Our Earth is about 4.5 billion years old. The rock record or ‘geological time’ extends back to 4 billion years ago. Of course it cannot go back right to the formation of Earth, because the young Earth was hot and most of the outer part was probably molten, but even so this is a vast period of time.

How do we know how old the rocks are?
We can think of geological time in 2 ways: relative and absolute. Relative time disregards years and only considers whether one event in the Earth’s history came before another event. Absolute time measures whether a geologic event took place a few thousand or a few million years ago.

We measure time in 2 basic units – a day, the interval required for the Earth to complete one revolution on its axis, and a year, the interval required for the Earth to complete one circuit of the Sun. In geological time, the problem is determining how many of these units of time elapsed when no-one was around to count them!

One clue to solving this problem is using the decay rates of radioactive minerals. Some minerals contain radioactive elements like uranium. Such elements decay over time, emit radioactivity and produce new ‘daughter’ elements. In fact, they form a reliable clock for the Earth. Uranium decays to produce helium and lead. The rates at which uranium and other radioactive elements decay are known, therefore there is a unique ratio of lead to uranium depending on the amount of time the decay has been going on. We can find out quite a precise age for a rock containing traces of uranium, by looking at the uranium-lead ratio in the crystals. Once we know the amount of daughter element (lead) and since we know the rate of decay, we can work back to the time when there was no daughter, but only the parent, and this give us the age of the rock. This process is called ‘radiometric dating’. Uranium is not the only element used in radiometric dating, there are others too, and between them we have been able to produce hundreds of dates for events in the Earth’s history.

Even before radiometric dating was invented, geologists were able to make a ‘geological timescale.’ This was done by looking at the relationships between the rocks – which were above and below, or cutting across others - and then placing them all in their correct order of formation. By using radiometric dating we have since been able to give many of the rocks an absolute age – that is the number of millions of years ago that they were formed. The oldest rocks so far have been dated at 3.8 billion years ago, but there have been some individual crystals dated at 4.2 billion years ago, almost right back to the birth of the Earth!

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