Fossil assemblages - characteristic groupings at any particular time. Index fossils - widespread geographically and limited in time. For example, refer to Figure 8.10 . Absolute Dating, Radioactivity and Radiometric Dating Absolute date - the actual number of years before the present. Radioactivity is the basis for absolute dating.
Some isotopes found in nature are unstable - their nuclei break apart or decay.
There are explicit rules governing the decay - both the type of decay and the rate of decay. Electron capture An electron combines with a proton to form a neutron. Figure 8.13 . The rate of decay of a radioactive isotope is measured by its half-life . The half-life is a precisely measured quantity which is constant in nature.