Introduction

Radioactive heating of the mantle and crust plays a key role in geodynamics as discussed in Section 4–5. The heat generated by the decay of the uranium isotopes 238U and 235U, the thorium isotope 232Th, and the potassium isotope 40K is the primary source of the energy that drives mantle convection and generates earthquakes and volcanic eruptions. Radiogenic isotopes play other key roles in the Earth sciences. Isotope ratios can be used to date the “ages” of rocks.

The science of dating rocks by radioisotopic techniques is known as geochronology. In many cases a rock that solidifies from a melt becomes a closed isotopic system. Measurements of isotope ratios and parent–daughter ratios can be used to determine how long ago the rock solidified from a magma and this defines the age of the rock. These techniques provide the only basis for absolute dating of geological processes. Age dating of meteorites has provided an age of the solar system of 4.55 Ga. The oldest rocks on the Earth were found in West Greenland and have an age of 3.65 Ga. Lunar samples returned by the Apollo missions have ages of over 4 Ga.

Quantitative measurements of the concentrations of radioactive isotopes and their daughter products in rocks form the basis for chemical geodynamics. Essentially all rocks found on the surface of the Earth have been through one or more melting episodes and many have experienced high temperature metamorphism.