How Do We Know the Earth Is 4.6 Billion Years Old?
― The scientist who found the answer first had to fight "lead dirt" in his lab

There is no record of the Earth's birthday anywhere. We still know its age because rocks hold a clock that never stops. But reading that clock was much harder than it sounds, and the answer took nearly a century of detours.

Only two things made it possible

But the road to putting these two together was far from smooth. Let's take it step by step.

The first answer was "tens of millions of years"

In the 19th century, the British physicist Lord Kelvin pictured the Earth as a hot ball that was cooling down. How long would a molten ball take to cool to today's temperature? From this calculation he estimated the Earth's age at 20 million to 400 million years. Late in life he cut it further, to a few tens of millions of years.

Geologists, and Darwin, who studied evolution, were troubled. Layers of rock seemed to need far more time to pile up, and living things seemed to need far more time to change. The physics and the evidence in the rocks did not agree.

What settled it was radioactivity, discovered in 1896. Uranium and other elements in rocks were giving off heat all the time. The assumption of a ball that only cools collapsed. And radioactivity carried not only heat but also a "clock."

How does the uranium clock tick?

A kind of uranium called uranium-238 turns half of itself into lead-206 in about 4.47 billion years. This time to fall by half is called the half-life. After the same time again, half of what is left changes. A quarter of the original remains.

Look at Figure 1. The solid line on the left shows the share of uranium still left. Move the slider below to advance time. In the column on the right, the lead part at the top grows and the uranium part at the bottom shrinks. If a rock had no lead when it solidified, the ratio of lead to uranium lets you work out the time since then.

In fact, uranium has another kind, uranium-235. It is a "fast clock" that halves in about 700 million years and turns into lead-207. Reading two clocks of different speeds in the same rock lets the two answers check each other.

What got in the way was lead, found everywhere

In the late 1940s, a young American researcher, Clair Patterson, set out to use this clock on meteorites. But every time he measured, the lead values came out scattered. Lead was already all around him, long before it was in the sample.

It was in dust, glassware, water, air, and even his own hair. At the time, lead was also mixed into petrol, so lead drifted in the air itself. Patterson scrubbed his rooms thoroughly and cleaned his equipment with acid. Even that was not enough, so he built a low-dust lab fed with filtered outside air.

It is said that in 1953 he was finally able to measure accurately the lead in an iron meteorite that fell in Arizona, in the United States. The age of the Earth he published in 1956 was about 4.55 billion years. As Figure 2 shows, that is more than 100 times older than Kelvin's answer. Later measurements have hardly changed this value.

Summary

The Earth's age was found by reading the speed at which uranium turns into lead in rock as a clock, and applying it to meteorites that were never remade. The first answer, worked out from cooling, was far too short because radioactivity, which gives off heat, was not yet known. The right answer took the patience to remove invisible lead dirt.

The Earth's birthday was remembered by a stone that fell from the sky.
To read it, first the dust in the room had to be wiped away.

To learn why the Earth's interior still hasn't cooled, see "Why Hasn't the Earth's Interior Cooled After 4.6 Billion Years?". For how fast the slow-moving continents travel, see "How Far Do the Continents Move in a Year?".

* This article is a popular-science explanation. The numbers given are rough guides to help you understand how things work. Age values differ slightly between sources.