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How does Geological Time tick?

Geological time is immense. Most humans live for less than one hundred years, a mere blink of the eye when compared to the lifespan of our planet. For hundreds of years, scientists thought that the earth was only a few thousand years old, using ancient texts and biblical events as their basis for this conclusion. Modern science has in recent years been able to date the earth to just a little older than that. Scientists now think that the earth is about 4.5 billion years old, that’s four thousand five hundred million years old! These numbers can be pretty scary, most of us don’t know how to put a value on a million, let alone a billion. Maybe we can start to get a grip on the length of time that we’re talking about when we’re dealing in geology.

How big is a million?

I’m pretty sure that everyone reading this has been alive for a million seconds. Lets work it out.

1,000,000 seconds divided by 60 gives us the number of minutes in 1 million seconds

Answer = 16,666.67 minutes

Divide 16,666.67 by 60 to give us the number of hours in 1 million seconds

Answer = 277.78 hours

Divide 277.78 by 24 to give the number of days in 1 million seconds

Answer = 11.57 days

For a million seconds to pass in your life it takes eleven and a half days. Perhaps that gives you some idea of just how big one million is.

A million minutes of your life takes very nearly two years to pass.

A million hours is beyond the lifespan of most humans, over one hundred and fourteen years.

A million days takes us back in time to the heyday of Ancient Greek civilisation and the first Olympic Games, over two thousand seven hundred years into the past.

Most people just can’t get their head around the huge expanse of geological time. If you’re one of them, don’t worry about it, so am I! I find it easier to look at the numbers as just numbers and use the names of the different divisions given to geological time.

So how did scientists work out the real age of the Earth? Before the advent of modern radiometric dating techniques geologists used basic observations to work out the relative ages of rocks. Relative ages don’t give you a definite figure of how old a piece of rock is, but allow you to say that this piece of rock is older or younger that another piece of rock.

This technique is still used today by geologists out 'in the field' as a method for trying to tell the story of a particular location.

One of the most important rules to learn is the law of Superposition. That’s just a fancy way of saying the order in which things were made. If you take flat layers of rock you can assume that the layers at the bottom were made first and are older than the layers that lie on top of them. Simple.

Look at this picture. Which of these layers formed first? Easy, the bottom dipping layers. But a weird thing has happened. The top layers are lying at a completely different angle to those bottom ones, it looks like they’ve chopped them off. That’s almost exactly what happened. Those older lower layers were tilted and worn down before the younger flat beds were laid down on top of them. This structure is called an unconformity and it represents a length of time that is not recorded by rock creation. It is a gap in the rocks, a missing piece of time.

Cross cutting relationships are another field method for comparative dating. Faults and dykes are features that cut through existing rocks. This means that the rock must have existed before the fault or dyke cut through it, so those features must be younger than the rocks around them.

We can also use observations of current processes to estimate the length of time it takes to build up a layer of rock. Look at how long it takes mud to build up in the bottom of a pond or lake or a layer of lava to pour out of a volcano. This gives us some idea of the range of times it can take to make different rocks.

There is a method of thought used in geology called Uniformitarianism. What this simply states is that everything that happens on the earth today has always happened and will continue to happen in the way id does now. That means that volcanoes have always erupted in the same way that they do now, and limestones were made in the same way in the past as they are today.

This idea overtook the previous notion of Catastrophism that was used to make the rocks fit the Biblical time scale. Events like the great flood that caused Noah to build his ark and enormous earthquakes all created the rocks in one sudden go. The flood was also blamed for the extinction of the dinosaurs and all the other fossils that were found that had no modern day counterpart.

Fossils are another way of finding out roughly how old rocks are. You can use fossils to compare different layers of rocks. Finding one type of fossil found in a layer of rock and another type of fossils in a layer of rock higher or lower you can infer that one fossil is older or younger than the other. You can then start to build up a catalogue of fossils that give an idea of how old different fossils are in comparison to one another.

These methods all work really well for field observations, but they don’t get us any closer to finding a definite age of the earth. To find the answer to that question we need to leave the field and get into the laboratory.

The Clocks in Rocks

In the 1950’s, scientists discovered that the rocks themselves had their own internal clocks. Radioactive elements were incorporated into the fabric of the earth, back at the formation of the universe during the Big Bang. Radioactive elements are unstable and decay over time. This decay process starts as soon as a rock is made. That means the moment that at igneous or metamorphic rock cools down and crystallises or a sedimentary layer is deposited the radioactive clock starts ticking.

To understand the process of radioactive decay, we need to understand something about the structure of atoms. Atoms are made out of three different particles, protons, neutrons and electrons. Protons and neutrons are found in the centre of atoms, the nucleus, with the electrons orbit the nucleus in a cloud like structure. These particles carry an electrical charge, protons carrying a single positive charge, neutrons are electrically neutral and electrons have a negative charge. In a complete atom the number of protons and electrons are equal, making a balanced charge, the atom has no electrical charge. Protons and neutrons also have a weight value and when these are added together you get a value for the weight of an element.

Each element has its own fingerprint based on the number of protons within the nucleus. The number of protons for an atom never changes, that is the key identifying feature of an element. The number of neutrons in an atom can change and this changes the weight of an atom. This gives rise to things called isotopes, variations of elements based on a different number of neutrons in the nucleus. Some of the isotopes of elements are unstable and spontaneously decay to a different, stable element.

It is these unstable isotopes and the new stable elements that are used for determining the absolute age of rocks. The unstable isotopes are called "parent isotopes" and the elements that they decay into are called "daughter elements". Each radioactive isotope decays at its own constant rate, determined by how long it takes for half the amount of parent material to become a daughter element. This is called the half-life of the radioactive element.

Using this half-life, we can work out how long the radioactive material has been decaying.

Some of the more common radioactive elements used for measuring the age of rocks are these.

Uranium-238 decays to Lead-206 with a half-life of 4.5 billion years

Argon-40 decays to Potassium-40 with a half-life of 1.25 billion years

So by measuring the amount of, for example, uranium in a rock and comparing it to the amount of lead in the same sample, we can work out how long the uranium has been decaying for. That will give us an overall age for the rock.

This process of radiometric dating has revolutionised the way geologists think about time and has opened up a window into the early earth. Using these methods, scientists have been able to find out amazing things about the earth and universe.

The oldest rocks on the earth come from Greenland and have been dated to 3.7-3.8 billion years old. Other really old rocks have been found in Western Australia, Southern Africa and the Great Lakes of North America and these have been dated to between 3.4 and 3.6 billion years old.

These very oldest earth rocks are metamorphic rocks that were originally lava flows or layers of sediments. Metamorphism resets the internal radioactive clocks, so the original rocks must have been even older.

The oldest minerals found on earth are tiny crystals of zircon from Western Australia and these have been dated to an incredible 4.0 to 4.2 billion years old.

Some moon rocks and meteorites have been studied. Meteorites in particular can give us real clues to the age of the Solar System, as certain meteorites are accumulations of the original dust that formed our planets. These have been dated to 4.4 to 4.6 billion years old, putting the age of the earth somewhere in between.

The current scientific best age of the earth has been calculated to be 4.54 billion years old, an enormous amount of time. As science progresses and technology can help us to ask and answer more questions we will start to find out more about the immense history of our planet. Geologists have had to come up with methods for breaking that time down into more manageable chunks.

 

 

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