DEFINITION OF AGE OF THE EARTH

Why do geologists say that the Earth is 4.6 billion years old? For

many hundreds of years, most people in European, Western, and

other cultures believed the Earth to be about 6,000 years old, based

on interpretations of passages in the Torah and Old Testament. However,

based on the principles of uniformitarianism outlined by James

Hutton and Charles Lyell, geologists in the late 1700s and 1800s

began to understand the immensity of time required to form the geologic

units and structures on the planet and argued for a much

greater antiquity of the planet. When Charles Darwin advanced his

ideas about evolution of species, he added his voice to those calling

for tens to hundreds of millions of years required to explain the natural

history of the planet and its biota. In 1846 the physicist Lord

Kelvin joined the argument, but he advocated an even more ancient

Earth. He noted that the temperature increased with depth, and he

assumed that this heat was acquired during the initial accretion and

formation of the planet, and has been escaping slowly ever since.

Using heat flow equations Kelvin calculated that the Earth must be

20–30 million years old. However, Kelvin assumed that there were no

new inputs of heat to the planet since it formed, and he did not know

about radioactivity and heat produced by radioactive decay. In 1896

Madame Curie, working in the labs of Henri Becquerel in France,

exposed film to uranium in a light-tight container and found that the

film became exposed by a kind of radiation that was invisible to the

eye. Soon, many elements were found to have isotopes, or nuclei of

the same element with different amounts of neutron in the nucleus.

Some isotopes are unstable and decay from one state to another,

releasing radioactivity. Radioactive decay occurs at a very specific

and fixed average rate that is characteristic of any given isotope. In

1903 Pierre Curie and Albert Laborde recognized that radioactive

decay releases heat, a discovery that was immediately used by

geologists to reconcile geologic evidence of uniformitarianism with

Lord Kelvin’s calculated age of the Earth.

In 1905 Ernest Rutherford suggested that the constant rate of

decay of radioactive isotopes could be used to date minerals and

rocks. Because radioactivity happens at a statistically regular rate

for each isotope, it can be used to date rocks. For each isotope an

average rate of decay is defined by the time that it takes half of the

sample to decay from its parent to daughter product, a time known

as the half-life of the isotope. Thus, to date a rock we need to know

the ratio of the parent to daughter isotopes and simply multiply by

the decay rate of the parent. Half-life is best thought of as the time

it takes for half of any size sample to decay, since radioactive

decay is a nonlinear exponential process.

The rate of decay of each isotope determines which isotopic

systems can be used to date rocks of certain ages. Also, the isotopes

must occur naturally in the type of rock being assessed and

the daughter products must be present only from decay of the parent

isotope. Some of the most accurate geochronologic clocks are

made by comparing the ratios of daughter products from two different

decay schemes—since both daughters are only present as a

result of decay from their parents, and their ratios provide special

highly sensitive clocks.

Isotopes and their decay products provide the most powerful

way to determine the age of the Earth. Most elements formed during

thermonuclear reactions in pre-solar system stars that experienced

supernovae explosions. The main constraints we have on

the age of the Earth are that it must be younger than 6–7 billion

years, because it still contains elements such as K-40, with a halflife

of 1.25 billion years. If the Earth were any older, all of the parent

product would have decayed. Isotopic ages represent the time that

that particular element-isotope system got incorporated in a mineral

structure. Since isotopes have been decaying since they were

incorporated, the oldest age from an Earth rock gives a minimum

age of the Earth. So far, the oldest known rock is the 4.03-billionyear-

old Acasta gneiss of the Slave Province in northwest Canada,

and the oldest mineral is a 4.2-billion-year-old zircon from western

Australia. From these data, we can infer that the Earth is between

4.2 and 6 billion years old.

The crust on the Moon is 4.2–4.5 billion years old, and the

Earth, Moon, and meteorites all formed when the solar system

formed. The U-Pb isotopic system is one of the most useful for

determining the age of the Earth, although many other systems give

identical results. Some meteorites contain lead, but no U or Th parents.

Since the proportions of the various lead isotopes have

remained fixed since they formed, their relative proportions can be

used to measure the primordial lead ratios in the early Earth. Then,

by looking at the ratios of the four lead isotopes in rocks on Earth

from various ages, we can extrapolate back to when they had the

same primordial lead ratio. These types of estimates give an age of

4.6–4.7 billion years for the Earth, and 4.3–4.6 billion years for meteorites.

So, the best estimate for the age of the Earth is 4.6 billion

years, a teenager in the universe.

generally found together in mixtures and each one decays

into several daughter products (including radium) before

turning into lead. The 230Th/234U disequilibrium method is

one of the most commonly used uranium-series techniques.

This method is based on the fact that uranium is much more

soluble than thorium, so materials such as corals, mollusks,

calcic soils, bones, carbonates, cave deposits, and fault zones

are enriched in uranium with respect to thorium. This

method can be used to date features as old as Precambrian.

Uranium-lead dating also uses the known original abundance

of isotopes of uranium and the known decay rates of

parents to daughter isotopes. This technique is useful for dating

rocks up to billions of years old. All naturally occurring

uranium contains 238U and 235U in the ratio of 137.7:1. 238U

decays to 206Pb with a half-life of 4,510 Ma through a process

of eight alpha-decay steps and six beta-decay steps. 235U

decays to 207Pb (with a half-life of 713 Ma) by a similar series

of stages that involves seven alpha-decay steps and four betadecay

steps. Uranium-lead dating techniques were initially

applied to uranium minerals such as uraninite and pitchblende,

but these are rare, so very precise methods of measuring isotopic

ratios in other minerals with only trace amounts of uranium

and lead (zircon, sphene) were developed. The amount of

radiogenic lead in all these methods must be distinguished

from naturally occurring lead, and this is calculated using their

abundance with 204Pb, which is stable. After measuring the

ratios of each isotope relative to 204Pb, the ratios of 235U/207Pb

and 238U/206Pb should give the same age for the sample, and a

plot with each system plotted on one axis shows each age. If

the two ages agree, the ages will plot on a curve known as concordia,

which tracks the evolution of these ratios in the Earth

v. time. Ages that plot on concordia are said to be concordant.

However, in many cases the ages determined by the two ratios

are different and they plot off the concordia curve. This occurs

when the system has been heated or otherwise disturbed during

its history, causing a loss of some of the lead daughter isotopes.

Because 207Pb and 206Pb are chemically identical, they

are usually lost in the same proportions.

The thorium-lead dating technique is similar to the uranium-

lead technique and uses the decay from 232Th to 208Pb

(with 6He4), with a half-life of 13,900 years. Minerals used

for this method include sphene, zircon, monazite, apatite, and

other rare U-Th minerals. The ratio of 208Pb/232Th is comparable

with 207Pb/235U. This method is not totally reliable and

is usually employed in conjunction with other methods. In

most cases, the results are discordant, showing a loss of lead

from the system. The Th-Pb method can also be interpreted

by means of isochron diagrams.

Potassium-argon dating is based on the decay of radioactive

potassium into calcium and argon gas at a specific rate

and is accomplished by measuring the relative abundances of

40K and 40Ar in a sample. The technique is potentially useful

for dating samples as old as four billion years. Potassium is

one of the most abundant elements in the Earth’s crust (2.4

percent by mass). One out of every 100 potassium atoms is

radioactive 40K, with 19 protons and 21 neutrons. If one of

the protons is hit by a beta particle, it can be converted into a

neutron. With 18 protons and 22 neutrons, the atom becomes

40Ar, an inert gas. For every 100 40K atoms that decay, 11

become 40Ar.

By comparing the proportion of 40K to 40Ar in a sample,

and knowing the decay rate of 40K, the age of the sample can

be estimated. The technique works well in some cases, but it

is unreliable in samples that have been heated or recrystallized

after formation. Since it is a gas, 40Ar can easily migrate

in and out of potassium-bearing rocks, changing the ratio

between parent and daughter.

Fission track dating is used to determine the thermal age

of a sample, the time lapsed since the last significant heating

event (typically above 215°F, or 102°C). Fission tracks are

paths of radiation damage made by nuclear particles released

by spontaneous fission or radioactive decay of 238U. Fission

tracks are created at a constant rate in uranium bearing minerals,

so by determining the density of tracks present it is possible

to determine the amount of time that has passed since

the tracks began to form in the mineral. Fission track dating

is used for determining the thermal ages of samples between

about 100,000 and 1,000,000 years old, and it is also used

for estimating the uplift and erosional history of areas, by

recording when specific points cooled past 102°C.

Thermoluminescence is a chronometric dating method

based on the fact that some minerals, when heated, give off a

flash of light. The intensity of the light is proportional to the

amount of radiation the sample has been exposed to and the

length of time since the sample was heated. Luminesence is

caused by heating a substance and thus liberating electrons

trapped in its crystal defects. The phenomenon is used as a

dating technique, especially for pottery. The number of

trapped electrons is assumed to be related to the quantity of

ionizing radiation to which the specimen has been exposed

since firing, because the crystal defects are caused by ionizing

radiation, and therefore related to the sample’s age. Thus, by

measuring the amount of light emitted on heating, an estimate

of the age of the sample is obtained.

There are a number of other isotopic systems that are

used for geochronology but they are less commonly used or

less reliable than the methods described above. Geochronologists

also incorporate relative and correlation dating techniques,

such as stratigraphic correlation of dated units, to

explore the wider implications of ages of dated units. A paleomagnetic

timescale has been constructed for the past 180

million years, and in many situations it is now possible to

determine the age of a particular part of a stratigraphic column

or location on the seafloor by knowing which geomagnetic

period the position is located within. Finally, geochronologists

use structural cross-cutting relationships to determine which

parts of a succession are older than or younger than a dated

sample. Eventually the geochronologist is able to put together

a temporal history of a rock terrane by dating several samples

and combining these ages with cross-cutting observations and

correlation with other units.

See also CARBON-14 DATING; DENDROCHRONOLOGY;

PALEOMAGNETISM; RADIOACTIVE DECAY; STRATIGRAPHY.

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