Bunch-Hellemans, Estimating the age of Earth

Aristotle assumed that Earth always existed, but most peoples throughout history have had different ideas. Eastern philosophy emphasizes cycles of creation and destruction, and the Judeo-Christian belief is that Earth was created once at a specific time. Curious people could use the evidence of the Bible to work out what that time was. The most famous timetable was developed by Archbishop James Ussher in 1650 and further refined by John Lightfoot in 1654. This chronology reported that Earth was created at 9:00 a.m. on October 26, 4004 BCE. The chronology was for the next 200 years or so printed in the margins of the Authorized, or King James, Version of the Bible. Many people in England and other English-speaking countries thought it had always been a part of the Bible. Charles Darwin, for one, was surprised to learn that the Biblical chronology was a recent work of humans.

Mikhail Lomonosov in the 18th century was among the first to see things differently. He thought from geological evidence that Earth must be hundreds of thousands of years old. Perhaps the first attempt to determine the age of Earth on scientific grounds was made by comte de Buffon, who assumed that Earth was originally hot and had slowly cooled. By measuring the rate at which iron balls cool, Buffon decided in 1779 that Earth is about 75,000 years old.

James Hutton in 1785 claimed, as Aristotle indicated, that Earth has “no vestige of a beginning.” Hutton’s followers, Darwin and Charles Lyell, did not try to determine Earth’s beginning, but each made informed guesses as to how long ago certain fossils formed. Darwin dated the end of the Cretaceous period at 300,000,000 years ago (a bit too far back; the currently accepted time is 65,000,000 years ago). Lyell erred on the other side in 1867, placing the start of the Ordovician period at 240,000,000 years ago; we now think it was about 500,000,000 years ago.

William Thomson (Lord Kelvin) disagreed with both these dates. Using an improved version of Buffon’s idea about Earth’s cooling, he proposed in 1862 that Earth’s crust formed between 20,000,000 and 98,000,000 years ago. He also calculated how long the Sun could have existed, assuming that it got its energy as a result of gravitational contraction. This figure was 100,000,000 years. Geologists did not accept his work, although they had a hard time refuting it.

In 1896, however, radioactivity was discovered. Radioactive elements produce heat, making meaningless Thomson’s calculations based on cooling that ignored radioactive heating. Radioactivity also provided the solution to finding out how old rocks really are. In 1904 Bertram Boltwood and others proved that one element changes into another during radioactive decay.

By 1907 Boltwood realized that he could calculate the age of a rock by measuring the ratio of uranium, a fairly common element, to lead, its final decay product. He applied his method to a mineral from Glastonbury, Connecticut, and got an age of 410,000,000 years. (More recent calculations, based on an improved knowledge of isotopes, give the Glastonbury mineral as only 265,000,000 years old.) Other radioactive elements can be used to date rocks that contain no uranium. Using various adaptations of Boltwood’s method, scientists have dated rocks from all periods of Earth’s past. Granite from Canada is the oldest known rock, at 3,960,000,000 years old. Zircon crystals, which are especially tough, are the oldest known mineral at about 4,300,000,000 years old. The current best estimate for the age of Earth, based on evidence from meteorites, is 4,600,000,000 years.