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 in the RRB