GeologyRocks

About the author

Jon
University of Edinburgh

Introduction

How do we know the age of the Earth? How do we know the age of rocks we find? There are two main ways to date rocks - absolute dating and relative dating. After the rocks have a date assigned to it, they can be put in some sort of order, and the relative dates worked out. This is the geological timescale.

The Ageing Earth

The Earth has aged a great deal since the inception of modern geology. Before the 19^th Century, Biblical interpretations of the age of the Earth were the accepted view. The most common figure was 6000 years old, based on counting the ages of people mentioned in the Bible, particularly the Old Testament. One scholar, John Lightfoot, stated the moment of creation was 9:00 AM, September 17th , 3928 BC.

In 1862 Lord Kelvin calculated an age of 20 to 40 million years, based on cooling of the Earth from a molten state. Other methods of trying to date the Earth include calculating evaporation rates from the oceans, using sedimentation rates, erosion rates and tidal forces between the Earth and the Moon. These methods gave ages of between a few million years and 1 billion years.

It wasn't until radioactive decay was discovered that the true age of 4.6Billion years (4,600,000,000 years!) was found.

Absolute Dating

Absolute dating is getting an actual date, i.e. this rock is 5.67 million years old. The main method for this is radioactive dating. So, here's a bit of particle physics...

Elements have isotopes, that is the same element with a different number of neutrons in the nucleus, but with the same number of electrons and protons. Sometimes these isotopes are unstable and decay to a more stable configuration. During decay the nucleus spontaneously disintegrates, producing energy and particles. There are two main methods of decay and both of them involve the loss of particles from the nucleus.

• Alpha Decay - loss of a helium nucleus (two protons and two neutrons) from the nucleus of the parent
• Beta Decay - a neutron turns into a proton and an electron and the electron is emitted.

There are two other types of decay, gamma and electron capture (or beta-minus), but gamma occurs along with the others and electron capture only occurs in man-made isotopes.

Each isotope has a unique half-life, which is a constant and related to the decay constant. The half-life is the time taken for half the radioactive isotope to be reduced by half. This leads to exponential decay from which the age of the isotope can be calculated. For some proof click here and if you don't like maths, just look at the graph.

The graph (figure 1) shows the number of radioactive nuclei (N) in an mass of isotope, against time (t).

Figure 1: Graph depicting the number of neutrons against time for a radioactive substance.

The main problem with this method is knowing N₀ - the original amount of the material. This is done using other methods, which are too advanced for this page. If N₀ is known then a rock may be dated if it contains radioactive isotopes which have formed with the rock. This only applies to igneous and in some contexts, metamorphic. If you try and date a sedimentary rock, you'll get a host of conflicting ages, as each particle may have come from a different source rock and hence be different in age.

Relative Dating

Relative dating is simply saying that this bit is older than that bit. This is most useful in a section of rock, where it is clear which rock is at the bottom (and hence is older) and which is the uppermost (and is the youngest). This method cannot be used worldwide, because every country does not have the same rocks at the same time. To get over this we can use things which only exist over a short (geologically speaking) period of time and have a worldwide distribution - fossils, or more precisely a zone fossil. A zone fossil needs to be:

• Common
• Rapidly evolving
• Widespread
• Easily identifiable

Graptolites make good zone fossils. If you find a fossil in one place which existed x million years ago in Scotland and the same fossil exists in America, the rocks are the same age. This is how most of the early dating was done.

One of the main laws in geology is the law of superposition. This simply means that stuff on top is younger than the stuff below, if they are the right way round. Another law is the cross-cutting law. If a rock is cut by another rock, then it is older than the rock which has cut it. The diagram below uses these principals to get the geological history.

Figure 2: Cross section of some geology, the history is detailed below. Redrawn from Plummer & McGeary (1997)

A brief history goes something like this:

• A - deposited
• B - intruded
• A & B - tilted and eroded
• C - deposited as D cuts A, B and C
• D - intruded
• D & C eroded, again due to flat surface
• E - deposited
• E - probably eroded

This means that not all the rock record is there, some of A, D and E may be missing. There may be other beds which have been completely eroded off. If you can't quite see why this is, try the step-by-step graphical approach, by clicking this link.

The whole sequence can be absolutely dated using the two intrusions, B and D. These, in turn, can give relative dates to the others by using the cross-cutting relationships seen in the diagram.

The Geological Timescale

The geological timescale covers the whole of the Earth's history. It starts at about 4.6 Billion (4,600,000,000) years ago or 4600Ma (Ma - Million years before present) with the formation of the solar system and continues right up to the present day. It is split into 4 Eras:

• Precambrian - 4600Ma to 550Ma
• Paleozoic - 550Ma to 250Ma
• Mesozoic - 250Ma to 65Ma
• Cenozoic - 65Ma to Present day

The eras are split into periods, which are split into epochs. These are shown in the table below. The names used are the British names and some different names may be used in other countries, especially North America. The dates shown are the ages in which the epoch started. The bold dates are the start of periods

Era	Period	Epoch	Age
Cenozoic	Quaternary	Holocene	10,000 Ya
		Pleistocene	1.6Ma
	Neogene*	Pliocene	5.14Ma
		Miocene	23.5Ma
	Palaeogene*	Oligocene	35.5Ma
		Eocene	56Ma
		Palaeocene	65Ma
Mesozoic	Cretaceous	Late Cretaceous	97Ma
		Early Cretaceous	146Ma
	Jurassic	Late Jurassic	155Ma
		Middle Jurassic	175Ma
		Early Jurassic	205Ma
	Triassic	Late Triassic	230Ma
		Middle Triassic	242Ma
		Early Triassic	251Ma
Paleozoic	Permian	Zechstein	260Ma
		Rotliegendes	290Ma
	Carboniferous	Stephanian	302Ma
		Westphalian	313Ma
		Namurian	323Ma
		Viséan	341Ma
		Tournaisian	353Ma
	Devonian	Late Devonian	371Ma
		Middle Devonian	380Ma
		Early Devonian	409Ma
	Silurian	Prídolí	411Ma
		Ludlow	423Ma
		Wenlock	430Ma
		Llandovery	439Ma
	Ordovician	Ashgill	443Ma
		Caradoc	464Ma
		Llandeilo-Llanvirn	476Ma
		Arenig	493Ma
		Tremadoc	510Ma
	Cambrian	Merioneth	515Ma
		St Davids	528Ma
		Caerfai	550Ma
Precambrian	Contains Archean and Proterozoic Eons		4600Ma

* - the Neogene and Palaeogene are collectively known as the Tertiary.

The table above has dates between periods in large, bold font. Don't worry to much about memorizing the others, not many geologists can list them off the top of their head. If you can say which period was 300Ma, then that's good enough!

Conclusion

The earth is 4.6 Billion years old. The rocks on the earth can be dated using relative dating, which involves dating a rock with respet to another rock; or absolute dating, which is giving a numerical date to a rock. Relative dating is done using fossils, the law of superposition or cross-cutting relationships. Absolute dating is done using radiometric methods which uses radioactive elements, such as Uranium or Potassium to date a rock. All rocks can be arranged in the geological timescale which stretches from the Precambrian to the most recent Cenozoic.

References

C. Plummer and D. McGeary, 1997. Physical Geology.

R. Muncaster, 1995. Nuclear Physics and Fundamental Particles.