Chapter I
Kelvin's Clock
In 1862 William Thomson, later Lord Kelvin, calculated how long the Earth would take to cool from molten rock to its present temperature. His answer, revised downwards over the years to a few tens of millions of years, was far too short for the geologists and for Darwin, whose natural selection needed hundreds of millions. Kelvin's physics seemed unanswerable. He had assumed, though, that the Earth has no internal source of heat, and that heat leaves its interior only by conduction through solid rock.
Chapter II
The Rocks Keep Time
Radioactivity supplied both the missing heat and a way to measure time. In 1904, lecturing at the Royal Institution with Kelvin in the audience, Ernest Rutherford pointed out that Kelvin's limit held only if no new source of heat were discovered. By his own account, the old man beamed at him. Rutherford also suggested that the helium building up in uranium minerals from alpha decay could date them. His first estimate was about 40 million years for one mineral, and helium ages soon ran to hundreds of millions of years.
Helium leaks out of rocks, so these ages were too low. In 1907 Bertram Boltwood at Yale found that lead is the stable end of uranium's decay chain, and dated minerals from the lead they contained, at up to 2.2 billion years. Arthur Holmes, a student in London, used the method in 1911 to put the first numbers on the geological periods, and argued in 1913 for an Earth of at least 1.6 billion years. Geologists who had fought Kelvin for more time now found they had more than they wanted.
Chapter III
Four and a Half Billion Years
Measuring the age of the Earth itself was harder. The Earth's surface is constantly recycled, so no rock survives from its formation. The answer lay in meteorites, which formed with the planets and have been unchanged since. At Chicago and then at Caltech, Clair Patterson spent years building a laboratory clean enough to measure tiny amounts of lead without contamination. In 1953 he measured the lead isotopes in the Canyon Diablo meteorite, and in 1956 published an age for the Earth and meteorites of 4.55 billion years. Along the way he discovered that the lead contamination he fought in his laboratory was everywhere, and spent the rest of his career campaigning against leaded petrol.
Rocks date the Earth. Willard Libby found a clock for living things. Cosmic rays make carbon-14 high in the atmosphere, and plants and animals absorb it until they die. Afterwards it decays with a half-life of about 5,700 years. In 1949 Libby dated wood from Egyptian tombs of known age and got the right answers. Archaeology had its first absolute calendar. Hans Suess, a chemist who had helped build the shell model of the nucleus, showed in 1955 that burning fossil fuels was diluting the carbon-14 in the air, and later used ancient bristlecone pines to calibrate the clock against tree rings.
Chapter IV
A Closer Look: Reading the Clocks
Radiocarbon. If a sample holds a fraction of the carbon-14 it had when it died, its age is
| Carbon-14 remaining | Age |
|---|---|
| 50% | 5,730 years |
| 25% | 11,460 years |
| 10% | 19,000 years |
| 1% | 38,000 years |
| 0.1% | 57,000 years |
Living carbon holds only about one carbon-14 atom per trillion carbon atoms, so after about nine half-lives there is too little left to measure reliably, and radiocarbon dating stops at about 50,000 years. Laboratories still report "conventional" radiocarbon ages using Libby's original half-life of 5,568 years, about 3% short of the modern value, and let calibration absorb the difference.
Uranium and lead. Uranium-238 decays to lead-206 with a half-life of 4.47 billion years, and uranium-235 decays to lead-207 with a half-life of 0.70 billion years. Today there is one atom of uranium-235 for every 137.88 of uranium-238. The ratio of the lead each has made in time depends only on :
where divided by the half-life. No measurement of the amount of uranium is needed, only the ratio of two lead isotopes.
| Age (billion years) | Ratio of lead-207 to lead-206 made |
|---|---|
| 4.00 | 0.425 |
| 4.50 | 0.597 |
| 4.55 | 0.618 |
| 4.60 | 0.639 |
The ratio climbs quickly with age because uranium-235 was once far more plentiful. At the Earth's birth there was one atom of uranium-235 for every three of uranium-238. A shift of 50 million years changes the ratio by more than 3%, which is why Patterson could pin down 4.55 billion years so precisely.
Chapter V
Deep Time
Radiometric dating turned the history of the Earth and of life into a calendar. It gives the ages of the oldest minerals, 4.4 billion years, of the first fossils and the great extinctions, and of human ancestors in the Rift Valley of Africa. Radiocarbon, calibrated by tree rings, corals and cave deposits back 55,000 years, dates the spread of modern humans and the rise of farming, and the bomb pulse now dates cells in the human body. The Moon, formed in a giant impact, is still not dated to everyone's satisfaction.