THE AGE OF THE EARTH

Key Concepts

(1) Some cultures have believed the Earth to be very young; others, infinitely old.

People have long asked the question, ``How old is the Earth?'' The correct answer is not immediately obvious; historically speaking, the answers proposed have ranged from ``about 6000 years'' to ``eternally old''. The traditional Judeo-Christian-Islamic view, reached by taking an absolutely literal view of Old Testament chronology, is that the world is very young, being created around the year 4000 BC; this would make the Earth only a little older than the oldest written records.

A biblical chronology can be approximated by adding up the `begats' from Adam to Solomon. Then the historical books of the Old Testament permit us to extend the chronology from the reign of Solomon to the destruction of the First Temple. The Temple was destroyed by the Babylonian king Nebuchadnezzar, which allows us to tie the biblical chronology to the historical records of the Babylonians. St. Augustine of Hippo (author of `The City of God') used this method to estimate that the Earth was created in the year now known as 5500 BC.

This method was long used by scientists as well as saints. Johannes Kepler used biblical chronology to estimate that the world was created in the year 3993 BC; Isaac Newton, using similar techniques, estimated an age five years greater. Before the 18th century, computing biblical chronologies was the accepted method (at least in Europe) of calculating the age of the Earth. The ultimate in precision, using this technique, was attained by James Ussher, the Anglican Archbishop of Armagh, Ireland. In the mid-seventeenth century, Ussher wrote an immensely erudite work in which he concluded ``The beginning of time...fell on the beginning of the night which preceded the 23rd day of October, in the year 4004 BC''.

Let's not be too Eurocentric here; it should be noted that most Eastern religions asserted that the Earth was very old - perhaps infinitely old. In the Hindu worldview, for instance, the Earth is involved in an eternal cycle of creation and destruction. Each incarnation of the Earth lasts for a period of time called the `day of Brahma', equal in length to 4.32 billion years. In Jainism (a religion founded in the 6th century AD), the Earth is regarded as eternal: ``Know that the world is uncreated, as time itself is, without beginning and end.''

(2) Geological evidence indicates that the age of the Earth is large, but finite.

This is not intended as a tutorial on comparative religion; I have cited the religious views given above to point out that none of them is totally outrageous at first glance. Both the view that the Earth is young and that it is eternally old are acceptable if you don't look at the evidence extremely closely. About the time of the `rebirth of astronomy', however, there was a rebirth of geology and paleontology. During the 18th century, geologists and paleontologists became vividly aware that the rich and varied geological history of the Earth couldn't be compressed into a mere six millennia. Scientist became aware of: One intriguing argument for the age of the Earth was provided by Edmund Halley (of comet fame). Halley stated that oceans are constantly becoming saltier with time, as rivers continuously wash small amounts of dissolved salt into the sea. The Earth, Halley argued, cannot be extremely young (as the Bible claims) because then the oceans would still be fresh water. On the other hand, it cannot be infinitely old, because then the oceans would be saturated with salt, like the Dead Sea or the Great Salt Lake.

Actual numerical estimates of the Earth's age, based on scientific principles, had to wait until the 1890's. In that decade, Lord Kelvin (of temperature fame) assumed that the Earth was originally a molten sphere of rock, and asked how long it would take to cool to its current temperature. His answer was that the Earth is 20 to 40 million years old. (This answer is actually an underestimate; radioactivity in rocks keeps the Earth warmer than it would otherwise be.) John Joly, an Irish geologist, following Halley's suggestion, estimated that the Earth's oceans must be 80 to 90 million years old, if they started out salt-free. (This answer is actually an underestimate; salt is actually taken out of the ocean by sea spray and in the corpses of dead sea animals, which keeps the salinity lower than it would otherwise be.)

Despite the fact that the ages estimated by Kelvin and Joly were underestimates, they still, by being thousands of times longer than the biblical age, caused a crisis of faith among the late Victorians. Most Christians have adopted the view that the chronology of the mythical story of Genesis (especially the six days of creation) are to be taken as a metaphor, or parable. 

(3) The oldest Earth rocks are 4 billion years old (as determined by radioactive dating).

Modern calculations, using the technique of radioactive dating, tell us that the Earth is actually over 4 billion years old. Radioactive dating is based on the fact that some isotopes of some elements are unstable; they emit elementary particles and gamma-ray photons to form a lighter, stable element. This process of transmutation is called radioactive decay. (It is the particles and photons emitted by decaying atomic nuclei that keep the Earth warm and screwed up Kelvin's estimate of the Earth's age). For example, 238U (uranium with 92 protons and 146 neutrons in its nucleus) is radioactive. It decays into 206Pb (lead with 82 protons and 124 neutrons in its nucleus) with a half-life of 4.5 billion years. The decay of a nucleus is a random event, which cannot be predicted. All that can be said is that a particular uranium nucleus has a 50% chance of decaying during one half-life.

Suppose you started with a cluster of 8000 uranium atoms.
After 1 half-life (4.5 billion years), you would expect to have 4000 uranium atoms and 4000 lead atoms.
After 2 half-lives (9 billion years), you would expect 2000 uranium atoms and 6000 lead atoms.
After 3 half-lives (13.5 billion years), you would expect 1000 uranium atoms and 7000 lead atoms.
After 4 half-lives (18 billion years), you would expect 500 uranium atoms and 7500 lead atoms.
And so forth...

In principle, you determine the age of a rock by measuring the ratio of uranium to lead that it contains. If it has lots of uranium and very little lead, you conclude it is much younger than one half-life of uranium. If it contains equal numbers of uranium and lead atoms, you conclude it is one half-life (4.5 billion years) old.

In practice, things are more complicated, of course. One complication is that if a rock is ever melted at any time during its history, it becomes differentiated. Uranium is extremely dense (19,000 kg/m3), so it sinks to the bottom. Lead is slightly less dense (11,000 kg/m3) so it will float in a separate layer on top of the uranium. The ``radioactive clock'' is reset every time a rock is melted. Thus, when geologists talk about the age of a rock, they mean the time that has elapsed since the rock last solidified. (More information about radioactive dating is available here.)

Rocks on the Earth have a wide range of ages, testifying to the constant volcanic activity on our planet. The very oldest rocks to be created on the Earth have been obliterated by the heavy volcanic activity early in the Earth's history. Probably, the Earth and Moon formed simultaneously, about 4.5 to 4.6 billion years ago.

updated: