Geologists often trot out some incredible facts without a second thought – “This rock is 4.4 billion years old…” said geologist Simon Wilde in 2001 as he introduced the oldest known piece of the Earth, but how did he know?
The Clock Inside the Rock
It turns out that the most accurate way to date rocks depends on zircon, a semi-precious stone that can be found in a range of colours, as well as in a clear form that resembles diamond. Its physical properties have led in recent years to it becoming one of the best ways of dating rocks accurately. Zircon is everywhere, as very small grains in most types of rock.
For a period beginning about 4.5 billion years ago when it was formed, the Earth suffered an intense bombardment by meteorites, which generated a huge amount of heat, melting the materials that made up its surface. When molten rock cooled, some collections of atoms, including zircon, formed crystals which have survived unchanged until today. Unchanged, that is, apart from one useful characteristic. The lattice of a zircon crystal occasionally accepts a uranium atom instead of another atom of zircon, so that scattered through a crystal of zircon at the moment of its formation are a certain number of uranium atoms. But uranium atoms decay at a known rate into lead atoms. What’s more, when zircon crystals form they contain no lead atoms. From the moment of its crystallization, a zircon crystal will steadily acquire an increasing proportion of lead atoms as the uranium atoms decay.
So a zircon crystal in newly formed rock is like a clock set to zero. The ‘ticks’ in the clock are the uranium atoms contained in the crystal as they change to lead atoms. These changes don’t happen at regular intervals but they do happen in a statistically predictable way. Uranium has what is called a half-life, which means the time it takes for half the uranium atoms in a sample to change into lead.
Calculating the Clock ‘Ticks’
The ‘ticks’ of this geological clock are very long indeed, as we shall see.
One type of uranium, called uranium-235, has a half-life of 704 million years, while another type, uranium-238, has a half-life of 4.7 billion years.
So if a grain of zircon starts with 100 U235 atoms, say, and then one by one each atom decays to become a lead atom, after 704 million years the ratio of lead to uranium will be equal and there will be 50 of each. A scientist measuring this particular grain of zircon will be able to say that it is 704 million years since it was formed. After another 704 million years, half of the 50 uranium atoms will have become lead and the ratio will be 75% lead to 25% uranium and the scientist will know that the rock was formed just over a billion years ago.
Using the two types of uranium atoms and their ratio to lead atoms, geologists can work out how long ago the process started and therefore how old the crystal is.
The Oldest Rocks
In 2000 in Western Australia, at a site called Jack Hills, a deep purple crystal of zircon was discovered, less than a quarter of a millimetre across. It was given the prosaic name W74/2-36 and the uranium/lead ratio showed it to be 90 million years older than any previously dated rock on Earth, at 4,404 million years old. Although the crystal was so tiny, a wide range of other investigations could be made, including oxygen isotope measurements and rare Earth analyses, measurements that provided information about the physical processes that led to the formation of these very old rocks in the area, and even revealed that water was involved in the way the Earth’s crust was formed an that there were therefore oceans as well as land at that very early stage in the Earth’s history.