| Geologic Time Scale | Plate
Tectonics | Radiometric
Dating | Deep Time | Geological
History of New Zealand |
Radiometric measurements of time
Since the early twentieth century scientists have found
ways to accurately measure geological time. The discovery
by the French physicist, Henri
Becquerel, in 1896 paved the way of measuring absolute
time. Shortly after Becquerel's find, Marie
Curie, a French chemist, isolated another highly radioactive
The realisation that radioactive materials emit rays indicated
a constant change of those materials from one element to another.
The New Zealand physicist Ernest
Rutherford, suggested in 1905 that the exact age of a
rock could be measured by means of radioactivity. For the
first time he was able to exactly measure the age of a uranium
mineral. When Rutherford announced his findings it soon became
clear that Earth is millions of years old.
These scientists and many more after them discovered that
atoms of uranium, radium and several other radioactive materials
are unstable and disintegrate spontaneously and consistently
forming atoms of different elements and emitting radiation,
a form of energy in the process. The original atom is referred
to as the parent and the following decay products are referred
to as the daughter. For example: after the
of a rubidium-87atom ejects an electron, it changes into a
strontium-87 atom, leaving an additional
Major radioactive elements used for radiometric dating.
||10 - 4,600 million
||0.05 to 4,600 million
||Muscovite, Biotite, volcanic rocks.
||10 - 4,600 million
||Muscovite, Biotite, Metamorphic or Igneous
||100- 70,000 years
||Wood, Charcoal, Peat, Bone, Tissue, Carbonates,
Water containing dissolved carbon.
is a very special element. In combination with
it forms a component of all organic compounds and is therefore
fundamental to life. Willard F. Libby of the University of
Chicago predicted the existence of carbon-14 before it was actually detected
and formulated a hypothesis that radiocarbon might exist in
Willard Libby and his colleague Ernest Anderson showed that
collected from sewage works had measurable radiocarbon activity
whereas methane produced from petroleum did not. Perseverance
over three years of secret research to develop the radiocarbon
method came into fruition and in 1960 Libby received the Nobel
Prize for chemistry for turning his vision into an invaluable
The basic principle
Carbon has three naturally occurring
with atoms of the same atomic number but different
weights. They are 12C, 13C
and 14C. C being the symbol for carbon and the
isotopes having atomic weights 12, 13 and 14. The three isotopes
don't occur equally either, 98.89% of carbon is 12C,
1.11% is 13C - and only one 14C atom
exists in nature for every 1,000,000,000,000 12C
atoms in living material. 12C and 13C are both stable
whereas 14C is unstable or radioactive.
The radiocarbon dating method is based on the rate of decay of the radioactive or unstable 14C which is formed in the upper atmosphere through the effect of cosmic ray neutrons upon nitrogen 14.
The reaction is as follows:
(n is a neutron and p is a proton)
After formation the three carbon isotopes combine with oxygen
to form carbon dioxide. The carbon dioxide mixes throughout
the atmosphere, dissolves in the oceans, and via
enters the food chain to become part of all plants and animals.
In principle the uptake rate of 14C by animals
with the atmosphere.
As soon as a plant or animal dies, they stop the metabolic function of carbon uptake and with no replenishment of radioactive carbon, the amount of 14C in their tissues starts to reduce as the 14C
The half-life of 14C
Libby and his colleagues first discovered that this decay
occurs at a constant rate. They found that after 5568 years,
half the 14C in the original sample will have decayed
and after another 5568 years, half of that remaining material
will have decayed, and so on. This half-life (t 1/2) is the
name given to this value which Libby measured at 5568±30 years.
This became known as the Libby half-life.
After 10 half-lives, there is a very small amount of radioactive
carbon present in a sample. At about 50 000 to 60 000 years,
the limit of the technique is reached (beyond this time, other
radiometric techniques must be used for dating). By measuring
the 14C concentration or residual radioactivity
of a sample whose age is not known, it is possible to obtain
the number of decay events per gram of Carbon. By comparing
this with modern levels of activity (1890 wood corrected for
decay to 1950 AD) and using the measured half-life it becomes
possible to calculate a date for the death of the sample.
1950 AD is the marker date used in this technique. As a result
of atomic bomb usage, 14C was added to the atmosphere
artificially. This affects the 14C ages of objects younger
Any material which is composed of carbon may be dated. Herein lies the true advantage of the radiocarbon method.
Here are some of the materials that can be successfully dated using this method:
||Lake muds (gyttia) and sediments
||Marine, estuarine and riverine
||Metal casting and ores
||Avian egg shell
||Textiles and fabrics
||Corals and foraminifera
||Rock art works
||Iron and meterorites
||Resins and glues
||Paper and parchment
Potassium-Argon (K-Ar) dating is the most widely applied technique of radiometric dating. Potassium is a component in many common minerals and can be used to determine the ages of igneous and metamorphic rocks. The Potassium-Argon dating method is the measurement of the accumulation of Argon in a mineral. It is based on the occurrence of a small fixed amount of the radioisotope 40K in natural potassium that decays to the stable Argon isotope 40Ar with a half-life of about 1,300 million years. In contrast to
a method such as Radiocarbon dating, which measures the disappearance of a substance, K-Ar dating measures the accumulation of Argon in a substance from the decomposition of potassium.
Argon, being an inert gas, usually does not leech out of a mineral and is easy to measure in small samples. This method dates the formation or time of crystallisation of the mineral that is being dated; it does not tell when the elements themselves were formed. It is best used with rocks that contain minerals that crystallised over a very short period, possibly at the same time the rock was formed. This method should also be applied only to minerals that remained in a closed system with no loss or gain of the parent or daughter isotope.
Uranium-Lead (U-Pb) dating is the most reliable method for
dating Quaternary sedimentary carbonate and silica, and fossils
particulary outside the range of radiocarbon. Quaternary
geology provides a record of climate change
and geologically recent changes in environment. U-Pb geochronology
is used for determining the age of emplacement of igneous
rocks of all compositions, ranging in age from Tertiary to
Early Archean. U-Pb ages of metamorphic minerals, such as
zircon or monazite are used to date thermal events, including
terrestrial meteoritic impacts. U-Pb ages of zircon in sediments
are used to determine the provenance of the sediments.
The Fission track analysis is based on radiation damage
(tracks) due to the spontaneous fission of 238U.
Fission-tracks are preserved in minerals that contain small
amounts of uranium, such as
Fission-track analysis is useful in determining the thermal
history of a sample or region. By determining the number of
tracks present on a polished surface of a grain and the amount
of uranium present in the grain, it is possible to calculate
how long it took to produce the number of tracks preserved.
As long as the mineral has remained cool, near the earth surface,
the tracks will accumulate. If the rock containing these minerals
is heated, the tracks will begin to disappear. If the rock
is heated high enough, >120°C for apatite, all tracks will
disappear. Zircons will loose their tracks at higher temperatures
of 200°. The tracks will then begin
to accumulate when the rock begins to cool. If a rock cools
quickly as in the case of a volcanic rock or a shallow igneous
intrusion, the fission-track ages will date this initial cooling.
If the mineral formed at depth or was deeply buried after
formation, the fission-track age will reflect this later heating
and cooling. Fission-track analysis has been successfully
applied to many diverse areas of the earth sciences: volcanology,
tectonics, and impact of extraterrestrial bodies.
laboratory at The University of Waikato, Hamilton, New Zealand is a national radiocarbon facility
undertaking both Standard Radiometric Dating and Accelerator Mass
Spectrometry Dating (AMS). On their site go to Radiocarbon
WEB Info to find information
presented jointly with Oxford University on the development of the
Very good information about Potassium-Argon Dating
can be found on the website of the University of California: http://id-archserve.ucsb.edu/Anth3/Courseware/Chronology/09_Potassium_Argon_Dating.html
For useful info on corrections to some misconceptions regarding
radiometric dating try:
Bowman, S. (1990). Radiocarbon Dating. Berkeley and Los Angeles: University
of California Press. http://www.radiocarbondating.com/
Hamblin, W. K. and Christiansen, E. H. (1998).
Earth's dynamic systems. New Jersey: Prentice-Hall Inc.