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Seminars on Science
Earth: Inside and Out
LEARNING THE AGE OF THE EARTH
By E. A. Mathez
Biblical age and the seeds of doubt
Prior to about the middle of the 18th century, the question of the age of the Earth was
primarily the concern of theologians and philosophers, and the estimated ages varied as
widely as the belief systems. Aristotle, for example, regarded the Earth as eternal, but
Christian scholars, beginning in about the second century, used events recorded in
religious and historical texts and astronomical observations to arrive at young ages.
Among these, the most frequently cited date for the beginning of the Earth is 4004 BC--at
the “entrance of night preceding the twenty third day of Octob.”, to be exact. This
estimate originated from James Ussher (1581-1656), Archbishop and Primate of Ireland.
Ussher’s date was first published in 1650 as the subject of a two thousand page tome in
which he worked backward through the history of Israel and the family trees described in
the Old Testament, supplementing these with historical records from other cultures. The
date, however, owes its fame less to the scholarly nature of Ussher’s investigation than to
the fact that it appeared as an annotation in some editions of the King James Authorized
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Version of the Bible. The citation gave Ussher’s date an aura of truth and official
sanction.
By the 18th century, estimates based on observation and scientific reasoning began to
emerge. Among them was that of the French diplomat and naturalist, Benoît de Maillet
(1656-1738). De Maillet knew of the presence of sea shells (actually fossils) in rocks
high in the mountains distant from the sea, and he also knew that the level of the
Mediterranean Sea was falling. Since the shell-bearing rocks must have formed in water,
he hypothesized that the Earth was once covered by a “universal ocean” and that sea level
has since been dropping. The drop, he speculated, was due to the continuous evaporation
of water from the ocean and its loss to space. He then reasoned that the time required for
sea level to reach its present position could be calculated if the rate at which sea level was
falling could be estimated. Based on observations he had made during his extensive
travels around the Mediterranean, he estimated this rate to be about 8 centimeters per
century. This method led him to surmise a very old Earth indeed, on the order of billions
of years. This was so at odds with the biblical age that de Maillet cast his account as a
dialogue between an Indian philosopher by the name of Telliamed (de Maillet spelled in
reverse) and a French missionary. The manuscript circulated widely during his lifetime
but was only published, and then in modified form to make it more consistent with
Christian orthodoxy, ten years after his death.
Another approach was devised by the renowned French naturalist, Keeper of Jardin du
Roi, and author of 36 of the massive 44 volume encyclopedia, Histoire Naturelle, Comte
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de Buffon (born, Georges-Louis Leclerc, 1707-1788). In Buffon’s conception, the Earth
began as a completely molten body that has cooled, from outside in, to its present
temperature, so that determining the age of the Earth amounted to determining the
cooling time. For this purpose, he devised an experiment whereby he measured the time
necessary for different size spheres of iron (and later ones made of iron and non-metallic
material) to cool from near the melting temperature to room temperature. He then
extrapolated this rate to an iron body the size of the Earth, computing that such a body
would cool from a just-molten state to the ambient temperature in 96,970 years and 132
days. The estimate would fall to 75,000 years with later experiments that included nonmetallic materials more similar to the rocks that constitute the Earth and a correction for
the heat added by the Sun. Buffon also attempted to estimate age by calculating the time
necessary to produce observed thicknesses of sedimentary rock layers, based on an
estimate of the rate at which mud accumulates. The ages arrived at in this way ranged to
nearly three million years. Perhaps if 75,000 years could be imagined, so could three
million; in any case, the latter estimates appeared only in unpublished manuscripts.
Buffon’s experiments aimed at deducing the age of the Earth from heat loss anticipated
an argument that would be put fourth in the next century by Lord Kelvin.
Emergence of the revolutionary concept of an old Earth
As we might foresee from Buffon’s work, “natural philosophers” (or “natural historians”)
of the late 18th century began to see the Earth differently, to recognize, in particular, that
the history of the Earth can be deduced from rocks rather than from biblical studies. The
most influential advocate of this approach was the Scottish physician, businessman,
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farmer, chemist, naturalist, and philosopher James Hutton (1726-1797), whom many
regard as the father of geology. Hutton received an M.D. degree in 1749 but developed
an interest in farming, eventually settling, in 1754, on the farm he inherited from his
father 40 miles southeast of Edinburgh. There he remained until his retirement in 1768,
and it was during this time that he developed his theory of the Earth. The rest of his life
was spent in Edinburgh, where he quickly became entrained into intellectual and social
circles that provided fertile ground for debate and refinement of ideas.
Hutton conceived of the Earth as a machine, a dynamic place driven by internal heat and
consequently in a continuous state of renewal. He realized that erosion must operate
continuously on the land to produce sediments deposited in the ocean. According to his
theory, the Earth’s interior is hot and at great depth it is molten. Thus, as new sediment
covers old, the old sediment is buried and exposed to heat, which causes it to turn to rock.
The extreme heat of the interior provides an expansive force. These forces uplift and
deform the overlying rocks, which are then subjected to erosion and ultimately deposited
in ocean as sediment again, continuing the cycle. They also produce granite “plutons,”
large bodies of molten rock that forced their way up into the overlying rocks, and molten
material that occasionally breaches the surface to form volcanic eruptions.
Hutton’s theory predicted that there should exist places where the rocks that had been
thrust up, deformed, and then eroded are now covered by younger, undeformed sediment.
Hutton knew of a red sandstone that cropped out on the coast north of his farm, and he
also knew that along the coast to the east was a gray, gritty sedimentary rock, now known
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as greywacke. Seeking to test the prediction, he sailed along the coast searching for the
contact between the two rocks, which he found at the now-famous Siccar point. Here in
graphic juxtaposition nearly flat-lying beds of the Devonian Old Red Sandstone cover
near-vertical beds of Silurian graywacke . These old surfaces of erosion are known as
unconformities. This particular one, which can be seen in a variety of places in Scotland,
has become known as the “Hutton Unconformity”. Hutton understood that the
unconformity represents a hiatus in deposition, a gap in time, a missing part of the
geologic record during which time the region experienced uplift, tilting, erosion,
subsidence, and re-flooding. But these processes, as he could see around him, proceeded
infinitely slowly. Furthermore, they had to have occurred over and over again to account
for the surface relief and varied character of the rocks from one place to another. This led
directly to the concept of an old Earth. From the study of the rocks, Hutton wrote, “we
have the satisfaction to find, that in nature there is wisdom, system, and consistency.”
And then he added what has become probably the most famous quotation in geology,
“But if the succession of worlds is established in the system of nature, it is in vain to look
for any thing higher in the origin of the earth. The result, therefore, of our present
enquiry is, that we find no vestige of a beginning—no prospect of an end.” These are the
final words of Hutton’s most important treatise, Theory of the Earth; or an Investigation
of the Laws observable in the Composition, Dissolution, and Restoration of Lad upon the
Globe, which appeared in 1788 in the first volume of Transactions of the Royal Society of
Edinburgh. Unknowable perhaps, but the great age of the Earth would soon become a
fixture of geologic thought, and it emerged as the first revolutionary concept of the new
science.
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The geological approach developed by Hutton and others of the time was amplified in the
early part of the 19th century mainly through the writings of the Scottish geologist,
Charles Lyell (1797-1875). His treatise, Principles of Geology…, published in three
volumes between 1830 and 1833, would influence several generations of thinkers.
Although Lyell himself generally remained aloof from the debate over age, he
nonetheless presented important geological arguments for its great antiquity. The nature
of these arguments can be illustrated by insights Lyell gained on a trip to Italy. There he
visited Pozzuoli, near Naples, where he saw near the sea three standing columns of a
2,000 year old Roman ruin. High up on the columns are borings left by mollusks,
demonstrating that the columns had once been submerged. But since they are still
standing, the columns must have sunk and then been uplifted slowly. To Lyell this was a
graphic demonstration that geologic processes need not proceed as sudden upheavals but
could be slow and steady. Continuing his journey to Sicily, Lyell studied the volcano
Etna. He realized that if this towering volcano had grown to its present size at the same
rate at which it has been producing historic lava flows, it must be far older than all of
human history. In a previous study of mollusk fossils, Lyell had established that the
more ancient the rock the greater the proportion of extinct to living mollusks. But at Etna
he found that the entire volcano was built on strata containing fossils nearly identical to
the mollusks living in the Mediterranean today. This must mean that the fossils beneath
the lava are not only older than the volcano, but that despite their similarity to living
organisms they also must be very old in terms of human history.
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Among those influenced by The Principles of Geology was Lyell’s close friend Charles
Darwin (1809-1882). In the first edition of the Origin of the Species…, Darwin took a
novel approach to determining age. First, he estimated the total amount of material
removed from a geological dome that had been partially eroded to make The Weald, a
valley not too distant from his home in the south of England. Then he estimated the rate
at which the rocks may have eroded, based on the erosion rate of sea cliffs. From this
rate and knowing the amount of material removed, he arrived at an age of 300 million
years for the time needed to erode The Weald from a contiguous dome to its present,
incised character. Darwin’s analysis rested on certain arbitrary assumptions that he could
not defend on geologic grounds, so the analysis was withdrawn from the third edition of
Origin of the Species. Another method was adopted by John Phillips (1800-1874),
professor of geology at Oxford. Phillips computed the time necessary to accumulate the
total thickness of Cambrian and younger sedimentary rocks by measuring that thickness
and then dividing it by the estimated rate of sedimentation. He arrived at an age of the
Earth’s crust of 96 million years. This basic approach was to be adopted by a number of
geologists in the decades to follow, but it involved too many assumptions and
uncertainties to yield a definitive result.
It may have been partly for this reason that the 19th century discussion of the age of the
Earth was dominated by the physicist William Thomson (to become Lord Kelvin, 18241907). As the author of more than 600 scientific papers and books and one of the most
celebrated of British scientists, Kelvin’s ideas carried great authority, and he delivered
them with considerable force of intellect. Kelvin attempted to determine age from
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“known physical laws” rather than from observations of complex natural phenomena and
processes, such as those that geologists were making. In an article published in 1862, he
argued that since an initially molten Earth must have cooled to its present temperature, by
knowing initial temperature and cooling rate one could calculate age. For the latter
computation one needed the Earth’s internal temperature, which was obtainable from
knowledge of the thermal gradient (approximately known at the time from the
measurements of temperature variations in mines) and thermal conductivity of rocks
(which Kelvin had set out to measure). His best estimate for the age of the Earth was 98
million years. This was remarkably similar to Phillips’ estimate. Several other lines of
geologic reasoning were to converge on this age. For example, in 1899 John Joly (18571933), Professor of Geology at Trinity College, Dublin, estimated an age of 90-99 million
years based on the sodium content of the ocean, the assumption being that sodium builds
up slowly as the continents erode. Such consistency naturally led many thinkers of the
time to accept the Earth as being about 100 million years old, although by 1897 Kelvin
had reduced his estimate to “more than 20 and less than 40 million years ago, and
probably much nearer 20 than 40.” Thus, at the end of the 19th century there was
considerable discord between geologists who, following in the footsteps of Lyell and
Phillips, attempted to deduce age from the sedimentary and paleontological records, and
other scientists who sought to calculate age from physical arguments. Indeed, estimates
for the age of the base of the Cambrian varied from 3 million to 2.4 billion years.
Radioactivity and the age of the Earth
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Unfortunately for Kelvin’s view, not all was known. In particular, he assumed that the
only source of heat in the Earth is primordial heat (that of the original, hot planet).
Radioactivity had been discovered in 1896, and the New Zealand physicist Ernest
Rutherford (1871-1937), then professor of physics at McGill University and whose later
experiments with alpha particles would lead to the first theory of the nuclear atom,
pointed out in 1904 that radioactivity constitutes another source of heat. This meant that
the observed temperature gradient of the Earth, if due to radioactive rather than
primordial heat, had remained constant for a long time and thus that the age of the Earth
could be much greater than indicated by Kelvin’s computations. Work by Rutherford and
a colleague at McGill, Frederick Soddy, had suggested that the decay of elements
produces helium (i.e., alpha particles are helium ions), leading Rutherford to further
suggest in 1905 that age might be calculated from the amount of helium trapped in
radioactive minerals. In the meantime, the physicist Bertram Boltwood of Yale
University had discovered that uranium ores invariably contain lead in addition to helium
and hypothesized that lead is the end product of radioactive decay of uranium. He
proceeded to compile data on 43 uranium-bearing minerals from 10 localities and
calculated locality ages ranging from 410 to 2,200 million years. On the other side of the
Atlantic, the physicist Robert J. Strutt (1875-1947) of Imperial College had also taken up
the subject. He measured the composition of 22 radioactive mineral specimens and,
based on their relative helium contents, also calculated old ages.
One of Strutt’s students was Arthur Holmes (1890-1965). In 1911, Holmes, following up
on Boltwood’s work, obtained additional analyses of uranium-bearing minerals and
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showed that the calculated locality ages were consistent with relative ages obtained from
geological relations. Holmes, whose interests would soon gravitate to geology and
eventually become professor of the subject at the University of Edinburgh, probably more
than anyone else is responsible for development of the geologic time scale through the
dating of rocks and minerals. For example, in a popular 1927 booklet, The Age of the
Earth: An Introduction to Geological Ideas, he listed mineral ages from twenty-three
localities. Finding them to be consistent with geological evidence, Holmes went on to
estimate the Earth to be between 1.6 to 3 billion years old.
Like Kelvin’s, however, this first approach to dating rocks using the decay of uranium to
lead was inherently flawed. The problem was the assumption that all the lead in rocks
decayed from uranium. In fact, of the four stable isotopes of lead, three form by
radioactive decay (they are “radiogenic”), and one of these decays from thorium, not
uranium. (238U and 235U decay, respectively, to 206Pb and 207Pb, and 232Th decays to
208
Pb. It should be kept in mind, however, that not all 206Pb, 207Pb, and 208Pb are
radiogenic. The fourth isotope,
204
Pb, is not radiogenic at all. Therefore, it is common
practice to use the ratios of the radiogenic to non-radiogenic isotopes--i.e., 207Pb/204Pb
and 206Pb/204Pb--as measures of the amounts of radiogenic lead present in samples.) This
was not known, however, until the experimental chemist Francis W. Aston, working at
Cambridge University, used a mass spectrograph (a devise which had been developed
earlier to distinguish isotopes by mass) to show that lead consists of three principal
isotopes, 206Pb, 207Pb, and 208Pb (the relative abundance of 204Pb is much lower and was
not detected in this early attempt). A decade later, Alfred Nier and his colleagues at the
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University of Minnesota built an improved instrument that was capable of quantitatively
determining relative isotopic abundances and applied the technique to the study of lead
and uranium ores. In a series of papers published between 1938 and 1941, they reported
systematic variations in 207Pb/204Pb and 206Pb/204Pb ratios in different samples and
suggested that the variations were due to the mixture of “primeval” lead--i.e., the lead in
the first-formed Earth--and lead subsequently formed by radioactive decay.
The insight that ushered in a solution to the problem of the age of the Earth was
developed by E.K. Gerling in Moscow in 1942, and independently in 1946 by F.G.
Houtermans in Göttingen and by Holmes in Edinburgh. Since 238U decays to 206Pb with a
half-life of 4.468 billion years, and since 235U decays to 207Pb with the much shorter halflife of 703.7 million years, the 207Pb/204Pb and 206Pb/204Pb ratios of a rock change at
different rates with time. Gerling, Houtermans and Holmes all realized that they could
calculate the age of the Earth from the measured 207Pb/204Pb and 206Pb/204Pb ratios of a
sample if (a) they knew the original 207Pb/204Pb and 206Pb/204Pb ratios of the Earth—i.e.,
the primeval composition--and (b) the sample had neither gained nor lost lead or uranium
by exchange with its surroundings through time—i.e., the only change was the decay of
uranium to lead held in the sample. They further recognized that they could test the
veracity of the latter assumption by comparing the two chronometers in different samples
of known age. Using this approach, estimates of the age of the Earth crept higher, with
most falling between 3 and 4 billion years. The problem, however, was in not knowing
the primeval composition. What could that composition be, and how could it possibly be
determined with no samples as old as the planet itself?
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The answer came from meteorites, and this remaining piece of the puzzle was provided
by Clair Patterson of the California Institute of Technology, and his coworkers in a series
of papers that appeared between 1953 and 1956. They began by supposing that the Earth
and meteorites had formed at the same time from the same material in the solar system.
If true, they reasoned, then the initial lead isotopic composition of meteorites should be
the same as the elusive primeval Earth composition. They then measured the uranium
and lead contents and isotopic ratios of the iron sulfide mineral troilite in the Canyon
Diablo iron meteorite, which fell 50 thousand years ago to form Meteor Crater in
Arizona. Troilite may contain small amounts of lead but never any significant uranium,
so they argued that with no uranium present to produce radiogenic lead, the troilite must
have the same 207Pb/204Pb and 206Pb/204Pb ratios as the initial Earth. In combination with
certain terrestrial samples, this composition yielded an age of about 4.5 billion years.
The next step was also important. In the 1956 paper, Patterson and his colleagues
evinced strong evidence for their earlier supposition that the Earth and meteorites were
all “components of the solar system” by showing that three chondritic meteorites and two
iron meteorites possessed that same age. We now know that the Earth and meteorites did
not all form exactly at the same time and in particular that the Earth may not have
reached its present size until nearly 100 million years after formation of the solar nebula
4.56 billion years ago (Chapter 1). Patterson and his colleagues were right, however: the
Earth and meteorites are all components of the solar system, and the age of the Earth is
now well-established by numerous studies using lead as well as other isotopic systems.
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References
Allègre, C.J., G. Manhès, and C. Göpel, 1995, The age of the Earth, Geochimica et
Cosmochimica Acta 59, 1445-1456.
Burchfield, J.D., 1975, Lord Kelvin and the Age of the Earth, Science History
Publications, New York.
Dalrymple, G.B., 1991, The Age of the Earth, Stanford University Press, Stanford.
Hallam, A., 1983, Great Geological Controversies, Oxford University Press, New York.
Holmes, A., 1946, As estimate of the age of the Earth, Nature, v. 57, 680-684.
Hutton, J., 1788, Theory of the Earth; or an investigation of the laws observable in the
composition, dissolution, and restoration of land upon the globe, Transactions
Royal Society Edinburgh, v. 1, 216-304.
Lyell, C., 1830-33, Principles of Geology, being an Attempt to Explain the Former
Changes of the Earth's Surface, by Reference to Causes Now in Operation, 3
vols., John Murry, London.
Murthy, V.R., and C.C. Patterson, 1962, Primary isochron of zero age for meteorites and
the earth, Journal of Geophysical Research, v. 67, 1161-1167.
Oldroyd, D., 1996, Thinking about the Earth: A history of ideas in geology, Harvard
University Press, Cambridge.
Patterson, C.C., 1956, The age of meteorites and the earth, Geochimica et Cosmochimica
Acta, v. 10, 230-237.
Patterson, C.C., G. Tilton, and M. Inghram, 1955, Age of the Earth, Science, v. 121, 6975.
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