E. R. SWANSON – spring 2009
A SHORT GEOLOGIC HISTORY OF THE EARTH
The Geologic Time Scale
The concept of relative dating and the discovery that fossils could be used to determine the age
of rocks quickly led to the construction of the geologic time scale.
This time scale, based on fossils,
was divided into eras according to the life forms (fossils) found in rocks, and these time scale names
make more sense if you understand that the
zoic refers to life
paleo = old, meso = middle
and ceno = recent.
The major divisions of the geologic time scale are the
Paleozoic, Mesozoic and
These eras are divided further into time periods with names typically derived from the
regions were rocks of the various time periods were first described.
The Paleozoic Era, for example, is
divided into the Cambrian, Ordovician, Silurian and Devonian periods, all defined in England.
Devonian rocks were first described in Devonshire.
Younger Paleozoic periods include the
Pennsylvanian and Mississippian rocks from the U.S.A., and Permian rocks from the Perm province of
There are no hard and fast rules however.
Most of the world refers to our Pennsylvanian and
Mississippian rocks as Carboniferous for the abundant coal beds in these rocks.
(from the Latin
meaning chalk), the last period in the Mesozoic, is so named because limestone
(including chalk) is abundant in rocks of this age.
All of these periods are based on major changes in
the fossil record, although not as great as the radical fossil changes (mass extinctions) found between
rocks of the Paleozoic, Mesozoic and Cenozoic Eras.
And what about rocks that are older than Paleozoic?
What are they called, how are they
divided, and are these rocks older than life?
Well, these rocks are all older than the first Paleozoic
period, the Cambrian.
For this reason, Pre-Paleozoic rocks are said to belong to
Some evidence for life can be found in Precambrian rocks, but fossils in Precambrian rocks are
extremely rare and primitive.
Also, most Precambrian rocks are igneous, which would not have had
fossils, or they are sedimentary rocks that have been metamorphosed, a process that would have
destroyed any existing fossils.
Fossils, therefore, are not particularly useful in dating Precambrian
rocks, but dating based on the decay of radioactive elements is particularly helpful here.
Just as we
know less about the distant reaches of the Universe than about our own solar system, we know very
much less about Precambrian earth history than we do about the history of the Cenozoic Era, a time
closer to our own.
This represents a fairly significant lack of knowledge because the Precambrian Era
represents 88% of Earth’s history, compared to one 1.4% of the geologic time belonging to the
Things ancient, therefore, tend to get lumped together, and we play fast and lose in the
Precambrian talking about 4.3 or 4.4 billion as if 100 million years was next to nothing when, in fact,