Some Early Attempts to Date the Earth
Chronological Analysis of the Scriptures- Literal interpretation of the scriptures led
some people to conclude that the Earth was created approximately 6,000
years old. In fact, Archbishop Usher of Ireland calculated that the Earth
was created at 9 AM on October 26, 4004 BC! The presence of marine fossils in
rock layers, sometimes high in the mountains, seemed to provide “evidence” of a
It was believed that prior to the
Great Flood, Earth’s surface was flat and its climate was mild. Mountains,
erosion, and variations in climate were considered to be punishment for the sins
committed by humanity.
Uniformitarianism- The ideas of
James Hutton, Charles Lyell, and Charles Darwin required
significantly longer amounts of time (millions of years) for uniformity of
geological processes and for organic evolution.
Buffon's Iron Sphere Experiments- On the basis
of iron sphere cooling experiments, Frenchman Georges de Buffon
estimated that the Earth would have needed 75,000 years to cool to its present
In 1749, Frenchman G.L. de Buffon calculated a
cooling time for the Earth of 75,000 years based upon experiments with
heated steel spheres.
approach is admirable, but Buffon's assumptions are flawed. First, the
Earth is not made of steel. Silicate minerals have lower heat
conductivity than steels and are better insulators leading to slower cooling
rates. Second, the calculations did not incorporate the heating effects of
now know that Buffon's estimate is very low, but his work was important
because it challenged the young Earth concept.
Strata Thickness- In the late 1800’s, a British geologist estimated that
75 million years has lapsed since the beginning of the Cambrian.
This estimate was based upon the maximum known thickness of strata (from Cambrian to
present) divided by the average rate of sedimentation in modern environments.
Sea-water composition- In 1899, Irish scientist J. Joly used the salinity of
ocean water to determine the age of the earth. He calculated the modern rate of salt
delivery to the oceans, and suggested that the present salinity of ocean water would take
at least 100 million years to develop.
Several of these attempts are
flawed in that it is “sketchy” to
extrapolate the modern rates of geologic processes far back into Earth's past because geologic
rates can vary over time.
Lord Kelvin’s Attempt to Date the Earth
In the 1860's, English physicist Lord Kelvin disagreed with Charles Lyell’s
proposition that the earth behaves in a uniform, unchanging manner. Lyell's extreme
form of uniformitarianism would have required a perfect balance between heat
production and heat loss. Kelvin argued that this was physically impossible
(the concept is akin to a perpetual motion machine).
Kelvin knew that the Earth gets hotter with increasing depth
(the geothermal gradient), and took this observation as evidence that the Earth is cooling off. He believed
the Earth started off as a molten mass and subsequently transformed to a hot
solid mass during
On the basis of cooling rate calculations,
Kelvin estimated the Earth's age at 20-30 million
Kelvin’s age of the earth was displeasing to many geologists, who
believed the earth was much older.
Today we know that Kelvin's method was flawed in two aspects:
In 1899 American geologist T.C. Chamberlain challenged Kelvin’s
assumption that the earth started as a molten body. Instead, Chamberlain
proposed a model of “cold accretion” for the Earth. That is, the earth accreted from
small, cold chunks of material and then heated up at a later time.
2) Kelvin did not recognize the
important role of radioactive decay as an internal heat source.
In 1896, French physicist Henri Becquerel discovered
radioactivity: the spontaneous emission of particles and energy from unstable
nuclei of elements.
Isotope: A version of an atom that differs from other atoms of the
same element only in the number of neutrons. Different isotopes of
an element have similar chemical properties (undergo similar chemical reactions)
but have different physical properties (such as evaporation rates).
NOTATION FOR EXPRESSING ISOTOPES
Stable Isotope: An isotope that persists forever because it has a
“stable” ratio of protons to neutrons. For example, carbon-12 is a stable
Radioactive (or unstable) Isotope: An isotope that decays into
another element because it has an “unstable” ratio of protons to neutrons.
For example, carbon-14 is a radioactive isotope.
During radioactive decay, the radioactive parent isotope
changes to a
stable daughter isotope giving off heat in the process. There are 3 types of radioactive emissions:
Alpha ray: Equivalent to two protons and two
(essentially a helium nucleus, 42He).
Beta ray: A free electron is
released when a neutron converts to a proton.
Gamma ray: Consists of a photon (a packet of energy).
Some radioactive parent isotopes decay directly to a daughter
isotope. However, some radioactive atoms decay to the daughter atom through a series of intermediate steps
(called a decay series). The 238U decay series is a good
Radioactive decay is a spontaneous and statistical process. It is
impossible to predict when a particular radioactive parent atom will
decay to a daughter atom.
However, we can predict what fraction of the parent
atoms will decay over a certain amount of time because each radioactive isotope
has a constant rate of decay (unaffected by temperature, pressure, or chemical
The half-life is the amount of time required for one half of the
parent to decay to daughter.
Initially, there are many radioactive parent atoms so there are
more radioactive emissions. As decay proceeds and there are fewer parent atoms
and fewer emissions. By the 1st
half life, 50% of the parent atoms will have decayed to daughter. By the 2nd
half life, another 50% of the remaining parent will have decayed (leaving
25% parent and 75% daughter).
Radioactive Dating of Minerals
Absolute age dating is based upon the
decay of radioactive (unstable) isotopes.
At the blocking temperature, the radioactive parent isotope starts
decaying to the stable daughter isotope.
Since the decay rate is constant over
time, the parent:daughter ratio can be used to calculate the age
of the mineral or rock.
Dating basically depends upon 3 measurements:
1) the amount of unstable
parent isotope in the mineral
2) the amount of stable
daughter isotope in the mineral
3) the decay constant (l) of the particular radioactive parent isotope.
parent:daughter ratio is the key to age-dating. In young rocks, the
ratio will be _____. In old rocks, the ratio will be _____.
The radiometric age of a rock
is given by the equation:
t = ln(D/P + 1)
where D is the number of
daughter atoms, P is the number of parent atoms remaining, and
l is the decay constant.
The relationship between half-life and decay constant is:
t1/2 = .693/l.
Therefore the radiometric age equation can be rewritten as:
t = ln(D/P +1)
A final equation can be written in terms of N0,
the total amount of parent initially present in the mineral.
= P + D, we can write:
P = N0
Isotope Systems Commonly Used for Age-Dating
48.8 billion years
4.5 billion years
1.25 billion years
mineral is analyzed and found to contain 440,000 atoms of 235U
and 760,000 atoms of 207Pb.
Determine the age of the mineral.
Which Isotopic System Should We
It depends! To obtain
accurate dates, there must be enough measurable quantities of both parent
and daughter atoms in the mineral.
For example, isotopes with very long half lives are no good for
dating rocks younger than about 100 million years. This is because, in just
100,000,000 years of time, not enough parent will have decayed for daughter concentrations to be
In summary, we use isotope
systems with long half-lives to date old rocks, and isotopes with short
half-lives to date young rocks.
The Potassium-Argon system is susceptible to resetting by metamorphism.
The reheating cause leakage of gaseous 40Ar from rocks, essentially
resetting the isotopic clock. Therefore, the K-Ar system is useful for dating the age of
To obtain accurate dates, any pre-existing (background) amount of daughter
isotope must be subtracted out. Otherwise, the
rocks age will appear “older” than it really is.
When isotopic ages obtained from different chemical systems are in
agreement, they are considered “concordant”.
If they do not agree, isotopic ages are considered to be
discordant, and a reasonable explanation is needed. Metamorphism is a
likely reason that two isotopic systems may provide
What kind of rocks can be dated?
Igneous rocks- Isotopic age dating of igneous rocks can yield the
age of crystallization of magmas and lavas.
Metamorphic rocks- Since isotopic systems are often “reset” by increases in temperature,
dating of metamorphic rocks allows us to measure
the timing of metamorphic events.
Sedimentary rocks- Sedimentary rocks are generally not datable
using isotopic methods because the grains in sedimentary rocks may come from many
different rocks of different ages. Isotopic age dating would not give the
age of the sediment deposition or lithification, but rather the age of the
Geologists look at cross cutting
relationships first to constrain an age, and then use isotopic ages to get
the absolute age.
Absolute age dating is very time consuming and expensive
so you want to choose your rocks carefully!
What Else Has Radiometric Dating Told Us?
It has established the 4.6 billion year age of the
Earth, moon, meteorites,
and the Solar System.
It allows us to assign numerical ages to the geologic time column,
and provides a “double-check” on the relative ages determined by fossil
It allows correlation of Precambrian rocks, most of
which do not contain any fossils.
It provides a way to measure rates of geologic processes, such as
cooling rates and erosion rates.
Accuracy of Isotopic Dates
The accuracy of isotopic dating is affected by:
1) The limits of our abilities to determine decay constants
(usually this introduces only a small degree of uncertainty).
2) Uncertainties of the instrumentation (mass spectrometer).
3) The “degree of weathering” of the sample. Fresher is
Overall, a good uncertainty range is ± 0.2 to 2.0%.
On ages of 100,000,000 years, this may reflect in an uncertainty of
± 5 million years.
Special Type of Age Dating
Carbon-14 is produced in the upper atmosphere
when high energy cosmic
rays interact with oxygen and nitrogen nuclei causing nuclear reactions:
147N + neutron
→ 146C + proton
The atmospheric 14C is incorporated into carbon dioxide
molecules (CO2). Organisms acquire 14C from the air and
water (along with 13C and 12C), and they acquire the
environmental ratios of these isotopes. However, when organisms die, they stop
acquiring any carbon and the 14C starts to decay back to 14N
via beta decay. The 14C:14N
ratio decreases over time, and this ratio can be used to calculate a material's
All organic matter
(bones, shells, wood, charcoal, cloth, and limestone)
contains 14C and can be dated with this technique.
Carbon-14 has a relatively short half life of 5,730 years. It is good for dating
young rocks and artifacts. Beyond 60,000 - 80,000 years, there is too
little Carbon-14 left in the sample and this technique cannot be used.
A Final Type
of Age Dating: The Fission-Track Method
Fission-track dating is a more recent application of the decay of
radioisotopes, but this technique does not use the ratio of parent to daughter isotope to
obtain an age.
Most 238U undergoes alpha decay. However, a very small
proportion of 238U nuclei undergo fission and the nucleus splits to
form two smaller but very energetic nuclei that move away from each other. When
this happens in a mineral, the two departing nuclei leave behind a trail of
destruction in the crystal lattice. The trail is called a fission track.
The density of fission tracks in a mineral increase with age
and can be used to calculate the mineral's age.
Fission track dating is ideal for samples from “recent” times back
to 100,000,000 years. Beyond 100,000,000 years, the density of the tracks
becomes so great (saturated) that they cannot be counted reliably.
Fission tracks can “anneal” or heal with reheating, and so
this method is
affected by metamorphism.
The oldest dated rocks on Earth (Canada, China,
Wyoming) are dated at 3.96 billion years. Continental crust had at least partially formed by this
The oldest known mineral grains are detrital (not in
situ) zircon grains in Australia that were dated at 4.3-4.4 billion years.
The moon rocks have been dated at 3.5 - 4.2 billion years.
Meteorites are dated at 4.3 - 4.6 billion years.