Radioactivity and Geology
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Each of the 92 naturally-occurring elements is defined by the number of
protons in its nucleus. Carbon, element #6, has 6 protons; oxygen, element
#8 contains 8.
While the number of protons is fixed for each element, the number of
neutrons can vary. Only hydrogen exists in a state with no neutrons; all other
elements have neutrons in their nuclei. Neutrons help stabilize atomic nuclei.
The various flavors of each element are termed isotopes of that element.
For the lighter elements, the most common isotope will possess equal
numbers of protons and neutrons, e.g., carbon-12, with 6 of each. Starting
around element 19/20, the number of neutrons gradually outpaces the
number of protons. Uranium-238, contains 92 protons & 146 neutrons.
Certain isotopes of different elements are unstable over time. These
isotopes are termed radioactive.
Unstable atomic nuclei spontaneously eject energetic protons, neutrons,
alpha particles (bare helium nuclei) and gamma rays. The ejected particles
and radiation carry energy away from the nucleus, which is then deposited in
the surrounding matter.
When an atomic nucleus decays and releases 1 or more protons, it
transmutes into a different element. Uranium, for example, will eventually
decay into lead.
While the decay of any specific atom cannot be predicted, the rate at which
a bulk amount of a radioactive isotope decays is a measurable quantity.
Decay rates are measured in terms of the isotopic half-life, defined as the
time it takes for ½ of the atoms of a radioactive element to decay.
For example, half of a block of uranium-238 will decay into lead in 4.5 billion
years (one of the longest half-lives).
When molten rock solidifies, this radioactivity “clock” turns on. By comparing
the relative amounts of parent and daughter nuclei, a formation age can be
assigned to the rock.
Geologists have used this technique to date rocks from all over the Earth and
the Moon. Archeologists have used isotopes of carbon to date organic matter
back over a 100,000 years.