How are the ages of the Earth and universe calculated?
In a NutshellMany independent measurements have established that the Earth and the universe are billions of years old. Geologists have found annual layers in glaciers that can be counted back 740,000 years. Using the known rate of change in radio-active elements (radiometric dating), some Earth rocks have been shown to be billions of years old, while the oldest solar system rocks are dated at 4.6 billion years. Astronomers use the distance to galaxies and the speed of light to calculate that the light has been traveling for billions of years. The expansion of the universe gives an age for the universe as a whole: 13.7 billion years old.
Astronomers and geologists have determined that the universe and Earth are billions of years old. This conclusion is not based on just one measurement or one calculation, but on many types of evidence. Here we will describe just two types of evidence for an old Earth and two types of evidence for an old universe; more types can be found under Further Reading. These methods are largely independent of each other, based on separate observations and arguments, yet all point to a history much longer than 10,000 years. As Christians, we believe that God created the world and that the world declares his glory, so we can’t ignore what nature is telling us about its history.
Age of the Earth from seasonal rings and layers
If you’ve ever seen a horizontal slice of a tree trunk, you’ve seen how a tree forms a new growth ring each year. In years of drought, the tree grows less quickly so the ring is narrower; in good growing seasons the ring is thicker. A tree’s age can be found by simply counting its rings. By comparing the pattern of thick and thin rings to weather records, scientists can verify that the method is accurate. This method can even be used on dead trees that fell in a forest long ago. For example, the last 200 rings in the dead tree might match up with 200 rings early in the life of the living tree, so the two trees together can count back many years. In this way, multiple trees can be used to build a master chronology for a forested region. European oak trees have been used to build a 12,000-year chronology.1
The annual ice layers in glaciers provide a similar method that goes back much further in history. Each year, snowfall varies throughout the seasons and an annual layer is formed. Like the tree rings, this method can be verified by comparison to historical records for weather, as well as to records of volcanic eruptions around the globe that left thin dust layers on the glaciers. Scientists have drilled ice cores deep into glaciers and found ice that is 123,000 years old in Greenland2 and 740,000 years old in Antarctica.3 These annual layers go back much farther than the 10,000 years advocated by the young earth creationists. The Earth must be at least 740,000 years old.
Age of the Earth and solar system from radiometric dating
In your high school science classroom, you may have seen a large poster of the periodic table hanging on the wall. The periodic table shows the types of atoms that make up the world around us. An element in the periodic table can come in different flavors called isotopes. Some isotopes are unstable, and over time these isotopes “decay” into isotopes of other elements. For example, Potassium-40 is unstable and decays into Argon-40. As time passes, a rock will have more and more Argon-40 and less and less Potassium-40. Radiometric dating is possible because this decay occurs at a known rate, called the “half-life” of the radioactive element. The half-life is the time that it takes for half the radioactive sample to change from one element into the other.
Some isotopes have short half-lives of minutes or years, but Potassium-40 has a half-life of 1.3 billion years. Radiometric dating requires that one understand the initial ratio of the two elements in a given sample by some means. In this case, Argon-40 is a gas that easily bubbles out and escapes when it is produced in molten rock. Once the rock hardens, however, all the Argon-40 is trapped in the sample, giving us an accurate record of how much Potassium-40 has decayed since that time. So, if we find a rock with equal parts Potassium-40 and Argon-40, we know that half the Potassium-40 has decayed into Argon-40, and that the rock hardened 1.3 billion years ago.4
It’s hard to find rocks on the surface of the Earth that have not been altered over time. Most old rocks have been eroded by wind and water or submerged by continental plates. The oldest reliably dated rock formation is in Greenland, where several different isotopes were used to find an age of 3.6 billion years.5 Scientists also recently dated zircon grains (which resist erosion) in Western Australia to 4.4 billion years old.6 To find older rocks that haven’t been eroded, we need to look beyond Earth. Meteorites are rocks from the solar system that have fallen to Earth recently and haven’t suffered much erosion. Their pristine interiors give an age that dates back to their formation at the beginning of the solar system. Nearly all meteorites have the same radiometric age, 4.56 billion years old.7 Thus, the solar system, including the Earth, is about 4,560,000,000 years old.
- Davis A. Young, ”How Old Is It? How Do We Know? A Review of Dating Methods – Part One: Relative Dating, Absolute Dating, and Non-radiometric Dating” Perspectives on Science and Christian Faith, Vol 58 No 4 (2006), p. 264. (PDF)
- Roger C. Weins, "Radiometric Dating: A Christian Perspective", The American Scientific Affiliation (2002). See also North Greenland Ice Core Project Members, “High-resolution Record of Northern Hemisphere Climate Extending into the Last Interglacial Period,” Nature 431 (2004): 147–151, which reports ages back to 123,000 years. (web article)
- EPICA Community Members, “Eight Glacial Cycles from an Antarctic Ice Core,” Nature 429 (2004): 623–628.
- Young earth creationists reject radiometric dating methods, including claims that decay rates are not constant. For a critical review, see Randall Isaac “Assessing the RATE Project”, Perspectives on Science and Christian Faith, vol 59, no 2, June 2007, p.143-146. (PDF)
- See Wiens and references therein. (web article)
- Wilde et al. “Evidence from detrital zircons for the existence of continental crust and oceans on the earth 4.4 Gyr ago,” Nature (2001) 409, 175-178.
- See Davis A Young, ”How Old Is It? How Do We Know? A Review of Dating Methods—Part Two: Radiometric Dating: Mineral, Isochron and Concordia Methods” Perspectives on Science and Christian Faith, Vol 59, No 1 (2007) and references therein (PDF)