Geology Reference
In-Depth Information
Relatively Absolute: Combining Methods
for the Best Results
With the advancement of absolute dating techniques, you may think relative dating is no
longer needed. However, combining relative and absolute methods of dating is useful for
many reasons:
Absolute dating requires the presence of certain elements, which may not be found
in all the layers of rock that scientists wish to have dates for. In such cases, a sci-
entist may need to rely on relative dating for portions of a rock sequence that
can't be dated with absolute methods. The result is that geologists continue to
rely on relative dating to fill in gaps for periods when they cannot measure accur-
ate absolute ages.
Absolute dating of sedimentary rocks can be difficult because the sediments
provide dates of the mineral's formation from molten rock, not the formation of
the rock they are currently in. Therefore, a different approach must be used to
date layers of sedimentary rock. This approach is called bracketing.
Bracketing
means that the geologist takes a sample of dateable material
(for example, a mineral from a metamorphic or igneous rock) from immediately
above and immediately below the rock layer (or rock) she wants an age for. By
measuring dates on these samples, she can determine a minimum age and maxim-
um age for the rock layer in question. This approach is more specific than meth-
ods of relative dating but still does not give the exact age of the rock. Rather, it
provides a bracket of “somewhere between this age and that age.”
Methods of absolute dating require intensive work in the laboratory, many years of
training, or money to pay a special scientist to do the analysis for you. It would be
incredibly time-consuming and expensive to sample and date every single rock or
rock layer on the earth's surface. Instead, geologists choose to sample and study
specific parts of the rock record — layers that seem interesting or significant in
some way, such as indicating the boundary between two time periods when
something dramatic happened (like the extinction of the dinosaurs).
With the combined approach, scientists focus their time and money on specific, import-
ant questions that require absolute dates, and they still depend on relative dating for
other parts of the rock record.
