with a large concentration at 4.4 to 4.6 billion years. The younger ages reflect heating and collision events, to which the K-Ar method is particularly susceptible, whereas the older ages record events near or equal to the time of meteorite formation. Many meteorites have now been dated by the ⁴⁰Ar/³⁹Ar age-spectrum method, which reveals that many meteorites were heated after their formation. The metallic phases in iron meteorites cannot be dated reliably by the K-Ar method because of their nearly negligible potassium content and cosmic-ray effects. However, silicate inclusions in several iron meteorites have been dated by the K-Ar method at 4.5 ± 0.2 billion years (19).

Some of the most precise ages on meteorites have been obtained by the Rb-Sr isochron method. Table 7 lists some of these ages, from Faure’s (49) summary. Figure 3 plots the isochron for the meteorite Juvinas. Some iron meteorites containing small silicate inclusions have also been dated by the Rb-Sr isochron method; the results indicate that the least disturbed iron meteorites are of the same age (4.6 billion years) as the least disturbed stone meteorites.

Meteorites have also been dated by the Sm-Nd isochron method. Jacobsen and Wasserburg (69), for example, showed that 10 chondrites and the achondrite Juvinas all fall on an isochron of 4.60 billion years.

The results of radiometric dating on meteorites clearly indicate that these objects formed about 4.6 billion years ago. Because astrophysical considerations require that the formation of the planets and meteorites by condensation from the solar nebula was essentially simultaneous, we can infer with considerable certainty that the age of the most primitive meteorites also is the age of formation of the Earth. Even if we wished to deny this inference, we would still be forced to conclude that meteorites, which must at least post date the formation of the Solar System and the universe, are no less than 4.6 billion years old.

AGES OF LUNAR ROCKS

The Apollo missions, for the first time, gave scientists the exciting opportunity to study samples from another planet. Although all the samples provide important information about the history of the Moon, for data on the age of formation of the Moon we must again look at the oldest rocks.

The surface of the Moon can be divided into the lunar highlands and the lunar maria. The highlands are mountainous upland areas that still preserve some aspects of the original impact morphology of the earliest Moon. The maria, or “seas,” are younger, lowland areas that were flooded by lava after impact by asteroid-size bodies. The Apollo missions returned samples from both the highlands and maria.

Because of the severe impact history of the early Moon and the consequent heating and metamorphism of lunar samples, the conventional K-Ar method is not particularly useful in the study of lunar rock formation because it tends to date the latest heating and impact events rather than original rock ages. The ages of lunar rocks are known primarily from ⁴⁰Ar/³⁹Ar age-spectrum and Rb-Sr isochron dating; Table 8 lists some of these ages. As can be seen from this table, the rocks from each landing site give similar ages by both methods; this agreement cannot be a mere coincidence but must reflect the true ages of the rocks within the analytical uncertainties. Table 8, however, lists only data obtained before 1974; since that time, older rocks, from the lunar highlands, have been analyzed.

Numerous ⁴⁰Ar/³⁹Ar age-spectrum ages of highland rocks fall between about 4.0 and 4.5 billion years. The oldest ages, however, have been measured by the Rb/Sr isochron method on samples from the Apollo 17 site. These include mineral isochron ages of 4.55 ± 0.1, 4.60 ± 0.1, and 4.43 ± 0.05 billion years for three different rock types. In addition, ⁴⁰Ar/³⁹Ar age-spectrum analyses from the Apollo 16 site have now shown two rocks with ages of 4.47 and 4.42 billion years (see summary in 75), and Sm-Nd isochron ages of 4.23 ± 0.05 and 4.34 ± 0.05 billion years have been determined for two Apollo 17 samples (23).

The hundreds of radiometric ages on lunar rocks show clearly that the initial formation of the Moon was 4.5 to 4.6 billion years ago. There are, to be sure, some uncertainties about the exact chronology and events that led to the Moon we now see, but there is little doubt about when the Moon formed or about the date of the major volcanic events that produced the igneous rocks at the various Apollo sites.

MODEL LEAD AGE OF METEORITES AND THE EARTH

The generally accepted age of the Earth is based on a simple but elegant model for the evolution of lead isotopes. This model was developed independently by Houtermans (65) and Holmes (63), and first applied to meteorites and the Earth by Clair Patterson, now at the California Institute of Technology, in 1953. In his classic paper, Patterson (104) reasoned that if the Pb-isotopic composition were uniform in the solar nebula and, thus, uniform in the planetary bodies and meteorites at the time of their formation, and if these bodies contained differing amounts of uranium, then the Pb-isotopic composition of these bodies should fall on a straight-line isochron when the ²⁰⁷Pb/²⁰⁴Pb ratio is plotted against the ²⁰⁶Pb/²⁰⁴Pb ratio (Figure 8). The lower end of the isochron in Figure 8 represents the Pb-isotopic composition in a phase of iron meteorites (troilite, or iron sulfide) that contains no uranium; this point represents the initial Pb-isotopic composition of the Solar System.

Figure 8: Meteoric lead-isotope isochron showing that the age of meteorites and the Earth is about 4.55 billion years. After Murthy and Patterson (98) and York and Farquhart (136).

The Pb-isotopic compositions of iron and stone meteorites fall on an isochron age of 4.55 billion years (Figure 8). Note that this method, like the other isochron methods, is self-checking. Modern Earth leads, as represented by the Pb-isotopic compositions of some very young non-uranium-bearing minerals, also fall close to the meteoritic isochron,⁹ a result that we would expect if the Earth and meteorites formed contemporaneously. The ratios in lunar rocks have much larger values than those in terrestrial rocks and meteorites; they fall out of the field of Figure 8, but they do lie very close to the extension of the meteoritic isochron and, therefore, indicate a similar age.

If the Earth, the Moon, and meteorites were not genetically related and of the same age, there would be no reason for their Pb-isotopic compositions to lie along the same isochron. This is convincing evidence that the planetary bodies, including the Earth, all formed about 4.55 billion years ago. Note that Patterson’s (104) original estimate of the age of the Earth has changed very little over the past three decades. In a recent reevaluation, Tera (125) concludes that the age of the Earth is about 4.54 billion years. Tera also summarizes several other lead models for the Earth’s age; they all give results within the range 4.43 to 4.59 billion years. Thus, although there is still some debate about the exact age of the Earth and the Solar System, scientists are quibbling only about the first one- or two-tenths of a billion years. The age of the Earth is known to within about one part in 45, i.e., about two percent.

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⁹ Although modern Earth leads lie near the meteoritic isochron, many do not fall exactly on it, evidently because many have had complex (multistage) histories (e.g., 123).

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