Geoscience Reference
In-Depth Information
Geologic Time
Understanding geologic time is difficult because it is hard to
relate a diagram such as Figure 12.19 to the significant geologic
events used to mark major intervals of time. Such events in-
clude major periods of mountain building, catastrophic extinc-
tions, and the emergence of certain kinds of plants or animals
in the fossil record. In an effort to improve this understanding,
go to the
Geo Media Library
and select
Geologic Time
. This
animation illustrates, with images and text, some of the major
geologic events associated with past time intervals. The simu-
lation is interactive because you can explore various time peri-
ods on your own and view their unique qualities. After you visit
all the time intervals within this simulation, be sure to answer
the questions at the end to test your understanding of the geo-
logic timescale.
is generally considered to represent the time of the great ice
ages, which will be discussed in Chapter 17. The subsequent
Holocene spans the past 10,000 years and, as discussed in
Chapter 9, is generally considered to be the time of so-called
modern climate.
with the amount of the decayed end product. Using radiocar-
bon dating, for example, a geologist can compare the amount
of carbon-14 to the amount of nitrogen-14. If the ratio be-
tween the two is 1:1—that is, equal parts—then it means
that one-half of the radioactive isotope has decayed. In other
words, about 5730 years have passed since the carbon was
deposited.
“Telling” Geologic Time
You might wonder, How do geologists really know the age of
Earth and when major events occurred? The answer lies in our
understanding of the way in which rock elements radioactively
decay through time. Rocks are composed of countless atoms, all
of which contain protons and neutrons in their nuclei. Although
many of these atoms remain stable indefinitely, some do not.
These unstable atoms are called
radioactive isotopes
. In these
atoms, particles within the nucleus break apart and the atom
decays into a different element. For the purpose of analogy,
imagine a pet dog that suddenly woke up on Monday morning
and had become a cat, then the following Monday changed
again into a mouse.
In the context of calculating rock age, isotopic decay is im-
portant because radiation is emitted when this process (known
as
radioactivity
) occurs. Each isotope decays at a different con-
stant rate that is known by geologists. The reference timeframe
for the decay rate for any isotope is called its
half-life
; this
is the amount of time required for one-half of the isotopes in
any given sample to decay. For example, thorium-232 requires
14.1 billion years to change through its decay series (including
radium-228 and radon-220, among others) into the stable iso-
tope lead-208. In contrast, the half-life of the radioactive carbon
isotope (carbon-14) is 5730 years, during which time it converts
to the stable isotope nitrogen-14. In this fashion, radioactive
isotopes provide a running time clock for the history of Earth,
and geologists use
radiometric dating
to calculate age.
If a geologist is interested in the age of a rock sample,
he or she simply compares the amount of the original isotope
Putting Geologic Time in Perspective
Although understanding radiometric dating provides con-
fidence in the ages reported in Earth history, it does not re-
ally aid with the comprehension of deep geologic time. If
you want to put geologic time in some kind of perspective,
consider that virtually all scientists believe that anatomically
modern humans—that is, people looking basically like you
and me—have lived on Earth for about 150,000 years. Sounds
like a long time, yes?
Here is another analogy that helps explain deep time.
Imagine that all geologic time is contained within a single calen-
dar year, with time beginning on January 1 and extending until
December 31, as shown in Table 12.2. In this analogy each day
is equivalent to 12.6 million years, each hour is 525,000 years,
each minute is 8750 years, and each second is 146 years. The
year begins with the formation of Earth. From that point until
the middle of April, no life-forms inhabit Earth. In other words,
Earth is a lifeless planet for the first 4.5 months of the “year.”
Nothing
except single-cell organisms such as amoebas and sim-
ple bacteria exist from early May until late November, when the
first vertebrate animals (fish) develop. Subsequently, the first
land plants emerge on December 3. Reptiles develop on about
December 13, followed by the first mammals on December 18.
Dinosaurs dominate Earth until late December, when they be-
come extinct and mammals such as shrews and other small
rodents emerge and more fully evolve. Early hominids do not
evolve until 6 p.m on December 31, which means that anatomi-
cally modern people have existed for an extremely short period
of time within the year. Your life occurs within the last 0.05 sec
of the year. No wonder people have difficulty understanding the
concept of geologic time!
Radioactive isotopes
Unstable isotopes that emit radioac-
tivity as they decay from one element to another.
