Absolute time

Absolute time is established from the radioactive decay of elements. Radioactive decay proceeds at an exponentially constant rate. Exponential decay means that a constant proportion of the initial material decays in each time unit.

For example, suppose we start with 10 units and 1/10th of them decay per time interval; in the first time interval one unit will decay, and we shall have 9 units left. In the second time interval, a proportion equal to 1/10th of the remaining 9 units (i.e., 0.9 units) will decay; and we shall be left with 8.1 units. In the third time interval, a further 1/10th of the 8.1 units will decay, leaving 7.29 units (8.1 - 0.81) at the beginning of the fourth time interval ... and so on.

To estimate the age of a rock by the radioisotope technique, we need to be able to make two measurements. What are the two measurements? They are the radioactivity of the rock now and when it was formed. The radioactivity in a rock is obviously measureable now. The composition of the rock was originally fixed when it crystallized as an igneous rock from liquid magma, and the radioactivity of modern magma can be measured: this is a good estimate of the radioactivity present when the rock first formed.