The measurement of geologic time in terms of years was not possible
until the discovery of natural radioactivity. It was found that certain
atoms of a few elements spontaneously throw off particles from their
nuclei and break down to form atoms of other elements. These decay
processes take place at constant rates, unaffected by heat, pressure, or
chemical conditions. If we know the rate at which a particular
radioactive element decays, the length of time that has passed since a
mineral crystal containing the elements formed can be calculated by
comparing the amount of the radioactive element remaining in the crystal
with the amount of disintegration products present.
[Illustration: Figure 18. _Major subdivisions of the last 600
million years of geologic time and some of the dominant forms of
life._]
MILLIONS OF
YEARS AGO
0 Man
0-60 CENOZOIC QUATERNARY and TERTIARY Mammals
60-130 MESOZOIC CRETACEOUS
130-180 JURASSIC Dinosaurs
180-220 TRIASSIC
220-260 PALEOZOIC PERMIAN Reptiles
260-350 PENNSYLVANIAN, Amphibians
MISSISSIPPIAN
350-400 DEVONIAN Fishes
400-440 SILURIAN Sea scorpions
440-500 ORDOVICIAN Nautiloids
500-530 CAMBRIAN Trilobites
530- PRECAMBRIAN Soft-bodied
creatures
Three principal radioactive clocks now in use are based on the decay of
uranium to lead, rubidium to strontium, and potassium to argon. They are
effective in dating minerals millions or billions of years old. Another
clock, based on the decay of one type of carbon (_Carbon-14_) to
nitrogen, dates organic material, but only if it is less than about
40,000 years old.
The uranium, rubidium, and potassium clocks are especially useful in
dating igneous rocks. By determining the absolute ages of igneous rocks
whose stratigraphic relations to fossil-bearing strata are known, it is
possible to estimate the number of years represented by the various
subdivisions of the stratigraphic time scale.