Geologic time - chapter 9
(under construction - this will be revised within the next day or two)
We have already discussed geologic time at a very simple descriptive level and at a
philosophical level: we know the age of the earth in billions of years, and we know that
up until the 16th century Western civilization generally accepted the biblical account of
an earth that was only a few thousand years old. James Hutton was the first modern
scientist to look into the abyss of geologic time and recognize it for what it was: a
virtually infinite span, at least compared with the length of a human lifetime. Yet even
with this recognition he had no way of actually measuring that span, and it was not until
the twentieth century that the tools became available that would make this possible.
How, then, were geologists of the past two centuries able to construct the elaborate
table of eras, periods, and epochs that we know as the geologic time scale? Before
absolute dating was possible, several important principles were devised that allowed
geologists to determine the relative ages of different rock layers. These principles of
relative age dating, together with other rules for correlation of rocks of similar ages
over a broad area, made it possible to construct a cataloguing system that could be
applied over large areas of the globe. Long before geologists and paleontologists knew the
beginning and ending dates of the Cambrian or Ordovician periods, they were able to
determine whether a particular sedimentary rock was deposited in the Cambrian, the
Ordovician, the Silurian or the Devonian. When the discovery of the phenomenon of
radioactive decay made it possible to develop methods for radiometric dating of rocks, it
suddenly became possible to start attaching absolute dates to all of the time periods that
were already included in the time scale. In this chapter we focus first on the basic
principles and tools of stratigraphy (the study of layered sequences of sedimentary rocks)
and relative age dating; then on the geologic time scale; and then on the use of
radiometric dating techniques together with other tools like the sequence of reversals of
the earth's magnetic field.
- Relative age dating often relies on a small number ofl basic principles, some of which
have been around since Nicolaus Steno formulated them in Italy in the late 1600's. These
include the principle of original horizontality; the principle of [stratigraphic]
superposition; the principle of faunal (or biotic) succession; and the law of
cross-cutting relationships. Be prepared to explain these principles and be prepared to
use them in deciphering the age sequence in a block diagram.
- What is an unconformity and why is it significant? How are the major varieties of
unconformities (angular unconformity, nonconformity, and disconformity) distinguished from
each other?
- What is a geologic formation and what do geologists mean by the term
"correlation"? How is it possible to correlate rocks of a particular age with
rocks of a similar age in another area? What role do fossils play in correlation?
- Make sure that you are familiar with the major subdivisions of the geologic time scale,
including the age and significance of the Precambrian/Cambrian boundary; the Paleozoic,
Mesozoic, and Cenozoic eras; the age and significance of the Permian/Triassic boundary and
the Cretaceous/Tertiary boundary; the subdivision of the Cenozoic into Tertiary and
Quaternary periods; and the further subdivision of the Quaternary into the Pleistocene and
the Holocene epochs.
- Be prepared to explain the processes of radioactive decay, including beta-particle
emission, beta-particle capture, and alpha-particle emission. What are parent and daughter
isotopes, and what is meant by "half-life"? Be prepared to calculate the age of
a rock sample based on the ratio of parent to daughter product; This will be carefully
reviewed in class.
- What are the advantages and drawbacks of carbon-14 dating as opposed to other prominent
radioactive isotopes like rubidium/strontium 87, potassium/argon 40, and uranium 238/lead
206?
- How can radiometric dating be used, together with the principles of relative age dating,
to establish a time sequence for a particular set of rocks?
- What is the magnetic polarity-reversal time scale and what is its significance for
dating the ages of sediments and sedimentary rocks?