Thursday, September 07, 2006

Age of the Earth Series

Welcome to Paul Buede’s requested “Age of the Earth” series of lectures. This first lecture goes through, in broad terms, what radioactivity is, what half life is, and the main methods of radiometric dating.

In the next lecture, we will talk about the two most mathematically complex methods of dating – Isochron dating, and the Uranium-Uranium Concordia Discordia method. This lecture will be for enthusiasts, who are unhappy with the rather hand-wavy explanations I give in lecture 1.

In lecture 3, we’ll go through the common arguments that creationists give against radiometric dating, and show why they are false. In lecture 4, we’ll go through the main arguments creationists give for a young earth, and show why these are false. Then in lecture 5, we’ll have a brief discussion of the theology of an old earth – and how modern science deepens our understanding of creation, and our bond with a loving God.

Throughout, I encourage you to read the American Scientific Affiliation article, by Dr Roger Weins, called “Radiometric Dating – A Christian Perspective” – it will give you much needed background material for all lectures. I would also recommend the first chapter of “Science and Creationism” by Ashley Montagu, and the whole of G. Brent Dalrymple’s seminal text “The Age of the Earth”.

Lecture 1 – Radioactivity and Dating Methods – an Introduction

The Nucleus

Matter is made out of small units called atoms. Atoms have a nucleus in the centre - which is heavy and made out of sub atomic particles named "nucleons". Positive protons and neutral neutrons are examples of nucleons. Shells of electrons, which are negative, spin round the nucleus at very high speed in orbits (this is a simplification, electrons actually exist in a quantum state in these shells – they do not technically orbit).

The nucleus determines the nature of a substance - the number of protons determines what element a substance is, for example. The nucleus is also where radioactive decay happens. You see, protons actually repel each other electromagnetically - positive repels positive - driving the nucleus apart. However, the neutrons and protons attract each other, with a force called "the strong force", keeping the nucleus together.

Radioactive Decay

However, if the mix of protons and neutrons is wrong - or there are too many of each - the nucleus becomes unstable, because the forces aren't balanced right. That means that certain nuclei become unstable - and can even collapse and split apart. This is called radioactive decay.

There are 3 types of decay:

Alpha decay: This is when an "alpha particle" (made up of 2 neutrons and 2 protons) is ejected from the nucleus. This is especially common in heavy nuclei, that need to get rid of nucleons fast to become stable.

Beta decay: This is when a neutron turns into a proton and an electron - or a proton turns into a neutron and a positron (a positive electron). This occurs normally when the balance of neutrons and protons isn't right.

Gamma decay: This is a special type of decay where some of the excess mass of the nucleus is converted into energy, and given off as a burst of gamma radiation (a type of very very high energy invisible light).

Half life

The physics behind radioactivity is well researched - there is even a formula where you plug in the number of protons and neutrons and it'll tell you if the atom is radioactive (called the “Semi-empirical Mass Formula). There are also formulae to work out how quickly atoms will decay on average.

Of course, it is an average, because decay is a probabilistic/statistical process - but experimentally you'll find that decay rate is very very close to the average, because there are so many atoms making up the sample. Kind of like if the average height of men is 6 feet, if you take 2 men they might not have a 6 foot average - but if you take a million men, then it's likely to be close to the average. There are literally billions of atoms in a tiny spec of matter - meaning that radioactive decay follows the average pretty closely.

If we know how quickly atoms will decay on average, then we can tell how much will have decayed after a certain amount of time. We have a very useful measure, called the "half life", which is the amount of time it takes for 50% of a radioactive material to have decayed. In other words - if you have 1 kg of a substance with a half life of a year - if you wait a year, 500g of radioactive substance will be left, the rest will have decayed.

Decay Series

Now, what happens when an atom decays radioactively? Well, it turns into a different substance - because it'll have a different number of neutrons and protons left. This new substance is called the "daughter" of the decay (because it is literally born of the decay), and the radioactive element is called the "parent".

Sometimes though, when a parent decays, it decays into a daughter that is also radioactive! Then the daughter decays, giving birth to another daughter, which might also be radioactive. This can create a chain of radioactive parents and daughters, all the way down finally to a daughter that isn't radioactive, and then the chain stops. This happens often when the original parent is very heavy, like in the decay of Uranium.

At first, as the original parent starts to decay forming a chain, the chain grows. After a while (a few million years with Uranium) the chain settles down to a steady state. These decay chains can therefore be used to date rocks up to a few million years as the chain grows - but in older rocks, they can assure us that the rock is *at least* a few million years old, but they can't tell us exactly how old.

Radiometric Dating

Now, the half life of radioactive elements depends on the nuclear properties of the element. That means that it won't change over time - and since the nucleus is almost completely immune to high temperature and pressure - we know that even under very very extreme conditions the half life won't change that much.

Now, as rocks form inside the earth, and in lava flows, they often contain radioactive elements. And when they solidify, those elements decay inside the rock, and the daughter products are stuck within the confines of the rock. That means that millions of years later, we can pick up the rock, measure the proportion of daughter and parent elements, and estimate how old the rock is, using various methods.

Below I'll tell you about the various methods of dating rocks using radioactive material - and why we're sure they're accurate.

Types of Radioactive Dating

Uranium Dating

Uranium decays to lead, through a rather long decay series. That means that firstly, because of decay series, we often know for absolute sure that the rock is at least a few million years old (after all, what's the chances that exactly the right amounts of each daughter element in the series existed in the rock out of chance, to form a decay chain?)

Despite the reassurance of decay series, Uranium is still the crudest dating method - and the simplest. You take the amount of Uranium, the amount of lead, and plug it all into a formula to get a date out the other end. This method is usually used in Zircons – a type of mineral that chemically excludes lead. The reason for this is so we can be sure that all the lead within our sample is daughter element – it wasn’t there to start off with.

Now, clearly there is a source of error in this form of radiometric dating. What if some lead has been “leached” – what if it’s leaked away while the rock has been heated or partially metamorphosed? Well, these are valid sources of error, and hence this form of dating is less accurate than others. However, of course, this is a random source of error, and given enough samples, it ought to average itself out. It is also a source of error that predominantly gives artificially young ages – because if lead has disappeared, we will date the rock as younger than it really is.

Also, this method of dating is now rare, it has been overtaken by a method called the “Uranium-Uranium Concordia/Discordia” method – that takes into account the possibility of metamorphosis and leaching. We will talk about this method in broad terms later on in this series.

Potassium-Argon Dating

This type of dating is known as K-Ar (K and Ar being the letters representing Potassium and Argon in the periodic table). Now, this method of dating is very interesting for two reasons.

Firstly, Argon, the daughter element, is a rare inert (unreactive) gas - which means that it usually boils off and bubbles off into the atmosphere when a rock or crystal is formed. That means that we can be fairly sure that the original level of Argon was very very low in the rock. Furthermore, any atmospheric argon-40 that is trapped at formation will also be trapped with atmospheric Argon-36, in a ratio of 295 parts to 1 (the atmospheric ratio of Ar-40 to Ar-36). We can therefore measure the Argon-36 in a sample and work out how much Argon-40 is non-radiogenic, and remove this from our calculations.

Secondly, this type of decay takes place in salt crystals. That means that if the crystal has been disturbed, or cracked, or tampered with, it is fairly easy to see from it's structure - and also, all of the daughter argon is trapped in the crystal lattice, it cannot escape. Furthermore, no more Potassium can be added, the crystal structure is firm and does not allow this addition to take place.

That means that K-Ar dating is much more accurate than Uranium dating - yielding error margins as low as 1-2% in some cases. We also have a method known as the Ar-Ar Plateau method – which takes into account the possibility of metamorphism and leaking of argon during periods of heating. This method will be explained later in this series.

Rubidium Strontium Dating

This type of dating is one of the most accurate - and the most interesting. It's also the most complicated mathematically - so you'll have to forgive me for skipping over and simplifying the maths. It can only date old rocks though, at least over 10 million years old - because Rubidium has a very long half life (47 billion years!)

There are several different types of Strontium - only one type is created through radioactive decay. However, we also find the other types of Strontium in the rock - but we also know that the naturally occurring ratio of these different types of Strontium is constant at any one time on earth.

That means, with a bit of mathematics, we can come out with a straight line graph, (called an isochron, for enthusiasts), whose gradient is the age of the rock. But get this - if any part of the rock is tampered with, it won't be a straight line. If the rock has partially melted and resolidified, it won't be a straight line.

Basically, the results of this type of dating allow us to check all our assumptions that weren't possible to check with Uranium dating. We can know exactly how much Rubidium there was to start with. We can know if the rock has been tampered with. We can know if any daughter or parent element has been lost or added to any part of the rock. In other words, this method is self checking, and self calibrating - it's the near perfect method of telling time - and often yields errors as low as 1%.

If you ever hear scientists say that a rock is "unsuitable" for radiometric dating - chances are that they've tested it using Rb-St - and found that they didn't get a straight line, meaning that the rock has been tampered with and it's difficult to use it for dating. Unfortunately, creationists often twist this to an unwary public - and say that scientists throw away bad results and blame it on the rocks being unsuitable. Nothing could be further from the truth of course - in actual fact, it's because this dating method is so good that we can tell from it if our assumptions are correct, and therefore if a rock is suitable. We will go into more detail on isochron dating methods later in this series.

Dating Using Several Methods

We scientists aren't very often satisfied with one result. That's why we often date several rocks from any one site, and use as many methods of dating as possible to check our results. For example, the oldest rocks ever found date at about 4.2 billion years - and these were checked with all three methods of dating, all of which agreed to within bounds or error.

Final Quirky Little Proof of an Old Earth

I can't remember who came up with this one, but it's very interesting. There are many radioactive elements that we know about. Some occur naturally all the time - some do not generate naturally.

Of those that do not generate naturally, some have a half life of over 50 million years, and some do not. We find traces of every single one of the ones with a half life over 50 million years, and absolutely no trace at all of any of the ones with a half life below 50 million years.

Only the earth being many times old than 50 million years can explain how why we only find elements with long half lives - because the elements with low half lives have all decayed to nothing.