Age of the Earth

Scientists determined the Earth's age using a technique called radiometric dating. Radiometric dating is based upon the fact that some forms of chemical elements are radioactive, which was discovered in 1896 by Henri Becquerel and his assistants, Marie and Pierre Curie. The discovery gave scientists a tool for dating rocks that contain radioactive elements.

Many elements have naturally occurring isotopes, varieties of the element that have different numbers of neutrons in the nucleus. (The nucleus of an atom is made up of protons and neutrons.) For example, the element carbon, which always has six protons in its nucleus, has three isotopes: one with six neutrons in the nucleus, one with seven, and one with eight. Some isotopes are stable, but some are unstable or radioactive.

Over time, radioactive isotopes change into stable isotopes by a process known as radioactive decay. Some radioactive parent isotopes decay almost instantaneously into their stable daughter isotopes; others take billions of years. The rates of decay of various radioactive isotopes have been accurately measured in the laboratory and have been shown to be constant, even in extreme temperatures and pressures. These rates are usually expressed as the isotope's half-life--that is, the time it takes for one-half of the parent isotopes to decay. After one half-life, 50 percent of the original parents remains; after two, only 25 percent remains, and so on.

diagram showing proportion of parent atoms remaining over time

Decay curve of a radioactive element with a half-life equal to one time unit. Note that at time 0, the time of the mineral's formation, the crystal contains only parent atoms. At time 1, 50% of the parent atoms remain; at time 2, only 25% remain, and so on.

Radiometric dating works best on igneous rocks, which are formed from the cooling of molten rock, or magma. As magma cools, radioactive parent isotopes are separated from previously formed daughter isotopes by the crystallization process. Ideally, the mineral crystals in igneous rocks form a closed system--nothing leaves or enters the crystal once it is formed. This means that as radioactive parent elements decay, they and their daughters are trapped together inside the crystal. If, however, the rock is subjected to intense heat or pressure, some of the parent or daughter isotopes may be driven off. Therefore, scientists perform radiometric dating only on rocks or minerals that have remained closed systems.

One way to think about the closed system of the crystal is to compare it to an hourglass. The grains of sand in the top half of the hourglass are the radioactive parents, and those falling to the bottom are the stable daughters. At any moment, the ratio between them is a measure of the time elapsed, as long as the system remains closed. But if the hourglass were to break (become an open system), sand leaks out and the hourglass is no longer a reliable tool for telling time.

Once scientists have determined the parent-daughter ratio, they can use this measurement along with half-life of the parent to calculate the age of a rock containing the radioactive isotope. Radiometric dating has shown that very old rocks--3.5 billion years or older--occur on all the continents. Recently, rocks over 3.96 billion years old have been dated from northern Canada, Wyoming, and China.

The ages of these oldest rocks still don't tell us how old the Earth is, but they do establish a minimum age. We know the Earth must be at least as old as any rock on it. Unfortunately, none of the original rocks still exist, so scientists had to use less direct evidence to determine the age of the Earth.

One line of evidence involves rocks from outside the Earth--meteorites and moon rocks. Radiometric dating shows that almost all meteorites are between 4.5 and 4.7 billion years old. The oldest rocks and soils from the moon are about the same age--4.6 billion years old. Scientists assume that meteorites and moon rocks were not subjected to the extensive alteration that Earth rocks have undergone. Therefore, their ages indicate when they were formed. Because all parts of the solar system are thought to have formed at the same time (based on the solar nebula theory), the Earth must be the same age as the moon and meteorites--that is, about 4.6 billion years old.

Another line of evidence is based on the present-day abundances of the various isotopes of lead found in the Earth's crust. Natural lead is a mixture of four stable isotopes. Three of these isotopes (lead 206, 207, 208) result from radioactive decay of isotopes of thorium and uranium. The fourth, lead 204, is not the result of radioactive decay. This means that all of the lead 204 on the Earth has been around since the formation of the Earth.

Based on extensive sampling of the Earth's crust, scientists determined the present-day abundances of the four isotopes of lead relative to each other and to the parent isotopes that produced three of them. Because the original abundances of lead on the planet cannot be measured, scientists use meteorites to get at the Earth's original lead composition. Some meteorites contain the four lead isotopes but no uranium or thorium parents. This means that the lead composition in these meteorites has not changed since their formation, and scientists believe this is a reasonable approximation of the composition of the Earth's original lead, the so-called primordial lead.

Comparing the amounts of the four lead isotopes in primordial lead to their present amounts, scientists can determine how much lead has been added by radioactive decay since the Earth was formed. They can then calculate, using the half-life of each parent, how long it took to create the differences between the amount of present-day lead and primordial lead for each of the three isotopes. These calculations also yield an age of about 4.6 billion years for the Earth, which is consistent with the ages determined from meteorites and lunar rocks.

Sources

Dalrymple, G. Brent, 1991, The Age of the Earth--A Summary; in, The Evolution-Creation Controversey II--Perspectives on Science, Religion, and Geological Education: The Paleontological Papers, v. 5 (October 1999), p. 17-22.

Dott, Rober H., Jr., and Prothero, Donald R., 1994, Evolution of the Earth, 5th Edition: New York, McGraw, 569 p.

Wicander, Reed, and Monroe, James S., 1989, Historical Geology--Evolution of the Earth and Life through Time: St. Paul, West Publishing Co., 578 p.

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