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adiometric dating Radiometric dating uses the measurement
of radioactive decay of atoms to determine the ages of
volcanic rocks and some organic materials. Radioactive elements
are those with unstable nuclei: large atoms, or isotopes
of smaller atoms. Atoms can decay through the loss
of an alpha particle (consisting of two protons and two neutrons),
loss of a beta particle (electron), or the capture of a
beta particle (which turns a proton into a neutron).
Each of the radioactive elements decays at its own constant
rate (see table). After a certain amount of time, half
of the radioactive atoms will decay. This time is called the
half-life of the element. Beginning with 100 percent of a
sample of radioactive atoms, 50 percent will remain after
one half-life, 25 percent after two half-lives, 12.5 percent
after three half-lives, and so on. Each of the radioactive isotopes
is most useful for a different span of time. If it decays
very slowly, it cannot be used to determine the age of geologically
young material, as an immeasurably small amount
of decay will have occurred. If it decays rapidly, it cannot
be used to determine the age of geologically old material,
as an unmeasurably small amount of the original material
remains.
Radioactive Isotopes and Half-lives
R
The ratio of product to original isotope indicates the
time since the formation of the material that contains them.
The assumption must be made that the element that is the
product of decay was not originally present. This assumption
is frequently cited by creationists as an invalidation of radiometric
dating (see creationism), but geologists make this
assumption only when it is valid. For example, potassiumargon
dating may be performed on intact volcanic material
because the initial amount of argon (a gas) can be assumed
to be zero. The two uranium-lead dating techniques are not
performed on entire samples of volcanic rock, in which some
lead might initially have been present, but on zircon crystals.
Zircon crystals contain zirconium; uranium atoms can substitute
for zirconium, but lead atoms cannot. Therefore, when
zircon crystals form, they contain uranium atoms and no lead
atoms. Any lead atoms in a zircon crystal can be legitimately
assumed to have been produced by radioactive decay of uranium.
In fact, because zircon crystals contain both isotopes
of uranium, radiometric dating can be performed twice on
the same crystal to assure that the dates are consistent. Previously,
geologists had to selectively cull zircon crystals that
had appeared to be altered by weathering, since weathering
Radioactive isotope Half-life (years) Effective dating range (years) Product
Rubidium 87 ( 87Rb) 48.6 billion 0.01–4.6 billion Strontium 87 ( 87Sr) Thorium 232 ( 232Th) 14.1 billion 0.01–4.6 billion Lead 208 ( 208Pb) Uranium 238 ( 238U) 4.5 billion 0.01–4.6 billion Lead 206 ( 206Pb) Potassium 40 ( 40K) 1.3 billion 0.0001–4.6 billion Argon 40 ( 40Ar) Uranium 235 ( 235U) 71.3 million 0.01–4.6 billion Lead 207 ( 207Pb) Uranium 234 ( 234U) 245,000 1,000–800,000 Thorium 230 ( 230Th) Carbon 14 ( 14C) 5,730 100–100,000 Nitrogen 14 ( 14N)