Before we go into these discoveries, let us look at some theoretical foundations.
| Table I BASIC MEASUREMENT METHODS | |||
|---|---|---|---|
| Method | Material | Time Dated | Useful Time Span (years) |
| Carbon-14 | Wood, peat, charcoal | When plant died | 1000-50,000 |
| Bone, shell | Slightly before animal died | 2000-35,000 | |
| Potassium-argon | Mica, some whole rocks | When rock last cooled to about 300°C | 100,000 and up |
| Hornblende Sanidine | When rock last cooled to about 500°C | 10,000,000 and up | |
| Rubidium-strontium | Mica | When rock last cooled to about 300°C | 5,000,000 and up |
| Potash feldspar | When rock last cooled to about 500°C | 50,000,000 and up | |
| Whole rock | Time of separation of the rock as a closed unit | 100,000,000 and up | |
| Uranium-lead | Zircon | When crystals formed | 200,000,000 and up |
| Uranium-238 fission | Many | When rock last cooled | 100-1,000,000,000 (Depending on material) |
Time scale of the Earth, drawn to scale. See also the Holmes time scale on [page 46].
| Event Geologic Time | Period | Era | |
|---|---|---|---|
| Man appears[4] (2 million years ago) | |||
| Quaternary | CENOZOIC | ||
| Tertiary | |||
| Extinction of the dinosaurs (70 million years ago) | |||
| Cretaceous | MESOZOIC | ||
| Mammals appear (130 million years ago) | Jurassic | ||
| Triassic | |||
| Oldest known reptiles (300 million years ago) | Permian | PALEOZOIC | |
| Pennsylvanian | |||
| Mississippian | |||
| Devonian | |||
| Silurian | |||
| Ordovician | |||
| Cambrian | |||
| First abundant life in the sea (animals without backbones) (550 million years ago) | |||
| Algae and other microorganisms (1,900 million years ago) | Proterozoic | PRECAMBRIAN | |
| Archean | |||
| Oldest rocks in North America (2,800 million years ago) | |||
| First hint of life (bacteria?) (3,100 million years ago) | |||
| Oldest rocks (3,300 million years old) | |||
| Formation of Earth’s core (4,500 million years ago) |
THEORY OF NUCLEAR AGE DETERMINATION
We can think of the nucleus of an atom as a sort of drop—a bunch of [NEUTRONS] and [PROTONS] held together by very strong short-range forces. These elementary particles within a nucleus are not arranged in any fixed or rigid array, but are free to move about within the grip of these forces. These motions may be quite violent, but for most [NUCLIDES] found in nature, the nuclear forces are powerful enough to keep everything confined; thus the nuclei of these atoms hold together, and are said to be stable. If any one nucleus of a given [ISOTOPE] is stable, then all others are also stable, because what is true for one atom of a given kind is true for all others of the same kind.[5]
Some nuclides, both man-made and natural, are unstable, however. Their nuclei are in such violent turmoil that the nuclear forces cannot always hold them together, and various bits and pieces fly off. If we were to try to predict when one particular unstable nucleus would thus disintegrate, however, we could not succeed, because the instant any specific decay (or disintegration) event will occur is a matter of chance. Only if a large number of unstable nuclei of one kind are collected together can we say with certainty that, out of that number, a certain proportion will decay in a given time. It turns out that this proportion is the same regardless of any external conditions.
This property of nuclei to decay by themselves is called radioactivity. Radioactive nuclei decay at constant rates regardless of temperature, pressure, chemical combination, or physical state. The process goes on no matter what happens to the atom. In other words, the activity inside the nucleus is in no way affected by what happens to the [ELECTRONS] circling around it. (Only in very special cases can outside disturbances affect the radioactivity of a nucleus and then only slightly. For all practical purposes, rates of radioactive decay are constant.)
Most radioactive nuclides have rapid rates of decay (and lose their radioactivity in a few days, or a few years, at most); most of these are known today only because they are produced artificially. Some of them may have been present at the time the solar system was formed, but they have since decayed to such insignificant fractions of their original amounts that they can no longer be detected. Only a few radioactive nuclides decay slowly enough to have been preserved to this day, and so are present in nature. They are listed in Table II.