How Old is that Rock?

First - Some Background..
The Earth is reckoned to be about 4,500 million years old. 
We have found rocks that date back to around 3,500 million years ago.


All rocks belong somewhere on the 'geological column'. The column is a list, oldest rocks at the bottom, youngest at the top, of all the time that there has been while the Earth has been around. We haven't much evidence of what was going on for the first 3,800 million years, so the geological column mainly concentrates on the 700 million years that we know more about.
The good news is you don't have to learn the geological column - but you do need to be able to use it to match periods to ages and vice versa.

What the Syllabus says - and the details...

You need to know that geological events are dated and interpreted using: -

The concept of superposition of strata - we assume that younger rocks are lying on top of older ones, unless we have good evidence that the rock has been turned over by a tectonic event. In the diagram below the youngest rock would be the Dakota formation, the oldest rock the Kaibab formation.

Included fragments - a rock that includes fragments of another one (e.g. conglomerates contain clasts (pebbles) derived from other rocks) must be younger than the rock it has the bits from.

Cross cutting relationships - a dyke must be younger than the rocks it cuts through, a sill must be younger than the beds it was intruded between etc. 
See if you can work out the order in which the rocks below must have been made...

Dating (as opposed to ordering) is done with Fossils and Radioactive half lives - see information below.

Using Fossils to Date and Correlate Rocks

You need to know how to use the morphological changes of cephalopods and graptolites to put rocks into an age sequence...

Dating with Cephalopods

There are three types of cephalopods that developed over time; goniatites, ceratites and ammonites.  These have different suture lines, marks where the inside segment divisions met the shell. As explained at this excellent website the suture lines can be used to work out the rough age of a rock. First - look at the suture line...

...then look to see when when that type of cephalopod was around...

..and that will give you a rough age of the fossil, and therefore of the rock. For example, if your fossil has frills on both the saddles and lobes of the suture line, it must be an ammonite. Ammonites only existed between the start of the Triassic period and the end of the Cretaceous - so the rock must be between 245 and 65 million years old.

Dating with Graptolites

Graptolites have two basic parts to their skeleton; stipes (the 'leg-like' features) and thecae (the tooth-like feature in which the individual organisms lived). Graptolites were only present from the early (lower) Ordovician period to the mid Silurian. You can use the geological column above to see when these times were. The diagram below shows a number of changes that we can track for graptolites over time...

1. Graptolites had fewer stipes over time.

2. The thecae became more complex (hook like).

3. Early graptolites had the thecae on the inside, later ones had them on the outside and eventually only one side of the outside - probably to take best advantage of any food drifting past.

The specimen below, Didymograptus murchisoni, probably dates from the early mid-Ordovician - can you see why?

You need to be able to use ammonites and cephalopods as seen in specimens, drawings or photographs to interpret the history of a rock sequence including being 

Dating Using Radioactive Half Lives

The decay of radioactive materials provides a method for calculating the absolute age of some rocks and minerals. It works because the amount of radioactivity halves in a set period of time, called a half life. For example...

Potassium-40 is radioactive. Its atoms break by radioactivity to make Argon-40. The argon that is made is stable, it doesn't break down. We refer to the Potassium as the 'unstable parent' and to the argon as the 'stable daughter'...

The isotope potassium-40 has a half life of about 1.25 billion years. We know about how much potassium-40 there should have been on Earth when it was made (assuming we're right that the Earth was made from the remnants of an exploded star) but we only find about 8% of that expected amount. We can use this to put an age to Earth because...

When the Earth was made there should have been 100% of the expected amount.

After one half live, 1.25 billion years, there should have been 50%. This percentage figure is called the 'parent-daughter ratio'. 50% is a ratio of 1:1.

After two half lives there would have been 25%, a ratio of 1:3.

After three half lives there would have been 12.5%, a ratio of 1:7.

After four half lives there will only be 6.25%, a ratio of 1:15.

This places the age of the Earth at between 3 and 4 Potassium-40 half lives, so between 3.75 and 5 billion years. 

We can be rather more accurate by using a graph...

...by reading sideways from 8% to the curve, and then down from the curve to the time scale, we can see that this gives the age of the Earth at 4.5 billion years - which is where we started on this page! Element X is the Potassium-40, the graph would need different times for any other radioactive isotope.