Radioactive clocks
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.
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, MISSISSIPPIAN | Amphibians | |
| 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.