Determining Absolute Time

Absolute Time: numerical time using a specific units like years

Isotopes: Form of an element with more or fewer neutrons than other forms of the same element

Decay product: Element produced by the decay of a radioactive isotope

Decay-product ratio: Ratio between the mass of a radioactive element and the mass of the decay product

If an isotope is radioactive, it will break down naturally into a lighter element called a decay product.

This process occurs at a predictable rate and can be used to determine how old an object is.

Half-life: Time required for half of an element's atoms in a sample to change to the decay product.

In each half-life only half of the remaining radioactive atoms decay, no matter how large the sample is.

Look at the diagram below which represents the radioactive decay of uranium-238. The shaded area represents the decay product which is lead-206. The half-life of uranuim-238 is 4.5 billion years, since this object has gone through two half-lives it is 9 billion years old.

Look at the decay-product ratio This will help calculate the number of

half-lives that have occurred since the sample was formed.

Once you know that you can find the age of the sample.

If an object is made up of 50 % decay product then the object has been through 1 half-life.

75% decay product equals 2 half-lives 87.5% decay product equals 3 half-lives 93.76% decay product equals 4 half-

lives, and so on. Once you know the number of half-lives

you can determine the age by using the half-life in years listed on the back of the Reference Tables.

The chart on the next page gives information about the radioactive decay of carbon-14.

You could make a chart like this one for any radioactive decay question.

It will show how the half-lives, radioactive element (or decay product) and the age of the sample are related.

Try doing this to see if it will help you. You will want to have the front page of

your ERST handy

The sample to be dated must contain a measurable amount of a radioactive element and its decay product

A sample with the remains of living things will likely contain carbon-14

A geologist must choose the radioactive isotope whose half-life best measure the age of the sample

Example: Using carbon-14 can only get you back 50,000 years, which is relatively very recent

More recent rocks wouldn’t have enough uranium-238 which can measure the oldest rocks on Earth

These notes are based on pages 132-133 in the RRB