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Radioactivity, the discovery of time and theearliest history of the EarthAlex N. HallidayPublished online: 08 Nov 2010.

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Radioactivity, the discovery of time and theearliest history of the Earth

ALEX N. HALLIDAY

The discovery of radioact ivity about 100 years ago permitted the measurement of absolute

time in the distant past and transformed our understanding of the evolution of our planet

from the origin of the Solar System to the development of Homo Sapiens. Geologists were

already convinced that the Earth had to be very old but Lord Kelvin had placed stringent

limits on its antiquity by measuring how fast it appeared to be losing heat. Radioactivity not

only provides the heat necessary to keep the Earth geologically active for 100 times longer

than Kelvin had realized, it also equips the scientist with a powerful tool for deciphering the

absolute timing of events and rates of natura l processes in the ancient past. We can now date

individual microscopic mineral grains weathered from past continents, determine the rates at

which magma is accumulating beneath volcanoes, study the events surrounding the collapse

of the solar nebula and the formation of planets and test theories for climate change by

measuring the history of sea level, dust circulation and ocean temperature. However, the

largest challenge remains ® nding ways to decipher the ® rst 500 million years of Earth

history, a period from which not a single indigenous rock appears to have survived. Most of

the new insights are being gleaned from the record of the extinct short-lived nuclides that

were alive in the early Solar System.

1. Introduction

The discovery of radioactivity 100 years ago arguably had a

bigger eŒect on the Earth sciences than any subsequent

® nding. While the theory of plate tectonics is widely cited as

representing a revolution in Earth sciences analogous to the

eŒect that the theory of evolution had on the life sciences,

the discovery of radioactivity explained why the Earth

should still be active at all, provided us with the time scales

over which the Earth has evolved and gave us a means to

track the time-integrated histories of the components

within the Earth, even including the Earth’ s inaccessible

core. Numerous fundamental theories about the Earth

including those pertaining to the growth of the continents,

plate tectonics, the extinction of the dinosaurs, the history

of the atmosphere and oceans, the origin of the Moon and

the causes of ice ages would not be testable without

radioactive dating. In this article I ® rst set the stage with a

background to our level of understanding of the history of

the Earth prior to the discovery of radioactivity. I then go

on to explain how this ® nding changed our perspective, and

elucidate the principles of radioactive dating. The major

accomplishments of the many dating techniques are then

summarized before ® nishing with the biggest puzzle of

all Ð how the Earth ® rst started.

2. The discovery of time

Prior to the development of rad ioactive dating, our

understanding of the time frame for the Earth was rather

like a historian knowing that Albert Einstein lived some

time between Alexander the Great and Michael Jackson

without knowing exactly how much time had elapsed in

between these events. Without an absolute method of age

determination, the history of the Earth is, at best, relative.

Until about 200 years ago the Earth was generally assumed

to be young; its o� cial age was set at 4004 BC by Bishop

Ussher in 1650. This was calculated primarily from the age

of 4000 BC for the creation as proposed by Martin Luther

and his colleagues based on the genealogies in the Old

Testament. Ussher incorporated Johann Kepler’ s correc-

tion for the timing of the birth of Christ which he had

ascertained took place in 4 BC, assuming that the

darkening of the sky during the cruci® xion was brought

about by a solar eclipse.

Author’ s address: Department of Geological Sciences, University of

Michigan, 2543 C.C. Little Building, Ann Arbor, Michigan 48109-1063,

USA.

Contemporary Physics, 1997, volume 38, number 2, pages 103 ± 114

0010-7514/97 $12.00 Ó 1997 Taylor & Francis Ltd

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Fossils were generally considered to be tricks of nature

that were not intended to be understood, or they were

thought to be the result of the Great Flood. Society was

considered to be in such a state of decay that it was thought

that the Earth did not have long to last. In short,

humankind was considered to be central to the entire

history of the Earth. We now know that the Earth is a

million times older and that Homo Sapiens has only

developed within the last 0.01% of its history.

Breakthroughs in recognizing the enormity of geological

time came about as a result of trying to understand how

rocks and fossils form and by a (limited) appreciation of the

rates at which such processes must occur. For example,

Leonardo da Vinci (1451 ± 1519) expressed serious doubts

about the Flood as an explanation for fossils because he

could not understand how organisms such as clams and

snails could travel 250 miles inland from the Adriatic Sea to

the mountains of Lombardy, where he found well preserved

fossils, in just 40 days and 40 nights. He reasoned that the

rates at which such organims could migrate were far too

slow for such a distance and time. It was Nils Steensen

(`Steno’ ), a Danish medical practitioner working in Italy in

the seventeenth century, who wrote some of the most

insightful early comments on fossilization, originally based

on a report about an unusually large shark which was

washed up on the shores of Italy. He noted that the shark’ s

enormous teeth were rather like previously misidenti® ed

fossil shark’ s teeth found in the Mediterranean regions and

commonly known as `tonguestones’ because of their

strange shape. He then started reasoning from ® rst

principles about how a living organism could wind up

being a part of something as robust as a rock. Although

Steno later gave up geology, his writings attracted the

attention of Robert Hooke (famous for his law of elasticity)

who became immensely interested in fossils and the

developm ent of the Earth . Steno and H ooke both

recognized that Earth’ s landscape must be in a state of

continual upheaval. They do not appear to have questioned

its supposed antiquity. One of the ® rst to do so was the

Compte du BuŒon (1707 ± 1788) who, in 1778, reported the

results of experiments on the rate of cooling of molten Fe

from which he deduced that the Earth had to be about

75 000 years old, assuming that the Earth started as a hot

molten mass. He is reported to have later increased this

estimate to 3 million years. James Hutton (1726 ± 1797) and

Charles Lyell (1797 ± 1875) , two Scots who were easily the

most in¯ uential persons in establishing modern geological

thinking, argued vociferously for a very great age for the

Earth (at least many hundreds of millions of years), based

on the amount of sediment that had accumulated in the

rock record and the rates of sedimentation. Charles Darwin

(1809 ± 1882) joined forces with such geologists, using the

rates of biological evolution as supporting evidence.

However, this new awareness was temporarily dampened

by the arguments of one of the greatest physicists of the

time, William Thomson, better known as Lord Kelvin

(1824 ± 1907) .

Kelvin’ s thesis was on the ¯ ow of heat through the

Earth; in 1864 he concluded that the Earth had to be less

than 400 million years old, based on how long it should

have taken to cool to its current temperature. By 1898 he

had reduced this estimate to less than 40 million years.

Lyell and Darwin died, still convinced by their own

® ndings, but unable to explain Kelvin’ s, seemingly irrefu-

table, arguments. Following the discovery of radioactivity

by Henri Becquerel in 1896 , the Curies in 1903 , having

discovered and puri® ed Ra, showed that radioactivity was

exothermic and that there was, therefore, a previously

unidenti® ed source of heat within the Earth (® gure 1). Two

years later, Ernest Rutherford and Bertram Boltwood

published the ® rst radioactive ages of U minerals. They

used the amounts of U and He ( a particles), together with

the experimentally determined rates and laws of decay

previously established by Rutherford and Frederick Soddy

when working at McGill University, to deduce that the

minerals were about 500 million years old. This was

con® rmed by Boltwood’s U ± Pb ages in 1907. This was

well before the discovery of isotopes, the nucleus or the

neutron. By 1913 a very in¯ uential twentieth-century

geologist, Arthur Holmes, estimated that the oldest rocks

had to be at least 1600 million years in age.

It was J. J. Thompson’ s classic charge ± mass ratio

experiment and the resultant discovery of isotopes that was

critical to the further development of radioactive dating.

Soddy had already inferred that diŒerent isotopes might

Figure 1. Radiogenic heat production within the Earth as a

function of time (US billion years). Accretional energy and the

decay of short-lived nuclides such as244

Pu would have provided

heat for the very early Earth, but it is the long-lived radioactive

nuclides that have kept the planet active through most of

geological time. Based on a ® gure by R. K. O’Nions with kind

permission.

A. N. Halliday104

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exist. Thompson showed that the positive rays of Ne

produced lines corresponding to diŒerent masses. It was

left to his student, F. W. Aston, to devise an experiment to

separate partially these isotopes using diŒusion through a

porous medium. He showed that the diŒused Ne had a

diŒerent mass and built a mass spectrograph for measuring

isotopic compositions of a variety of elements. Once

several long-lived radioactive decay schemes had been

deciphered, and modern mass spectrometers had been built

by such great experimental physicists as the late Alfred

Nier, accurate isotopic ages could be obtained. For

example, armed with such a background, the late Clair

Patterson in 1955 published the results of Pb isotopic

analyses of meteorites which demonstrated that the Earth

and other Solar System objects had to be 4500 ± 4600

million years old. This wonderful experiment, the result of

which has been repeatedly con® rmed by new evidence,

® nally established the age of the Earth and placed the

rapidly expanding ® eld of radiogenic isotope geochemistry

® rmly on the map. Today, diŒering aspects of the ® eld are

of fundamental importance to almost every area of Earth

sciences.

3. Principles of isotopic dating

There is a wide variety of radioactive isotopic dating

methods in use today (table). In general the development of

these dating systems has followed technical progress in

mass spectrometry. Many of the elements of interest are

present at the trace level (parts per trillion to parts per

million) in most rocks. Making accurate measurements has

long been a challenge. The major issues have been the

sensitivity (the number of ions deteted per atoms used),

blanks, the mass resolving power, and the need for stable

Main isotopic decay systems in use in Earth sciences.

Parent Daughter(s) Half-life (years) Applications

Short-lived decay3H

10Be

14C

26Al

36Cl

81Kr

12 9I

21 0Pb

22 6Ra

23 0Th

23 1Pa

23 4U

Long-lived decay40

K87

Rb13 8

La14 7

Sm17 6

Lu18 7

Re23 2

Th23 5

U23 8

U

Extinct decay22

Na26

Al53

Mn60

Fe92

Nb10 7

Pd12 9

I13 5

Cs14 6

Sm18 2

Hf20 5

Pb24 4

Pu

3He

10B

1 4N

2 6Mg

3 6S,

3 6Ar

8 1Br

12 9Xe

20 6Pb

20 6Pb

20 6Pb

20 7Pb

20 6Pb

4 0Ca,

4 0Ar

8 7Sr

138Ba,

13 8Ce

14 3Nd

17 6Hf

1 87Os

20 8Pb

20 7Pb

20 6Pb

2 2Ne

2 6Mg

53Cr

60Ni

9 2Zr

107Ag

12 9Xe

1 35Ba

14 2Nd

1 82W

2 05Tl

1 36Xe ²

1 × 23 ´ 10

2 × 6 ´ 106

5 × 73 ´ 103

7 × 3 ´ 105

3 × 01 ´ 105

2 × 1 ´ 105

1 × 57 ´ 107

2 × 23 ´ 10

1 × 6 ´ 103

7 × 54 ´ 104

3 × 28 ´ 104

2 × 47 ´ 105

1 × 25 ´ 109

4 × 88 ´ 101 0

1 × 05 ´ 101 1

1 × 06 ´ 101 1

3 × 57 ´ 101 0

4 × 23 ´ 101 0

1 × 40 ´ 101 0

7 × 04 ´ 108

4 × 47 ´ 109

2 × 605

7 × 3 ´ 105

3 × 7 ´ 106

1 × 5 ´ 106

3 × 6 ´ 107

6 × 5 ´ 106

1 × 57 ´ 107

2 × 3 ´ 106

1 × 03 ´ 108

9 ´ 106

1 × 5 ´ 107

8 ´ 107

Ground water

Sediments, arc volcanoes

Palaeoceanography, archaeology

Sediments

Ground water

Ice cores

Hydrocarbons

Young sediments

Volcanoes

Spelaeothems, coral, volcanoes

Volcanoes

Spelaeothems, corals, volcanoes

Volcanic rocks, uplift ages, lunar bombardm ent

Mantle and crust evolution, marine carbonates, early Solar System

Very limited applications

Mantle and crust evolution, early Solar System

Igneous rocks, crusta l evolution

Mantle and crust evolution, early Solar System

Very limited applications

Mantle and crust evolution, early Solar System

Mantle and crust evolution, early Solar System

Early Solar System

Early Solar System

Early Solar System

Early Solar System

Early Solar System

Early Solar System

Early Solar System, terrestria l degassing

Early Solar System

Early Solar System, crustal evolution

Early Solar System, core formation

Early Solar System

Early Solar System, terrestria l degassing

² Spontaneous ® ssionogenic product

Radioactivity and discovery of time 105

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measurement systems with low noise that are linear

through a large dynamic range. The isotopic systems can

be broadly categorized into three groups.

(1) Short-lived decay systems are ideal for studying very

recent activity. These radio-nuclides are produced

today as a result of cosmic-ray interactions or as

intermediate daughters of the actinide series. In

many such cases, only the radioactive parent isotope

is measured (e.g.14

C).

(2) Long-lived decay schemes utilize nuclides with half-

lives within two orders of magnitude of the age of the

Earth and are therefore perfectly suitable for

deciphering events on geological time scales where

the parent and daughter isotopes must both be at

measurable levels (® gure 2).

(3) Extinct decay systems utilize short-lived isotopes that

were alive in the early history of the Solar System

and produced variations in the abundances of their

daughter isotopes. The isotopic compositions of such

daughter elements provide us with clues regarding

the ® ne-scale evolution of the earliest events in the

Solar System and constrain the nature of the stars

and supernovae that contributed to the molecular

cloud which collapsed to form the solar nebula.

Clearly, only the daughter element isotopic composi-

tions can be measured (® gure 2).

All isotopic dating methods are based on the principles

of radioactive decay elucidated by Soddy and Rutherford

in 1902, that is that decay is spontaneous and random, such

that the rate of decay decreases according to the number of

atoms available, but the probability of an atom decaying

per unit time is a constant called the decay constant k . In

dating rocks we conventionally count time backwards; so

recasting the equations of Soddy and Rutherford we have

[P] 5 [P]t 3 exp( 2 ¸t) ,

where [P] is the number of parent nuclides today and t is

some time in the past. Many studies utilizing cosmogenic

nuclides such as10

Be,14

C,26

Al and129

I merely measure the

number of atoms of the radioactive parent isotope in the

sample. The age since the radioactive parent isotope was

incorporated in the sample can then be calculated if one

knows the amount [P]t present when the sample formed.

This is determined in part using models of production

against time (a function of the cosmic-ray ¯ ux). Another

method commonly used in dating very young sediment is to

assume that the cosmic-ray ¯ ux and the sedimentation rate

have stayed constant and to measure the number of atoms

as a function of depth in the sediment.

Nearly all isotopic dating over long (geological) time

scales uses long-lived nuclides and relies on the measure-

ment of the amount of daughter as well as parent isotope.

Every parent atom that decays produces a (`radiogenic’ )

daughter atom D*:

[D* ] 5 [P]t 2 [P]

Therefore

[D* ] 5 [P][exp( t) 2 1].

We can denote the present abundances by their isotopic

labels. So, for the decay of238

U to206

Pb for example,

P ® 238U and D ® 206

Pb:

[206

Pb* ] 5 [238

U][exp( 238 t) 2 1]

which can be rearranged to give the age

t 5 ln[206

Pb* ]

[238

U]1 1 238 .

In practice there are a variety of inherent complications

to take into account. In nearly all isotopic dating, one has

to make allowance for the initial daughter isotope present

in the mineral or rock when it forms, This can be

corrected for adequately in some minerals with very high

parent element-to-daughter element ratios. For example,

the most accurate ages of ancient rocks in the continental

crust are obtained by U ± Pb dating of a common

zirconium silicate mineral called zircon. This mineral has

a high U-to-Pb ratio and is highly resistant to the eŒects

of natural resetting via diŒusion. Pb will not ® t into the

lattice of this mineral when it grows; so any initial Pb is

minor and corrections can be made with su� cient

con® dence that the ages are extremely accurate. In other

minerals and rocks it is better to monitor the abundance

of intitial daughter isotope more precisely by measuring

the parent and daughter isotopes relative to a non-

radiogenic isotope of the same daughter element. For

example, the above equations for U ± Pb then become

Figure 2. Short-lived nuclides decayed su� ciently fast that

they are now totally extinct except for the small amounts

produced anthropogenically, by spallation reactions with cosmic

rays and as intermediate daughter products of the chain decay of

Th and U. However, variations in the abundances of the daughter

isotopes of short-lived extinct nuclides can tell us much about the

early history of the solar system and the time intervals from

nucleosynthesis to collapse of the Solar nebula.

A. N. Halliday106

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[206

Pb]

[204

Pb] t

5[206

Pb]

[204

Pb]2

[238

U]

[204

Pb][exp( 238 t) 2 1]

and therefore

t 5 ln[206

Pb]

[204

Pb]2

[206

Pb]

[204

Pb] t

[238

U]

[204

Pb]1 1 ¸238 .

In order to determine an initial ratio such as ([206

Pb]/

[204

Pb])t, it is necessary to use what are termed isochron

methods (® gure 3). Samples that are all formed from the

same precursor material (or protolith) would be expected

to have uniform initial isotopic compositions when they

crystallize. Thus the individual minerals in a sample of

granite are usually expected to have had the same initial

Sr isotopic composition ([87

Sr]/[86

Sr])t, when they form.

However, because of chemical diŒerences they would have

distinct ratios of their radioactive parent to radiogenic

daughter element (i.e. the Rb-to-Sr ratio [87

Rb]/[86

Sr]).

Measuring the Sr isotopic compositions and Rb-to-Sr

ratios of a variety of such phases that all precipitated

from the same magma allows one to determine the age

and the initial Sr isotopic composition, by regressing the

data in ® gure 3, for which

t 5 [ln (slope 1 1) ]/ ¸87

and ([87

Sr]/ [86

Sr])t is de® ned by the intercept. So, in most

radioactive dating techniques, and particularly those used

for geological time scales, one needs to measure the

parent-to-daughter element ratio and the daughter iso-

topic composition and to know the decay constant to

determine an age.

There is one system which allows additional leverage.

There are two isotopes of U which decay to two isotopes of

Pb (table). This means that the two age equations:

[206

Pb* ] 5 [238

U][exp( 238 t) 2 1]

and

[207

Pb*] 5 [

235U][exp( 235 t) 2 1]

can be combined. It therefore becomes unnecessary to

measure the parent-to-daughter ratios, since the ratio

[238

U]/[235

U] is constant in nature (137.88 ). It follows that

[207

Pb* ]

[206

Pb* ]5

1

137.88

exp( 235 t) 2 1

exp( 238 t) 2 1.

This last equation can only be solved for t iteratively. All

three of these equations should yield the same age Ð a

unique and powerful property of U ± Pb dating. The

quantity [207

Pb*]/[206

Pb*] is simply a function of age and

not U-to-Pb ratio. In a similar manner, isochrons can be

used to determine the initial Pb isotopic composition and

age from a suite of co-genetic samples by plotting [207

Pb]/

[204

Pb] against [206

Pb]/ [204

Pb]:

slope 51

137.88

exp(¸235 t) 2 1

exp(¸238 t) 2 1.

It was using these techniques that C lair Patterson

determined that the age of Fe and silicate meteorites, and

the Earth, was between 4500 and 4600 million years (® gure

4).

Studies of extinct radionuclides, which may have been

present in the early Solar System, have been of great

interest to cosmologists and cosmochemists because they

provide clues about the timing and nature of nucleosyn-

thetic processes that contributed matter to the Solar

System, as well as a chronology for the accretion and

diŒerentiation of the inner planets and planetesimals. The

principle is that the decay of parent isotopes now extinct

should have left a record of isotopic variations in daughter

elements if there has been no isotopic homogenization over

the subsequent 4000 million years. Such isotopic eŒects are

only found in bodies that have remained isolated since

soon after the Solar System formed. The ® rst such anomaly

was discovered by John Reynolds who isolated Xe with an

excess of129

Xe from a primitive kind of meteorite called a

Figure 3. Isochrons provide a mechanism for getting around the

problem of the initial daughter isotope in radioactive dating.

Several samples of a suite of minerals or rocks which all formed

at the same time from the same material (e.g. diŒerent pieces of

granite from a common body which was once molten) will have

the same isotopic composition when the samples formed

originally. However, isotopic diŒerences will develop with time

as the radioactive parent isotope decays to the daughter isotope.

The change in the abundance of the isotope in the daughter

element will be a function of how much time has elapsed and

the ratio of the concentration of the parent element to the

concentration of the daughter element which will vary with

diŒerent mineral or rock samples. In an isochron diagram such

as that shown here, the data will de® ne a straight line, the slope

of which is proportional to the amount of time that has elapsed.

Radioactivity and discovery of time 107

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chondrite. These are thought to represent unprocessed

material of similar average composition to the solar

nebula, with the exception of very volatile elements which

have been lost. They probably represent the material which

was prevalent prior to accretion of the planets. He

hypothesized that the excess129

Xe was caused by the

former presence of129

I, which decays with a half-life of

only 16 million years. Although some argued that this

could simply re¯ ect heterogeneity in the early Solar System

with no implications for129

I, he later con® rmed his

hypothesis by showing that the [129

Xe]/[130

Xe] ratio was

correlated with the I-to-Xe ratio in the diŒerent phases of

the meteorite. So even though there was no129

I left, its

former presence could be ascertained. Demonstrating a

correlation between an isotopic anomaly and the relevant

parent-to-daughter elemental ratio has become a standard

test for the decay of a radioactive nuclide in situ. As with

other techniques it is necessary to use isochrons to

determine the abundance of the parent isotope at the time

that the object formed, and this in turn is a function of the

time elapsed since the nuclide was produced, and the

amount that was produced. Therefore it becomes a

complicated task deconvolving such eŒects. This work

has to be conducted extremely carefully. It is all too easy to

generate isotopic anom alies with bad measurements!

However, the scienti® c rewards of accurate measurements

are very great. We can deduce how quickly the various

bodies in the Solar System accreted, diŒerentiated and

formed metallic cores and atmospheres. Furthermore, the

deduced initial abundances of short-lived nuclides at the

start of the Solar System provide unequalled insights into

the processes of stellar evolution in our galaxy in the

vicinity of the molecular cloud which collapsed to form the

solar nebula.

4. Establishing a geological time scale

The list of major scienti® c problems that have been solved

by isotopic dating is enormous. We now have a well

established framework for the geological record. Much of

the eŒort these days is concerned with pushing the limits of

temporal resolution further in order to de® ne precisely the

rates of processes in the past.

The age of the Solar System is now ® rmly set at 4.57 Ga

(gigayears) ago by dating chondrites. Most non-primitive

meteorites (both the silicate types called achondrites and

the metal types called irons) represent disrupted fragments

of planetesimals (small planets) and are of similar age. One

achondrite, Angra dos Reis, has now been dated using U ±

Pb with a precision of 0.01% , permitting unprecedented

resolution of early Solar System evolution. The achondrites

have textures and chemical compositions implicating an

origin from molten rock (magma). The irons appear to be

segregates of metallic cores and also yield similar, very old

ages. All these diŒerentiated meteorites are generally

thought to be derived from small bodies that accreted,

melted and then were disrupted by impacts at an early

stage. The Earth probably accreted from a mixture of such

material.

Because of radioactive dating we have been able to

identify fragments of other planets without ever having

visited them! A class of quite diŒerent meteorites has been

discovered with surprisingly young isotopic ages. A group

of silicate meteorites called `SNC’ (pronounced `snick’ )

meteorites usually yield ages of about 1300 million years.

SNC meteorites are now identi ® ed by their O isotopic

compositions. This had become a standard mechanism for

recognizing which meteorites came from the same parent

body because O has a more heterogeneous and distinctive

isotopic composition in diŒerent meteorites than any other

element. The young isotopic ages suggest that SNC

meteorites were derived from a body that is su� ciently

large to have sustained magmatic activity until relatively

recently, geologically speaking. Small bodies such as the

Moon are no longer active. Conversely, the body must be

small enough that, when debris was knocked oŒits surface

by an impactor, it could, on occas ion, escape the

gravitational pull of the host planet and eventually wind

up on the surface of the Earth. The most likely candidate is

the planet Mars. We can measure chemical and isotopic

compositions of these meteorites and thereby retrieve

information on the evolution of Mars and its atmosphere.

Figure 4. Pb isotope isochron diagram for meteorites as

determined by the late Clair Patterson. The slope of the line

corresponds to an age of about 4500 ± 4600 million years. Most

samples from the Earth and Moon plot on the same line as other

Solar System objects (with a slight tendency towards the right).

There is no reason for this unless all these Solar System objects

formed at about the same time. Therefore, even though the oldest

rocks found on Earth thus far are only 4000 million years in age,

scientists believe the Earth is probably closer to 4500 million

years old.

A. N. Halliday108

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It is one of these SNC meteorites that has recently been

found to contain evidence that may indicate the former

presence of life on Mars.

The Earth is still very active with continents drifting

around, volcanoes erupting, erosion attacking the surface

and mountains continuously being built. A striking feature

of our planet is the existence of continents and ocean

basins. But how did they develop? The growth of the

continental portion of the crust (the outermost layer of the

Earth which is 20 ± 70 km thick) has been an important

area of study for many years with the development of

isotopic dating methods. Not only can we date speci® c

rocks, but also we can determine the integrated history of

the continents with radiogenic tracers and model Earth’ s

chemical evolution. For example, the two rare earths Sm

and Nd have such similar chemical properties that their

ratio is hardly changed by most geological processes.

However, a large fractionation in Sm and Nd takes place

with the production of continental crust; that is the Sm-to-

Nd ratio of the continental crust is low relative to the rest

of the silicate earth (® gure 5). Since147

Sm decays to143

Nd

(table), the time-integrated eŒect of this is unradiogenic Nd

with a lower abundance of143

Nd in the crust than in the

mantle. The Nd isotopic composition of continental rocks

of diŒerent ages gives us a clue about the extent of crustal

growth at diŒerent times (® gure 5). From this record, plus

the direct determination of formation ages of speci® c rocks

it has been found that the continental crust has a record

extending back to about 4000 million years. There is only a

trace of a record before that time. The mean age of the

crust turns out to be about 2000 million years.

From the changing volume of continental crust with

time it might seem easy to calculate a growth rate (® gue 6).

However, the problem is much more complex than this.

The continental crust is ultimately derived from the mantle;

the enormous underlying magnesium silicate reservoir

extending 2900 km down to the metallic Fe core. Con-

tinental crust is made of low-density material and buoy-

ancy prevents it from being returned to the mantle as a

discrete entity. Oceanic crust is more dense and is

continually being returned to the mantle by a process

called subduction Ð the pulling of ocean ¯ oor down to the

mantle. The places where this occurs are called subduction

zones and they are accompanied by several striking

features. The surface of the ocean ¯ oor is depressed,

producing deep-sea trenches such as the Marianas trench.

Melting takes place above the subduction zone, causing

extremely violent volcanoes. The world’ s largest earth-

quakes are the result of the fact that cold and dense ocean

¯ oor is forced down into the interior of the Earth at these

locations. This process of subduction creates a major

uncertainty over the history of continental growth because

a variable, but often unknown, amount of continental crust

is carried down into the Earth’ s interior as sediment on the

subducting oceanic crust `conveyor belt’ . Although it is

easy to determine the ages of continental rocks and

possible to determine the average time at which the current

continental masses were extracted from the mantle, we do

not know how much continental material has been

returned as sediment to the mantle at diŒerent times.

Therefore, we cannot know for certain whether the present

continental mass against age distribution also represents a

continental growth against age distribution. Hence there is

still considerable debate about crustal growth models with

Figure 5. The rocks of the Earth’s continental crust have a low

Sm-to-Nd ratio, relative to the total Earth. The Nd isotopic

composition of a sample of continental material, such as a

sedimentary rock, is a function of the average time at which the

continental crust from which it formed was separated from the

Earth’s mantle. With a variety of such samples, scientists can

determine the average age of the continents (about 2000 million

years). Based on a ® gure by G. Faure with kind permission.

Figure 6. The uncertainty over the amount of recycling of

continental material back into the mantle at subduction zones

precludes a unique model of crustal growth. The average age of

the crust alone does not de® ne how fast the continental crust

grew in the past.

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some arguing that the crust has not increased in volume

signi® cantly for the past 4000 million years because of

recycling (® gure 6).

The major surface features of the Earth such as

mountain belts and ocean basins are now understandable

because of isotopic dating. Ironically, at one stage, we knew

more about the age of the surface of the Moon than we did

about the age of the ocean ¯ oor. The application of K ± Ar

dating to the basalts of the ocean ¯ oor in the early 1970s

showed that all the ocean basins were young, the oldest and

deepest oceanic crust being less than 200 million years in

age, the youngest forming the new shallow ocean ridges.

These data con® rmed the theory of continental drift and

provided a clear picture of sea ¯ oor spreading in which new

oceanic crust is created at the world’ s ocean ridges.

To the layman, mountains are often thought of as robust

features that have survived longer than everything else. It

was easy to show with isotopic dating that the Earth’ s

mountain belts represent the youngest most active portions

of the continents, in a state of rapid uplift because of

continental movements. Isotopic methods can be used to

deduce the time at which diŒerent portions of the Earth

cooled through particular temperatures as they were

excavated from greater depths. Portions of the Alps and

Himalayas appear to represent rapidly uplifting crust

exhumed from tens of kilometres within the Earth over

just a few million years as a result of collisions between

continental plates.

A major issue in establishing a geological time scale is the

development of life. Here we hit a problem. The fossil

record is preserved in sedimentary rocks, and these are

extremely di� cult to date. Sediment carries the record of

isotopic heterogeneities produced by radioactive decay

within the protolith material that was eroded, so that the

isotopic `age’ of a sedimentary rock usually represents a

mixed age of the components. To make matters even more

di� cult, complex organisms with specialized cells (metazo-

ans) do not appear in the fossil record in rocks older than

about 700 million years. The period before this, represent-

ing 85% of Earth history, is almost completely barren of

fossils. Most of those that have been recovered are of very

simple life forms such as modern cyanobacteria. The cause

of this sudden diversi® cation of life and rapid development

of sophisticated plants and animals is unclear. However,

the build-up of free O in the atmosphere was a clear

prerequisite. Finding precise ways to date and calibrate the

fossil record and placing the ® rst 85% of Earth history into

a well understood sequence of events have been tricky

tasks.

A signi® cant advance in calibrating the sedimentary

record has been made with the development of U ± Pb

dating of carbonates. Some well preserved limestones, if

carefully sampled, can yield extremely accurate direct age

determinations. The reason for this is that carbonate

sediments are mainly formed by direct precipitation from

sea water in warm climates (such as the present-day

Bahamas). As such, they do not contain the fragments of

older rock which prevent the dating of most sedimentary

formations; the rock is entirely newly formed. Some

carbonates precipitate with a very high ratio of U to Pb,

permitting accurate age determination. This method has yet

to be evaluated thoroughly for the earliest carbonates,

partly because the proportions of carbonates decrease back

through time and partly because many early rocks are not

well preserved.

Providing ages for sedimentary rocks and fossils is

particularly important in trying to understand the causes of

biological extinctions. As the paleontological data have

amassed, it has become apparent that on several occasions

over the past 700 million years, a large proportion of the

biota became extinct. The causes of such `mass extinctions’

has been a subject of enormous controversy. The extremes

to the models are those that advocate a catastrophe which

eŒects the entire Earth simultaneously and those that

invoke gradual change in environment with a threshold

beyond which many species cannot survive. These models

can be tested by studying the rates of extinctions and

comparing the timing of extinctions with that of major

catastrophes. The two major catastrophes commonly

considered are impactors hitting the Earth and unusually

large volcanic eruptions. Of course the latter may be

precipitated by the former, so, even if an impactor hits the

Earth, it could be the ensuing volcanic activity that wipes

out most lifeforms. The most widely discussed extinction

event is that associated with the demise of the dinosaurs

about 65 million years ago. Scientists have discovered an

impact crater of exactly the correct age oΠthe coast of

Mexico and, by using very precise K ± Ar dating, have

demonstrated that vast outpourings of basalt occurred in

the Deccan of western India at the same time.

5. From long to short; from large to small

With the overall framework of the evolution of the Solar

System and Earth established, some of the largest issues

concern the precise rates of more recent geological activity.

The development and use of new, highly accurate mass

spectrometry methods for measuring the abundances of

intermediate daughters of the U and Th decay series, which

have largely superseded counting techniques, the use of low

blank laser furnaces to degas very young samples for K ± Ar

dating, together with the measurement of cosmogenic

nuclides using accelerator mass spectrometry have been

critical. These methods have been applied to an array of

problems related to the recent development of the Earth

including hom inid evolution, the m elting rates and

residence times of magma prior to eruption, the timing of

past sea level changes, ground water dating and theories for

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the causes of ice ages. The development of U ± Th

disequilibrium series dating using mass spectrometry

instead of the more traditional a counting techniques has

resulted in dramatic improvements in precision and sample

size. The short-lived nuclides226

Ra,230

Th,231

Pa and234

U

can all be determined using this method. The precise time

span over which radioactive disequilibrium has existed can

be used to determine the timing of chemical fractionations

such as Th from U. Such fractionations occur when magma

forms and starts to crystallize under volcanoes. We can

deduce the accumulation rates and residence times of

magma from such information. When coral grow from sea

water, they incorporate U but not Th into their carbonate

skeletons. Since corals live near sea level, their exact age

allows palaeoceanographers to deduce changes in sea level

back through time. These in turn are related to the amount

of ice held in glaciers and polar ice caps. Hence one can

determine the timing of ice ages with hitherto unattainable

accuracy and, from this, test models for the driving forces

behind changes in climate.

Scienti ® c breakthroughs in this discipline of science are

nearly always linked with technical developments in mass

spectrometry. For the past 25 years there has been a

fruitful symbiotic relationship between scientists and

instrument manufacturers which needs to be maintained.

For example, it is increasingly important to develop new

instrumentation for studying isotopic variations at the

microscopic scale. Even such major geological features of

the Earth as volcanoes, mountains and hydrocarbon

reservoirs relate to processes that take place on the scale

now being approached by modern material characteriza-

tion techniques such as transmission electron microscopy.

Matching such textural and chemical information with

precise isotopic data on the same scale represents the

toughest technical challenge in geochemistry. The closest

approach has come from the development of ion probes, a

form of secondary-ion mass spectrometry. Closely follow-

ing ion probes is the new technique coupling laser ablation

with multiple-collector, inductively coupled plasma mass

spectrometry. W ith such devices we can now date

individual mineral grains and tackle problems as diŒerent

as the origin of pre-Solar dust grains preserved in primitive

meteorites and the rates of mountain building in the

Himalayas.

In the midst of all the interest in how the Earth has been

behaving in detail, there is a persistent nagging de® ciency in

our most basic knowledge of Earth evolution. How did the

Earth start?

6. How did the Earth start?

For many years it has been a mystery as to why the oldest

rocks of the Earth were 4000 million years in age whereas

the Earth itself, like the rest of the planets, formed more

than 4500 million years ago. What happened in the

mysterious ® rst 500 million years of Earth’ s history Ð the

`Dark Ages’ of geological time? This is still an intriguing

and fundamental problem that probably represents the

largest challenge to radiogenic isotope geochemistry. Slow

progress is being made, and some important clues came

from sampling the Moon.

Major technical and scienti® c developments in isotopic

dating accompanied the attempts to put a person on the

Moon. The enormous unprecedented costs of rebuilding

laboratories, redesigning mass spectrometers and hiring the

best analytical geochemists to work on the returned samples

were trivial compared with the overall budget for the Apollo

missions. The results powerfully demonstrated what a huge

amount of information of diŒerent kinds could be gleaned

about the chemical and physical evolution of another Solar

System object by returning a few samples. In contrast with

the Earth, there are plenty of rocks preserved on the Moon

which formed prior to 4000 million years ago. Some appear

to be relics from the very earliest stages of its history.

We think that planets such as Earth built up with a run-

away accretional eŒect. The earliest planetesimals were

small. Most of the meteorites which land on Earth’ s surface

represent fragments of such planetesimals. As these early

planetesimals attracted each other under gravity, the ensuing

collisions resulted in the establishment of successively larger

bodies which exerted a still stronger gravitational pull. A

body the size of the Earth is expected to have taken 50 ± 100

million years to grow. Therefore the Earth should be, on

average, younger than the chondrites and early planetesi-

mals, resulting from a more protracted growth. The Pb

isotopic composition of the Earth, as sampled at the surface,

is broadly consistent with its having an age which is slightly

younger than these meteorites (® gure 4).

The Moon had a ® ery start and is thought to have been

formed by an impactor the size of Mars hitting the Earth

with a glancing blow. There seems to be no other way to

explain both the angular momentum of the Earth ± Moon

system and the chemical and isotopic compositions of lunar

rocks. The eŒect of this giant impact would be to melt and

vaporize partially both bodies and to provide angular

momentum to the hot debris encircling the spinning Earth.

The Moon formed from this debris but appears to have lost

large amounts of volatile material in the process. The

Moon can only have formed from a giant impactor at a

relatively late stage in Earth history because, as time

progresses, the number of impactors should decrease (as

the Solar System gets `cleaned up’ ) but the chances of

encountering a very large impactor increase. This is

consistent with the ages determined for lunar rocks. While

many aspects of lunar origin are still ® ercely debated, we

are con® dent from isotopic dating that the Moon is

signi ® cantly younger (by about 50 ± 100 million years) than

the collapse of the solar nebula.

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The Moon rapidly developed an ocean of molten rock

down to considerable depth, sustained by the energy of the

vast numbers of impactors gravitationally attracted to its

surface during the early stages of planetary accretion, when

there was abundant debris still ¯ ying around. Its surface

continued to be intensely bombarded until about 4000

million years ago. All magmatic activity had ceased by

about 3000 million years ago. The history provides us with

important clues about our own planet. It is clear that any

eŒects of bombardment on the Moon would have been far

worse on Earth, because the gravitational pull would be

much larger. Therefore the fact that the Earth lacks rocks

that are older than 4000 million years is understandable.

The entire surface of the Earth was almost certainly

decimated by impactors until about that time. Whatever

continents and ocean basins existed were largely wiped out.

But what did exist before? Are there ways of deciphering

the earliest history of the Earth without a geological

record? It turns out that there are. In particular, we can

examine the issues of when and how the Earth ® rst

developed a hydrosphere or atmosphere, a metallic core

and continental crust using four diŒerent short-lived

(ex tin ct) decay sy stem s:1 2 9

I ±1 2 9

X e ,2 4 4

Pu ±1 3 6

X e,182

Hf ±182

W and146

Sm ±142

Nd.

`Xenology’ is a complex subject. However, the critical

points are as follows. The Xe in the atmosphere is not as

enriched in the isotope129

Xe as that being degassed from

the Earth’ s mantle at ocean ridges (® gure 7). There is no

known cause of such a diŒerence other than the decay of129

I with a half-life of 16 million years. The iodine content

of the atmosphere is negligible. Since129

I is no longer

present or produced within the Earth in signi ® cant

amounts, the only possible causes of this diŒerence have

to be either the early release of Xe from the interior of the

Earth into the atmosphere, before all the Earth’ s original129

I had decayed away, such that the residue in the Earth’ s

interior became enriched in129

Xe relative to the atmo-

sphere or the accretion of our current atmosphere at a

relatively late stage from cometary materials (® gure 8). The

former model requires the atmosphere to outgas from the

interior of the Earth within 100 million years of the last

synthesis of129

I. Excesses of heavier isotopes of Xe such as136

Xe are also found (® gure 7) and are thought to have been

produced by the ® ssion of PU early in Earth history and the

® ssion of238

U throughout geological time. Despite the

evidence for the former existence of129

I in the Earth, the

quantities calculated to have been present, relative to non-

radioactive I (127

I) are low. One of the ways in which this

can be modelled involves an accretion of the Earth that

was more protracted than is generally assumed. Of course

xenology tells us nothing about many of the other

components of the atmosphere. If the Xe in the atmosphere

were acquired from cometary material, substantial amounts

of other volatiles would also be accreted. However, many

components would have been added much later: O by

photodissociation and photosynthesis, and Ar by the decay

of40

K.

The recent development of Hf ± W chronology has

allowed scientists to reassess the age of the Earth ’ s

inaccessible core (® gure 9). The core was previously

thought to have formed before the Moon because of

Figure 7. Samples of basalt from the world’s midocean ridges

contain greater abundances of129

Xe and136

Xe than are found in

the atmosphere. These can only be explained by the former decay

of129

I and244

Pu respectively.

Figure 8. Xe isotopic diŒerences between basalts and the

atmosphere provide evidence that the atmosphere segregated

from the interior of the Earth within the ® rst 100 million years of

its history (From a ® gure by W. S. Broecker, with permission).

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diŒerences between the Fe contents of the two bodies.

However, once the techniques were available it was easy to

show that core formation had to be similar in age to the

Moon and probably formed when the Earth had its own

globally encircling magma ocean. The metallic liquid,

which drives the Earth’ s m agentic ® e ld, must have

segregated from a silicate-and-metal-liquid em ulsion

(rather like oil-and-vinegar dressing) at a late stage when

there was widespread melting in the Earth. We know that

the Earth’ s metallic core did not segregate immediately

because of the similarities in W isotopic composition

between the silicate Earth (with high Hf-to-W ratio) and

primitive meteorites (with low Hf-to-W ratio). The eŒects

of the decay of182

Hf appear to have been negligible (® gure

9). The Earth’ s core is clearly not as old as the cores of early

Solar System planetesimals. The best estimate for the age of

both the core and the Moon is about 4470 6 40 million

years as opposed to 4570 6 10 million years for the origin of

other solar systems and the earliest diŒerentiated meteor-

ites.

The early growth of the continents has been tackled by

two approaches. The ® rst has been to use ion probes to try

to ® nd any grains of the highly resistant and refractory

mineral zircon which are older than 4000 million years and

were ultimately preserved in younger sedimentary rocks.

This search, using U ± Pb dating, has been partially

successful. A few grains have been found in an Australian

sandstone called the Mount Narryer Quartzite which yield

ages of 4100 ± 4200 million years. These are the oldest

surviving indigenous grains on Earth yet identi ® ed. Since

zircon mainly grows in continental crustal rocks, the

implication is that continental rocks did exist prior to

4000 million years ago.

The second approach has been to use Nd isotopes. As

explained above, the Nd isotopic composition of a rock

re¯ ects its time-integrated Sm-to-Nd ratio, which is

strongly fractionated during the production of continental

crust. Nearly all Nd isotopic studies have utilized the decay

of147

Sm to143

Nd. However, a shorter-lived isotopic system

has been used recently to try to detect earlier growth of the

continental crust. The decay of146

Sm to142

Nd (table) early

in the history of the Earth would result in anomalous

abundances of142

Nd if Sm and Nd were fractionated by

crustal production. The eŒect is likely to be extremely small

and the m easurement of1 42

Nd abundances at such

precision is di� cult. At the time of writing, only one group

has reported an isotopic diŒerence; this was for a single old

rock, and the result has yet to be convincingly replicated.

All other results are negative, indicating that the geochem-

ical and isotopic eŒects of production of continental crust,

if it existed in any signi® cant amount, were eŒectively

stirred back into the mantle.

In summary, we now believe that the Earth had a

protracted growth lasting at least 50 ± 100 million years

and that towards the end of this period it formed a core,

was decimated by an impactor the size of the planet Mars

to form the Moon, was heavily degassed and lost its

earliest atmosphere. At some stage it acquired a new

atmosphere by cometary impacts or degassing of the

interior of the Earth. Generation of continental crust must

have occurred, but it may have been very limited and

nearly all vestiges were probably destroyed by impact

melting of the Earth.

7. Concluding remark

With all its powerful and heuristic technique development,

radiogenic isotope geochemistry is still very much a

developing observational science. The new data provide

powerful constraints and lead to the development of new

models of Earth behaviour. However, the Earth, in turn, is

so much more complicated and hard to fathom than we

thought, that eventually many models fall apart, and we are

still left needing fresh methods, approaches and models for

® guring out exactly how it all works. Lyell and Darwin

would be astonished at the detailed picture of Earth

evolution that we now have. But with their sharp intellects I

would imagine it would take them less than 5 minutes to

point out what we still do not know about extinctions,

volcanic eruptions and climate change. We still have much

to sort out regarding the ® rst 85% of Earth history. The

® rst 10% needs considerable innovation and thought. Most

of the progress here will probably come from applying

additional short-lived (extinct) nuclide systems to the

Earth.

Figure 9. The W isotopic composition of the silicate portion of

the Earth is the same as that found in primitive solar system

material such as chondrites, despite a much higher Hf-to-W ratio

caused by segregation of the metallic W-rich core. The similarity

in W isotopic compositions indicates that the metallic core must

have segregated late, after all the live182

Hf of the early Solar

System had decayed.

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Acknowledgements

I am very grateful to Chris Ballentine, Michael Sanders and

Rob Van der Voo for criticizing earlier versions of this

paper. Research in radiogenic isotope geochemistry at the

University of Michigan is supported by the US Department

of Energy, National Aeronautics and Space Administration

and National Science Foundation.

Further readingMany of the above subjects have not yet been described in broad overview

form. The following are some readily understandable books and papers

which should be of help to those interested in further reading.

AlleÁ gre, C. J., Staudacher, T., Sarda, P., and Kurz, M., 1983, Nature, 303,

762 ± 766.

Broecker, W. S., 1985, How to build a habitable planet (New York: Eldigio

Press).

Edwards, R. L., Chen, J. H., Ku, T.-L., and Wasserburg, G. J., 1987,

Science, 236, 1547 ± 1553.

Faure, G., 1986, Principle s of Isotope Geology, second edition (New York:

Wiley).

Froude, D. O., Ireland, T. R., Kinney, P. D., Williams, I. S., Compston,

W., Williams, I. R., and Myers, J. S., 1983, Nature, 304, 616 ± 618.

Halliday, A. N., RehkaÈ mper, M., Lee, D.-C., and Yi, W., 1996, Earth

planet. Sci. Lett., 142, 75 ± 89.

Harper, C. L., and Jacobsen, S. B., 1992, Nature, 360, 728 ± 732.

Jeffery, P. M., and Reynolds, J. H., 1961, J. geophys. Res., 66, 3582 ± 3583.

Jones, C. E., Halliday, A. N., and Lohmann, K. C., 1995, Earth planet. Sci.

Lett., 134, 409 ± 423.

Lee, D.-C., and Halliday, A. N., 1995, Nature, 378, 771 ± 774.

Newsom, H. E., and Jones, J. H. (editors), 1990, Origin of the Earth

(Oxford University Press).

Patterson, C. C., 1956, Age of meteorites and the Earth, Geochim.

Cosmochim. Acta, 10, 230 ± 237.

Podosek, F. A., and Swindle, T. D., 1988, Extinct radionuclides, Meteorites

and the Early Solar System , edited by J. F. Kerridge and M. S. Matthews

(Tucson, Arizona: University of Arizona Press), pp. 1093 ± 1113.

Taylor S. R. and McLennan S. M., 1985, The Continental Crust: Its

Composition and Evolution (Oxford: Blackwell Scientific).

Toulmin, S., and Goodfield, J., 1965, The Discovery of Time (London:

Hutchinson).

Wasserburg, G. J., Papanastassiou, D. A., Tera, F., and Huneke, J. C.,

1977, Outline of a lunar chronology, Phil. Trans. R. Soc. A, 285, 7 ± 22.

Wetherill, G. W., 1980, Formation of the terrestrial planets, A. Rev. Astron.

Astrophys., 18, 77 ± 113.

Alex N. Halliday obtained his BSc in Geology

and his PhD, in Physics at the University of

Newcastle-upon-Tyne. He was a postdoctoral

fellow and faculty member at the Scottish

Universities Research and Reactor Centre in

East Kilbride for 10 years and has been a

Professo r at the University of Michigan since

1986. His principa l scienti® c interest is the use of

radiogen ic isotope geochemistry to study the

early evolution of the Solar System, the develop-

ment of volcanic magma chambers, the origin of

mineral deposits , the history of the atmosphere

and oceans and the evolution of the Earth’ s

mantle.

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