(704 million years); uranium 238–lead 206 (4.47 bil-
lion years); carbon 14-nitrogen 14 (5,730 years).
See alsoELEMENT.
radiocarbon dating By using radioactive isotopes,
it is possible to qualitatively measure organic material
over a period of time. Radioactive decay transforms
an atom of the parent isotope to an atom of a differ-
ent element (the daughter isotope) and ultimately
leads to the formation of stable nuclei from the unsta-
ble nuclei. Archeologists and other scientists use
radioactive carbon to date organic remains. The
radioactive isotope of carbon, known as carbon-14, is
produced in the upper atmosphere and absorbed in a
known proportion by all plants and animals. Once the
organism dies, the carbon-14 in it begins to decay at a
steady, known rate. Measuring the amount of radio-
carbon remaining in an organic sample provides an
estimate of its age. The approximate half-life of car-
bon 14 is 5,730 plus or minus 30 years and is good
for dating up to about 23,000 years. The carbon-14
method was developed by the American physicist
Willard F. Libby in 1947.
radiometric dating The use of radioactive isotopes
and their half-lives to give absolute dates to rock for-
mations, artifacts, and fossils. Radioactive elements
tend to concentrate in human-made artifacts, igneous
rocks, the continental crust, and so the technique is not
very useful for sedimentary rocks, although in some
cases, when certain elements are found, it is possible to
date them using this technique. Other radiometric dat-
ing techniques used are:
Electron Dating Spin Resonance
Electrons become trapped in the crystal lattice of min-
erals from adjacent radioactive material and alter the
magnetic field of the mineral at a known rate. This
nondestructive technique is used for dating bone and
shell, since exposure to magnetic fields does not
destroy the material (e.g., carbonates [calcium] in lime-
stone, coral, egg shells, and teeth).
Fission Track Dating
This technique is used for dating glassy material like
obsidian or any artifacts that contain uranium-bear-
ing material, such as natural or human-made glass,
ceramics, or stones that were used in hearths for food
preparation. Narrow fission tracks from the release
of high-energy charged alpha particles burn into the
material as a result of the decay of uranium 238 to
lead 206 (half-life of 4.47 billion years) or induced by
the irradiation of uranium 235 to lead 207 (half-life
of 704 million years). The number of tracks is pro-
portional to the time since the material cooled from
its original molten condition, i.e., fission tracks are
created at a constant rate throughout time, so it is
possible to determine the amount of time that has
passed since the track accumulation began from the
number of tracks present. This technique is good for
dates from 20 million to 1 billion years ago. U-238
fission track techniques are from spontaneous fission,
and induced-fission tracking from U-235 is a tech-
nique involving controlled irradiation of the artifact
with thermal neutrons of U-235. Both techniques give
a thermal age for the material in question. The spon-
taneous fission of uranium-238 was first discovered
by the Russian scientists K. A. Petrzhak and G. N.
Flerov in 1940.Potassium-Argon Dating
This method has been used to date rocks as old as
4 billion years and is a popular dating technique for
archeological material. Potassium-40, with a half-life
of 1.3 billion years in volcanic rock, decays into
argon-40 and calcium-40 at a known rate. Dates are
determined by measuring the amount of argon-40
in a sample. Argon-40 and argon-39 ratios can
also be used for dating in the same way. Potassium-
argon dating is accurate from 4.3 billion years (the
age of the Earth) to about 100,000 years before the
present.Radiocarbon Dating SeeRADIOCARBON DATING.Thermoluminescence Dating
A technique used for dating ceramics, bricks, sediment
layers, burnt flint, lava, and even cave structures like
stalactites and stalagmites. It is based on the fact that
some materials, when heated, give off aflash of light.
The intensity of the light is used to date the specimen
and is proportional to the quantity of radiation it has
been exposed to and the time span since it was
heated. Similar to the electron spin resonance (ESR)284 radiocarbon dating