Geologists (scientists who study rocks) divide the time
since the Earth was formed until today into chunks called periods. During the
various periods, different species of animals and plants lived on the Earth.
For example, the Cretaceous period, which lasted from 146 million years ago to
65 million years ago, was the final period of the dinosaurs. Some rocks can be
given a relative age by identifying the fossils they contain. The date of
formation of some rocks can be found by using RADIOMETRIC DATING.
Over many decades, palaeontologists have built up a database of
fossils called the fossil record. This shows when different species of animals
and plants lived in the history of the Earth. A fossil can be matched against
the fossil record to find the age of the rock it was found in. The information
in the fossil record is based on using stratigraphic layers to tell the age of
a rock and the characteristic life forms within it. Here are some fossils and
the periods they belong to.
1.8 million years ago (mya) to the present day
Radiometric dating is a way of measuring the age of a
piece of rock. Igneous rocks contain tiny amounts of radioactive chemicals. As
the rocks age, these elements gradually break down into elements that are not
radioactive. By knowing the rate at which the elements break down and measuring
their level of radioactivity, the age of a rock sample can be
calculated.
In argon-argon dating, the amounts of two forms of the element
argon in a rock sample are measured using a device called a mass spectrometer.
One form only is produced by radioactive decay as the rock ages, so comparing
how much there is of this in relation to the other form of argon reveals the
age of the rock.
