Geology Reference
In-Depth Information
Chapter 16
Getting a Grip on Geologic
Time
In This Chapter
Using stratigraphy to put relative dates in order
Assigning absolute dates
Recognizing the difference between eons, eras, periods, and epochs
Combining methods to add details to the timescale
Each time a rock forms, it preserves a snapshot of the earth processes that create it. This
means that within the rocks of earth's crust is a story of all the past geologic events in
earth's history. However, with the constant movement of plates (as described in Chapter
9) and changes to the earth's surface (described in Part IV), the history of events gets all
mixed up. The challenge for geologists is to place the events in the right order and inter-
pret the stories told in the rocks.
Geologists have created an accurate sequence for past geologic events by combining
methods of relative dating with modern techniques of absolute dating. By combining the
two, they constructed — and continue to revise — earth's geologic timescale. The geolo-
gic timescale of earth's history documents the past 4.5 billion years, beginning with
earth's formation (see Chapter 3). As new information is uncovered, the timescale is re-
vised, updated, and improved.
In this chapter I describe relative and absolute dating methods, explain how they are
used to interpret earth's past, and discuss how new research continues to refine and re-
vise the details of earth's history.
The Layer Cake of Time: Stratigraphy and
Relative Dating
