Geology Reference
In-Depth Information
Organizing rock layers into the order of their formation is the first step in constructing a
geologic history. When the events are in the proper order, the story in the rock layers
can be read. The challenge is figuring out the original order of rock layers that have
been tilted, deformed, eroded, and otherwise changed.
In this section, I describe how the study of rock layers, called
stratigraphy,
is used to un-
tangle sequences of rock formation and construct a relative timescale for these events.
Speaking relatively
One way of describing history is to figure out the correct order of past events in relation
to one another. This approach is called
relative dating.
Relative dating does not provide
numerical ages or dates for events. Relative dating techniques describe only when
events happen in relation to other events.
For example, if you describe the ages of your family members relative to you, you may
say that your sister is younger than you are and your brother is older than you are. You
describe each person's birth in relation to your birth: Your brother was born, then you
were born, and then your sister was born. In this way, you construct a relative sequence
of events without providing the actual age of each family member.
This same approach is used by geologists to build a relative history of the earth, stating
that one period or sequence of events occurred before or after another.
Sorting out the strata
Long ago, geologists realized that rock layers could be organized through time relative
to one another. This sequence of relative rock layers provided a context to interpret
earth's history. For this reason, the study of layered rocks is the foundation of relative
dating in geology.
The study and interpretation of rock layers (or
strata
) is called
stratigraphy.
Scientists
called
stratigraphers
compile information from rock layers all around the world and com-
pare that information in order to understand the sequence of rock layer formation and
details about the history of earth's surface.
Stratigraphers are interested in both the history of the rock layers and their formation
or origin. Therefore, these scientists have multiple ways of describing the rocks layers,
or
units:
