Figure 11.26: Zircon crystal. ( 15 )In general, radiometric dating works best for igneous rocks and is not very useful for deter-
mining the age of sedimentary rocks. To estimate the age of a sedimentary rock deposit,
geologists search for nearby or interlayered igneous rocks that can be dated. For example, if
a sedimentary rock layer is sandwiched between two layers of volcanic ash, its age is between
the ages of the two ash layers.
Using a combination of radiometric dating, index fossils, and superposition, geologists have
constructed a well-defined timeline of Earth history. For example, an overlying lava flow can
give a reliable estimate of the age of a sedimentary rock formation in one location. Index
fossils contained in this formation can then be matched to fossils in a different location,
providing a good age measurement for that new rock formation as well. As this process has
been repeated all over the world, our estimates of rock and fossil ages has become more and
more accurate.
Lesson Summary
- Techniques such as superposition and index fossils can tell you the relative age of
objects, which objects are older and which are younger. Other types of evidence are
needed to establish the absolute age of objects in years. Geologists use a variety of
techniques to establish absolute age, including radiometric dating, tree rings, ice cores,
and annual sedimentary deposits called varves. - Radiometric dating is the most useful of these techniques—it is the only technique that
can establish the age of objects older than a few thousand years. The concentrations
of several radioactive isotopes (carbon-14, potassium-40, uranium-235 and -238) and