Geology Reference
In-Depth Information
Rock layers below are generally older than rock layers above.
All sedimentary rock layers are originally formed in a horizontal position.
When a different rock is cutting through layers of rock, the cross-cutting rock is
younger than the layers it cuts through.
These principles and a few others that I describe in Chapter 16 guide geologists called
stratigraphers
in interpreting the order of rock layers so that they can form a relative or-
der of events in earth's history.
However, sometimes simply knowing that something is older than — or younger than —
something else is not enough to answer the question being asked.
Absolute dating
meth-
ods use radioactive atoms called
isotopes
to determine the age in numerical years of
some rocks and rock layers. Absolute dating methods may determine, for example, that
certain rocks are 2.6 million years old. These methods are based on the knowledge,
learned from laboratory experiments, that some atoms transform into different atoms at
a set rate over time. By measuring these rates of change in a lab, scientists can then
measure the amount of the different atoms in a rock and provide a fairly accurate age for
its formation.
If the process of obtaining absolute dates from isotopes seems very complex, don't
worry: In Chapter 16, I explain in much more detail how absolute dates are calculated
and how they are combined with relative dates to construct the
geologic timescale:
a se-
quence of earth's geological history separated into different spans of time (such as peri-
ods, epochs, and eons).
Witnessing evolution in the fossil record
The most fascinating story told in the rock layers is the story of earth's evolution. To
evolve
simply means to change over time. And indeed, the earth has evolved in the 4.5
billion years since it formed.
Both the earth itself and the organisms that live on earth have changed through time. In
Chapter 17, I briefly explain the biological understanding of evolution. Much of modern
understanding about how species have changed through time is built on evidence from
fossilized
or preserved life forms in the rock layers. Fossilization occurs through differ-
ent geologic and chemical processes, but all fossils can be described as one of two
forms:
Body fossils:
Remains of an organism itself, or an imprint, cast, or impression of
the organism's body.
