Igneous rocks may be dated using the radioactivity of some of their minerals. Crystals in lavas, sills and dykes
form in a very short time, geologically speaking. Radioactive isotopes decay into atoms of a daughter element which,
below a certain temperature, are locked in the crystal lattice. Over time, the amount of parent isotope decreases and
that of the daughter istope increases. The ratio of the parent to daughter isotope is used the calculate the age of
the rock.
The age of the rock is given as a number of years - it is therefore a method of absolute dating (as opposed
to relative dating using fossils and sedimentary structures).
Larger igneous masses, such as batholiths, take thousands or millions of years to cool, so radiometric dates are used
to show their cooling histories, rather than giving one specific date at which they formed.
Geopix Study Topic "Radiometric Dating"
- gives an outline of atoms, isotopes and radioactive decay
- explains how the half-life of a radio-isotope can be used to date rocks
- describes the main methods used for radiometric dating
- explains how the methods can be applied to igneous, metamorphic and sedimentary rocks
- describes the method of radiocarbon dating
The Radiometric Dating Study Topic contains the following pages:
'Radiometric Dating' is part of the Igneous Processes Study Guide CD.
To order this CD go to the Geopix Online Store
