Facts of the Matter
How Old Is Earth?
OVER the years, people have been curious about the age of Earth and have tried ingenious methods to figure it out. Earth's calculated age has increased with the passage of time as more and more precise methods are developed.
James Ussher, Anglican Primate of All Ireland, was a classical and biblical scholar who undertook a careful, critical synthesis of historical biblical, Middle Eastern and Mediterranean documents, Roman history and ancient calendars. In 1650 he published "Annals of the World," in which he concluded that Earth was created on Sunday, Oct. 23, 4004 B.C., a date that is not too distant from the earliest known writing.
Edmund Halley, of cometary fame, suggested a way in 1715 to make a physical estimate of Earth's age. He reasoned that rivers are continually washing small amounts of dissolved salts into the oceans, so over time the oceans should get more salty. However, he made no estimates due to lack of data.
George-Louis Leclerc Buffon, a French naturalist and the most prestigious scientist of his day, published "Epochs of Nature" (1778), in which he estimated the time it would take for a molten Earth to cool to its present state. He arrived at an age of about 75,000 years after conducting experiments with heated iron spheres and scaling their cooling rates to an Earth-size mass.
James Hutton published "Theory of the Earth," the foundation of modern geology, in 1788. Based on rates of sedimentation and the lack of deterioration of Hadrian's wall, among other things, he envisioned "no vestige of a beginning, no prospect of an end" and initiated the concept of "deep time," an age for Earth that was far, far older than human history.
Charles Lyell, Hutton's most famous student, wrote "Principles of Geology," which went through 11 editions between 1830 and 1872. The book had a profound influence on Darwin's evolutionary thoughts by conceptualizing the time needed for slow evolutionary changes to produce Earth's diverse species.
Lyell used several different methods to estimate various geological ages, although he did not attempt to put a number on the age of Earth as a whole.
In 1854 the German physicist Hermann von Helmholtz calculated that it would take 100 million years for the sun to condense down to its current diameter and brightness from the nebula of gas and dust from which it was born. He assumed that the sun was only glowing from the heat of its gravitational contraction because the process of nuclear fusion that powers the sun was not yet known.
In 1862, William Thomson, aka Lord Kelvin, calculated that it would have taken 98 million years to for Earth to cool from molten to present temperature.
Kelvin was unaware of the heat that radioactivity generates within Earth's interior because Becquerel did not discover it until 1896.
In a lecture in 1869, Darwin's greatest advocate, Thomas H. Huxley, attacked Thomson's calculations, suggesting they appeared precise in themselves but were based on faulty assumptions.
In 1899 John Joly, an Irish geologist, used Halley's method to estimate Earth's age to be between 80 million and 90 million years.
Like Halley, Joly was unaware of the various ways that salts are naturally removed from the oceans, part of the regulatory processes that keep salinity constant. The chemistry of ocean salinity is far too complex for this to be a viable way to estimate the age of Earth or the oceans.
No one really conceived of an age in the billions of years until Ernest Rutherford, a pioneer in the study of radioactivity, suggested in 1904 that alpha particles released by radioactive decay could be trapped in a rocky material as helium atoms, and could thus be used to estimate the time since the rock's formation.
Radioactivity works as a clock because certain isotopes are unstable and break down or decay. When they decay each radioactive isotope emits nuclear particles of a particular kind with a characteristic speed. By measuring the types of particles given off and counting their numbers, scientists can determine the amounts of various radioactive atoms in a sample of rock.
Each decaying atom leaves behind a unique "daughter" atom. Many daughter isotopes are also radioactive and can also be useful in the dating process.
Each radioactive isotope decays at a fixed rate that does not depend on the physical or chemical state of the atom. After a century of testing, no physical process has ever been discovered that changes the rate of decay for a given isotope.
The term "half-life" refers to the amount of time required for one-half of a collection of particular "parent" atoms to decay to the "daughter" atoms.
Because each isotope decays into a particular type of daughter atom at a unique rate, it is possible to determine the number of half-lives that have elapsed by comparing the relative amounts of parent and daughter in a sample of rock.
Events such as melting or baking at high temperatures might reset the clock by allowing some or all of the parent or daughter to escape, so geologists are aware that the radiometric dating gives the age of the most recent event in the rock's history.
There is a certain amount of uncertainty in any single measurement, so reliable measurements use multiple parent-daughter pairs. The ages determined from different sets of parent-daughter pairs plot on a straight line on a graph, indicating a correspondence of ages and a level of confidence that the ages are correct.
The oldest rocks found on Earth are nearly 4 billion years old. This does not represent the age of Earth because these rocks are metamorphic, meaning that they were created from pre-existing rocks that must be older still.
The final piece in the greatest age puzzle comes from the ages of meteorites and moon rocks. Our best understanding of the formation of the solar system suggests that all of the planets and other objects formed around the same time from the same kind of materials.
The average age of moon rocks and meteorites is 4.6 billion years, plus or minus 100 million years, as determined by hundreds of measurements using the full range of radioactive isotopes with suitably long half-lives.
Although we can never know exactly when Earth was formed, the best methods available today are remarkably consistent and point to an age of just a little more than 4.5 billion years.
The precision of the geologic time scale is being revised as the behavior of isotopes in Earth's crust is more clearly understood. The geologic column, which shows both relative and absolute time, represents only the present state of knowledge. Further revisions and modifications will fine-tune the numbers as research continues to improve our knowledge of Earth and its long history.
Richard Brill, professor of science at Honolulu Community College, teaches earth and physical science and investigates life and the universe. E-mail questions and comments to firstname.lastname@example.org