How Old is the Earth?

This segment documents how notions on the age of the Earth have changed, from the view held by Europeans in the seventeenth century that the Earth was created only a few thousand years ago, to the realisation in the nineteenth and early twentieth centuries that it is thousands of millions of years old.

The question of the age of the Earth has interested humans from at least the time of the ancient Egyptians. Some Asian cultures, such as the Hindus, considered the Earth to be thousands of millions of years old. However, until last century most Europeans believed the Earth to be only a few thousand years old. This view received support from the work of Archbishop James Ussher (1581-1656) of Ireland, who in his Annales Veteris et Novi Testamenti, published from 1650 to 1654, used the generations descended from Adam as listed in the Old Testament of the Bible to calculate that the Earth was created in the year 4004 BC. Ussher's chronology was inserted in the Authorised Version of the English Bible and so became very widely accepted by English speaking Christian people.

In the late eighteenth and early nineteenth centuries, when scientific studies of the processes shaping the Earth began in Europe, it became clear from the slow rates at which these processes occurred that the age of the Earth must be very great, and so efforts were made to determine its age using scientific methods. The earliest methods attempted to calculate how long it would take for the total thickness of sedimentary rocks in the Earth's crust to accumulate, but the great variability from place to place in thickness of rocks and observed rates of present-day sedimentation meant that the results were unreliable, with estimates ranging from as little as 3 million years to as much as 1,584 million years. In 1899, attempts were made to calculate how long it would have taken for the oceans to become salty from the accumulated salts washed into it by rivers, and an estimate of 80-90 million years was arrived at. A flaw in this method was that it failed to recognise that salt contained in sediments deposited in the ocean is naturally recycled when those sediments are later eroded on land. At about the same time, the British physicist Lord Kelvin (1824-1907) concluded that the Earth was between 20 and 40 million years old, based on calculations of how long it would have taken to cool from an initially molten state. Kelvin's calculations appeared conclusive at that time and gained wide acceptance, but the subsequent discovery that radioactive decay of elements within the Earth provided an internal source of heat meant that his assumptions on the rate of cooling were incorrect, and his calculations therefore meaningless.

The discovery early in the twentieth century that the rate of decay of each radioactive element is constant, and so can be used as a form of clock, enabled the first progress to be made in the reliable dating of rocks and hence of the Earth. Using this technique, called radiometric dating, geologists now accept that the Earth is about 4,600 million years old. This age is based on the radiometric dating of meteorites, which are believed to have been formed at the same time as the Earth and the rest of the Solar System.

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