The study of the sequence of occurrence of fossils in rocks, biostratigraphy,
reveals the relative time order in which organisms lived. Although this relative
time scale indicates that one layer of rock is younger or older than another,
it does not pinpoint the age of a fossil or rock in years. The discovery of
radioactivity late in the 19th century enabled scientists to develop techniques
for accurately determining the ages of fossils, rocks, and events in Earth's
history in the distant past. For example, through isotopic dating we've learned
that Cambrian fossils are about 540-500 million years old, that the oldest known
fossils are found in rocks that are about 3.8 billion years old, and that planet
Earth is about 4.6 billion years old.
Determining the age of a rock involves using minerals that contain naturally-occurring
radioactive elements and measuring the amount of change or decay in those elements
to calculate approximately how many years ago the rock formed. Radioactive elements
are unstable. They emit particles and energy at a relatively constant rate,
transforming themselves through the process of radioactive decay into other
elements that are stable - not radioactive. Radioactive elements can serve as
natural clocks, because the rate of emission or decay is measurable and because
it is not affected by external factors.
About 90 chemical elements occur naturally in the Earth. By definition an element
is a substance that cannot be broken into a simpler form by ordinary chemical
means. The basic structural units of elements are minute atoms. They are made
up of the even tinier subatomic particles called protons, neutrons, and electrons.
To help in the identification and classification of elements, scientists have
assigned an atomic number to each kind of atom. The atomic number for each element
is the number of protons in an atom. An atom of potassium (K), for example,
has 19 protons in its nucleus so the atomic number for potassium is 19.
all atoms of a given element contain the same number of protons, they do not
contain the same number of neutrons. Each kind of atom has also been assigned
a mass number. That number, which is equal to the number of protons and neutrons
in the nucleus, identifies the various forms or isotopes of an element. The
isotopes of a given element have similar or very closely related chemical properties
but their atomic mass differs.
Potassium (atomic number 19) has several isotopes. Its radioactive isotope
potassium-40 has 19 protons and 21 neutrons in the nucleus (19 protons + 21
neutrons = mass number 40). Atoms of its stable isotopes potassium-39 and potassium-41
contain 19 protons plus 20 and 22 neutrons respectively.
Radioactive isotopes are useful in dating geological materials, because they
convert or decay at a constant, and therefore measurable, rate. An unstable
radioactive isotope, which is the 'parent' of one chemical element, naturally
decays to form a stable nonradioactive isotope, or 'daughter,' of another element
by emitting particles such as protons from the nucleus. The decay from parent
to daughter happens at a constant rate called the half-life. The half-life of
a radioactive isotope is the length of time it takes for exactly one-half of
the parent atoms to decay to daughter atoms. No naturally occurring physical
or chemical conditions on Earth can appreciably change the decay rate of radioactive
isotopes. Precise laboratory measurements of the number of remaining atoms of
the parent and the number of atoms of the daughter result in a ratio that is
used to compute the age of a fossil or rock in years.
Age determinations using radioactive isotopes have reached the point where
they are subject to very small errors of measurement, now usually less than
1%. For example, minerals from a volcanic ash bed in southern Saskatchewan,
Canada, have been dated by three independent isotopic methods (Baadsgaard, et
al., 1993). The potassium/argon method gave an age of 72.5 plus or minus 0.2
million years ago (mya), a possible error of 0.27%; the uranium/lead method
gave an age of 72.4 plus or minus 0.4 mya, a possible error of 0.55%; and the
rubidium/strontium method gave an age of 72.54 plus or minus 0.18 mya, a possible
error of 0.25%. The possible errors in these measurements are well under 1%.
For comparison, 1% of an hour is 36 seconds. For most scientific investigations
an error of less than 1% is insignificant.
As we have learned more, and as our instrumentation has improved, geoscientists
have reevaluated the ages obtained from the rocks. These refinements have resulted
in an unmistakable trend of smaller and smaller revisions of the radiometric
time scale. This trend will continue as we collect and analyze more samples.
Isotopic dating techniques are used to measure the time when a particular mineral
within a rock was formed. To allow assignment of numeric ages to the biologically
based components of the geologic time scale, such as Cambrian...Permian...Cretaceous...
Quaternary, a mineral that can be dated radiometrically must be found together
with rocks that can be assigned relative ages because of the contained fossils.
A classic, real-life example of using K-40/Ar-40 to date Upper Cretaceous rocks
and fossils is described in Gill and Cobban (1973).