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!
Why is it important to know how old rocks are?
The rocks of the Earth and the fossils they contain, record important events in the Earth’s history. They tell us about the changes in the Earth’s climate, and if we can date the rocks accurately we can work out the rate of climate change. Rocks also record catastrophic events and through dating we can work out exactly when these were, and figure out what else might have been happening at the same time. For example, we know from the fossil record in the rocks that there was a mass extinction of the dinosaurs and many other kinds of animals, some millions of years ago, at the end of the Cretaceous and before the beginning of the Tertiary Period– this is usually called the ‘K-T boundary’. By dating rocks we have been able to match up this event with the impact of a large meteorite off the Yucatan coast of Mexico, and with the eruption of vast lava flows in India. Both of these events caused a catastrophic change in the climate and environment, and that was the end of the dinosaurs! Radiometric dating has given us an exact age for this - 65 million years ago.
When it comes to rocks younger than two million years old, many of the elements in radiometric dating cannot be used because their decay rates are too slow, but we can use the radioactive decay of carbon to give us absolute ages. Carbon occurs in organic remains like bone and wood, which can also be fossilized in rocks. ‘Carbon dating’ is therefore useful for dating events in the ice age, like the length of glacial periods and the date of the retreat of the last ice sheet covering the UK which was 13,000 years ago. We can even date the actual ice using the bones of woolly mammoths which are trapped and preserved in it.
Of course carbon can also be used for dating archaeological remains like boats, tools and skeletons – which is important for understanding the evolution of humankind.
Carbon dating used together with tree rings - a science called ‘dendrochronology’ - gives us real clues to the changes in our environment in the last few thousand years. Sections through preserved or fossilised tree trunks are carbon dated and these dates are then matched up with the growth rings in the tree trunk. This gives us actual dates for changes in climate. During warm periods, like interglacial periods, trees grow vigorously and produce thick rings, in colder periods they grow very slowly and form thinner growth rings.
Another organic matter which can help us with dating and give us clues to past climate is pollen. Pollen and spores are part of the reproductive system of plants, and they are produced by flowering plants, trees, moss and ferns. These microscopic grains are very resistant to decay. They are trapped in peat bogs and even occasionally in ice. Pollen can be carbon dated, which gives us an absolute age for the pollen grain, and also the surrounding peat or ice. Plants are quite fussy about the environment in which they can grow. Therefore the type of pollen grain we find, whether it is pine, birch, oak tree or grass for example, will help us work out exactly what type of climate prevailed when that patch of peat or ice was laid down.